An Introduction to GIS usingArcView

A Tutorial

Issue 1, Spring 1997 based on ArcView release 3.

Preface:This tutorial is aimed at non-geographers who are seeking to learn how to use a GIS in order toexamine spatial, geographic, characteristics of information within their own disciplines. It isintended for use primarily within the UK Academic Community and is thus based upon theArcView GIS and upon British data which is readily available within the community. Thestructure is such that it is hoped that local usage will substitute appropriate datasets, of specificinterest to the students, for those referenced in these materials where appropriate. It is intendedthat this framework is adequate to permit the addition of further units to take account of newfacilities within ArcView and, perhaps, to introduce more sophistcated techniques of spatialscience.

ArcView is a product which runs on a wide variety of platforms, some of which restrict thecapabilities of the tool. The initial units, 1 - 4, of this tutorial should work on all ArcViewsupported platforms although it is recommended that one of Windows 95, Windows NT, orUNIX, should be used if at all possible. Unit 5 will make extensive use of features which are notsupported on Windows 3.1 or on the Apple Macintosh platforms.

Inevitably, individual installations will make use of different filestore conventions. The authorsenvisage that local instructions will be provided alongside these materials to define the specificfilestore naming and access conventions that apply.

Within this material there are illustrations drawn from a variety of sources, much of which iscopyright. Whilst copies of these materials may be made for teaching and training purposes, nopart of these materials, printed or digital, may be used other than as an integral part of thetutorial. Thanks are gratefully expressed to the copyright holders for permission to reproducetheir material.

Contents:Unit 1: A Brief introduction to Geographical Information Systems. Especially for non-geographers; starts at the beginning. Should require 1 to 2 hours, depending upon the student'sfamiliarity with maps, etc. No computing facility is necessary but a ruler and a pencil are!

Unit 2: Introduction to ArcView and simple display. Runs ArcView, loads and manipulates somedatasets and introduces some basic presentation techniques. May require 4 hours.

Unit 3: Introduction to Analysis. Introduces some of the basic spatial analysis techniques, suchas statistical and overlay analysis. May require 4 hours.

Unit 4: Data - input, conversion, integration and management. Introduces some of theconsiderations involved in getting data into a GIS

Unit 5: Introduction to cell based analysis. Using the Spatial Analyst extension to ArcView,introduces more complex analytic techniques. This unit is not included in the first release.

Datasets required (Units 1, 2 and 3):

Bartholomew Great Britain Digital data for roads, railways, and drainage: roads_gb.shp,

railf_gb.shp, and river_gb.shp; Administrative Boundaries (County and District): counties.shpand dstricts.shp (1981 boundary data). Sample copies of these datasets are provided with thesematerials; seek local advice regarding their location on your system.

Acknowledgements:The production of materials such as these can only be achieved by means of a team effort. Alarge number of people and organisations have contributed to making this possible. Especialthanks are due to the following organisations:

ESRI (UK); Harper-Collins Cartographic (Bartholomew); The National Remote Sensing Centre(NRSC); AGOCG; Leeds Metropolitan University and the Universities of Newcastle upon Tyne,Sussex, and York.

The Advisory Group on Computer Graphics, AGOCG, provided a generous grant which enabledan editorial workshop, held in York in July 1996, and assisted with the editorial costs ofproduction. The Editors wish to thank the participants in the July Workshop, for theircontributions in completing tasks set at the workshop, and their institutions for supporting them:Martin France, Sarah Falconer, Rob Fletcher, Howell Harris, Kamie Kitmitto, Martin Ralphs,Phil Schwartz, Oliver Tomlinson, Rosemary Tomlinson, John T Wills, and Peter Wyatt.

The following assisted in the definition of the project and in the assessment of the materialsgenerated: Nigel Brown, John Castleford, Kamie Kitmitto, David Maguire, Anne Mumford, JimPetch, and Mike Worboys. Thanks are gratefully expressed for their contributions of time andguidance.

Undergraduate and Post-Graduate students studying courses at Leeds Metropolitan University,and at the University of York, have field trialled these materials and have made manyconstructive comments; their willingness to learn, their tolerance of incomplete materials, andtheir many helpful suggestions for improvements are gratefully acknowledged.

Grateful thanks are due to Helen Parker, of the Computing Service at the University of York,who provided invaluable secretarial support and made much of the finishing process a reality.

Spring 1997

The Editors:

T J Browne, University of Sussex

P J Halls, University of York

A Millard, Leeds Metropolitan University

A P Miller, University of Newcastle upon Tyne

Unit 1:A brief introduction to Geographical

Information SystemsThe aim of this unit is to provide a measure of spatial awareness together with a briefintroduction to Geographic Information Systems, GIS, and some of the techniques uponwhich it is founded.

1.0 Introduction

We live in a 4-dimensional world. Human beings have always had to consider how things arearranged spatially in such a world and how they might change with time, whether it waspalaeolithic hunters deciding how best to stalk their next meal or 20th Century planners decidingwhere to site a new supermarket. With population increase and industrialisation continuing toexert pressure on the land available it is more important than ever that information about spatialrelationships and trends can be collected, stored and processed efficiently. Although suchactivities have been going on for hundreds of years the basic system involved is always thesame.

Any system is essentially a dynamic entity which can receive inputs, process them in some wayand produce outputs. In the past, systems dealing with information have been largely paper-based with someone's brain doing the processing in the middle. If the information is spatially-based in some way then we have the essence of a Geographical Information System.

The term Geographical may be regarded by some as being too disciplinary specific. It couldeasily be replaced by terms such as spatial, environmental, land etc. But the termGeographical is now in such widespread usage that it is here to stay.

The development of information technology (IT) has already had a profound effect on manyaspects of life. Over the past decade or so the rapid advances in computing hardware havedramatically increased the potential for producing IT systems to effectively manage and processspatial information, which is invariably more complex than non-spatial information.

Many of the early computer-based Geographical Information Systems (in the rest of thisguide the term GIS refers specifically to computer-based geographical information systems)have been relatively technical and only really accessible to people with a high level of computerliteracy. The purpose of this Study Guide is to introduce you to a particular GIS called ArcViewthat has been designed specifically to make GIS accessible to a much wider audience.

You are advised to work steadily through the Guide although, depending upon your background,you may be able to skip through some sections relatively quickly. To help you consolidate yourunderstanding there are Self-Assessment Questions (SAQs) for you to try with the answers in theback of the guide.

For most of the Guide you will need access to a computer with ArcView installed. However,before actually sitting down at the keyboard it is worth considering briefly how we have dealtwith spatial information before the advent of GIS and some pointers to how GIS mirrors andmodifies this approach. If you are a geographer or familiar with using maps you may want to gostraight on to section 2, but if this is relatively new to you or your memory needs refreshing thenwork through this introductory section before you switch on the computer.

1.1 Using maps to record spatial information

1.1.1 Features

Plate 1.1 shows a portion of landscape consisting of a number of features such as buildings,roads, fields etc.

Plate 1 - Photo of Whitby, copyright (c) PJ Halls, 1975, Used with permission.

The traditional way of recording spatial information about this particular piece of landscapewould be on a paper map of the type shown in Fig. 1.1.

Fig. 1.1. Map of Whitby

In essence, the map can give us three types of information about the features that make up thelandscape:-

1) their spatial arrangement2) their shape3) their size

A map will always provide information on 1) about all of the features that it shows. However,depending upon their size and the scale of the map (see 1.1.2 below) some features may only beshown by a symbol (which tells us virtually nothing about their actual size and shape) at theappropriate place on the map. For example, in Fig. 1.1 the size and shape of the area of woodlandis clearly shown but the bus station symbol tells us nothing about its shape or size, except that itmust be a lot smaller than the woodland!

The meanings of the various symbols used are usually given in a key alongside the map. On thekey of Ordnance Survey maps, similar types of features are grouped into categories or themes(e.g. roads and paths, boundaries, water features etc.). During the period BC (Before Computer!),if it was necessary to analyse the themes that make up a map, they could be drawn as a series ofseparate overlays to make the analysis easier, as shown in Fig. 1.2

Fig. 1.2. Schematic 3D diagram of map from Fig. 1.1

1.1.2 Scale

For a map to be as accurate as possible the sizes of features and distances between them shouldbe in the same proportion as they are in the real world. This is done by drawing the map to scale.The number of times the real world distances are greater than the map distances gives themagnitude of the scale e.g. 1 cm on the map may represent 50000cm (i.e. half km) in the realworld, hence the scale would be 1:50000 . Scale can also be represented as a scale bar (see Fig.1.1).

When real world distances have been reduced to relatively small distances on the map it isknown as a small scale map whereas when the real world distance is represented by quite largedistances on a map it is known as a large scale map. Another way of considering this is that on astandard size of paper, say A4, a map covering the whole of England would be small scalewhereas, on the same piece of paper, a map covering a few streets in a town would be largescale. The boundary between large and small scale is not officially defined but in the UnitedKingdom, in general, anything at 1:10000 or larger (e.g. 1:2500, 1:1250) is large scale andanything smaller than 1:10000 would be small scale.

SAQ 1. What is the scale of the map in Fig. 1.3?

Fig. 1.3. Road lengths in kilometers.

It also needs to be remembered that scale influences the accuracy with which location can beshown. Although cartographers (people who produce maps) always aim for accuracy in showingthe location of one feature relative to another, the margin of error on the map associated with afeature's actual location on the ground is likely to increase as the scale of the map decreases.There are many reasons for this. Some are that cartographic conventions often result in, forexample, a road of a particular classification always being drawn at a standard line width andthat point features (e.g. windmills) are always drawn at a constant size. Sometimes features that,when represented at a small scale would overlay each other, are deliberately slightly displaced sothat they are all visible, e.g. a road, railway and river co-existing in a steep-sided valley.

1.1.3 Co-ordinates

To use a map effectively it is important to have some means of identifying the location of aparticular feature. When trying to indicate where something is, people often describe it inrelationship to something else "up the road, turn left and it's just opposite the supermarket". Thisis all very well if you are always coming from the same location but if your starting point variesthose instructions are not much use.

If you look at a model of the globe, you should observe vertical lines being drawn joining theNorth and South poles and a series of horizontal lines being drawn between these poles.

Fig. 1.4. Map of globe, showing latitude & longitude graticules

The vertical lines are lines of longitude and the horizontal lines are lines of latitude. Theequator is the most famous latitude and is given the arbitrary value of zero. The North and Southpoles are given the arbitrary value of 90. So all locations between the equator and the poles havea value lying somewhere between 0 and 90. If you look at an atlas, you will observe that thesouthern most tip of Cornwall, in south-west England has a latitude of approximately 50.Cartographers call latitudes north of the equator, Northings (Guess what those south of theequator are called!).

Some time last century it was internationally agreed to give the value zero to the vertical line, orlongitude that passes through Greenwich in London. Cartographers call Longitudes east ofGreenwich, Eastings. (Guess what those west of Greenwich are called!). We will convenientlyignore for now what happens when you have got half way round the world.

Using a latitude value and a longitude value it is possible to uniquely locate any position on theglobe. These lines do not, of course, actually exist on the earth, but over the centuries,cartographers have found them to be a convenient way of identifying locations.

However, it is often not very convenient to use these particular lines on large scale maps. In suchcircumstances, cartographers commonly use some form of grid system, tailored to the specificneeds of a particular country.

For those of you who can remember drawing graphs at school, you will recall that the horizontalaxis was often also called the 'x' axis and the vertical axis was often called the 'y' axis. Any pointwhich had an x and y value could be located, or represented on the graph. In a similar way,imagine the outline of the UK drawn as a series of x,y values on a graph. Also imagine that thex,y value of 0,0 (which is equivalent to the origin of a graph) is in the Atlantic Ocean just off theScilly Isles, which are located west of the Cornish mainland. Every location in the UK can nowbe given a unique x,y value, also known as a pair of coordinates.

This describes a simplified version of a special grid which the Ordnance Survey use for maps ofthe UK, and which they call the National Grid, (not to be confused with the other national gridwhich delivers electricity across the country!). In this grid, the UK is divided up into large

squares of 100km x 100km (called, not surprisingly, 100km squares) and the origin of thiscoordinate system (i.e. the x,y cordinates 0,0) is indeed just west of the Scilly Isles. The x valuesincrease moving eastwards to Kent and the y values increase moving northwards to the ShetlandIsles. Pinching terminology used above, locations east of this origin are called Eastings andlocations north of this origin are called Northings. So an x,y pair of coordinates can be identifiedfor all intersections between the horizontal and vertical lines in the grid.

But how are features located that do not conveniently fall at these 100km grid intersections? Thisis resolved by further dividing each 100km grid square into 10km grid squares. In turn, each10km grid square can be further divided into 1km grid squares, and so on. The extent to whichyou wish to continue such subdivision depends upon the accuracy at which you wish to representthe location of a point. This in turn will be influenced by whether you are using a large scale map(lots of detail) or a small scale map (not so much detail).

Consider first, just the Easting. The 100km line will be represented by a single digit. This will be0 or 1 or 2 etc. Representing the horizontal (i.e. x ) value of the bottom left hand corner of the100km grid square which contains the feature of interest. The 10km subdivisions of the 100kmEasting will be represented by a second digit, the 1km subdivisions of the 10km Easting will berepresented by a third digit and so on, with the 1m subdivisions represented by a sixth digit. Asimilar subdivision can be made for the Northing.

Fig. 1.5.

In such an example, the grid reference would therefore consist of twelve digits, the first sixrepresenting the Easting and the second six digits representing the Northing.

Such a grid reference can look uncomfortably indigestable and would only be used for very large

scale maps. For, say the Ordnance Survey 1:250,000 maps, a grid reference expressed as eightdigits, the first four for the Easting and the second four for the Northing be located. The eightfigure grid reference for the church with a spire in Fig 1.6 is 54561782

Fig. 1.6.

Purely for completeness, though at the risk of adding some confusion, two alternative but relatedways in which the Ordnance Survey express grid references should be noted, because it is oftenused on their paper maps. Rather than express a grid reference as e.g. 54561782 the Easting andNorthing 100km digits are written first, and then followed by the 10km and subsequent digitsfor first the Eastings and then the 10km and subsequent digits for the Northings. So this gridreference would be written as 51/254798. A further variant is that the 100km squares are given atwo letter code, so 51 is also known as TQ, giving a grid reference expressed as TQ254798. Butcomputers are much happier dealing with an all-digit representation with a logical hiararchyfrom 100km, 10km, 1km etc. so we shall not consider the alternative representations any further.

Fig. 1.7

SAQ 2. Describe what is found at grid reference 48905094 in Fig. 1.7.

SAQ 3. Using Fig 1.7 to identify the grid lines in Fig. 1.1, what is the grid reference forRuswarp Police Office in Fig 1.1. What is the grid reference for the pub (PH) in Fig. 1.7?

1.1.4 Representing height

Ignoring the 4th dimension (time) for the moment, we can say that a map, with length and width,is a 2-dimensional representation of a 3D world which in reality has not only length and widthbut also height. Therefore there needs to be some way of indicating how this 3rd dimension(height) varies. This can only be done if there is an agreed 'baseline' to measure the height from.On Ordnance Survey maps this is known as the vertical datum and is the mean sea level atNewlyn in Cornwall.

Height is represented on maps in a number of ways. The height of an individual point can begiven as a spot height or, as the height of a triangulation pillar, which is a concrete post oftensituated on the top of a hill and from which the height of other points visible from the hill can beaccurately measured (see Fig. 1.7). In fact, such trig. points are becoming increasinglyredundant as it is now possible to measure very accurately the location (including height) of anypoint on the Earth using stereo photogrammetry and, increasingly, satellite technology. Manymaps also have contour lines, each of which joins up all the points at the same height. The closertogether the contours are the steeper the slope is. With practice, by studying the contours on themap you can get a good picture in your mind of the general 3-dimensional landform.

Fig. 1.8

SAQ 4. Briefly describe the landform shown on the map in Fig. 1.8.

1.1.5 Map Layouts

The layout of a map can vary but if it is to be of maximum use to the person studying it there arecertain essential components that must be present. These would include whatever themes wereappropriate for that map together with the necessary grid for co-ordinates, scale, key, north arrowand, of course, a title!

1.2 The advantages and uses of a Geographical Information System (GIS)

We have seen how a paper map contains information about the size, shape and spatialrelationships of features in the real world. However, there may be other information about afeature or features that we need to know. For example, a traffic planner might need to know thenumber of vehicles per hour passing along a particular road or someone in business might need

to know how many sales she or he was making in a particular town. This type of information(known as thematic information) would not be shown on a general-purpose map and the personrequiring it would have to obtain it from another source and, if they wanted to represent itspatially in some way, draw a special map to do so.

In addition, there may be topographical information (which is about specific locations that couldbe shown on a map) that cannot appear on a particular map because of its scale. For example, anecologist might need to know the number, distribution and type of tree species growing in aparticular wood, which itself can only be shown as an area of green on the map.

A GIS provides an opportunity to store both these kinds of non-spatial information (also oftencalled descriptive or attribute information, as well as the spatial information contained within themap itself, in a single system. In current GISs these two types of information (spatial and non-spatial) are often stored in two separate databases (essentially, the term used for storing data in acomputer in a particular structured way). However, the two databases are linked or joinedtogether so that, for example, the characteristics of a line in the spatial database can be identifiedas representing a road and that it is a motorway; a point represents a church, and the church has atower, etc. etc. An analogy might be the way we mentally link information with particularfeatures on a map. The term georelational is sometimes used to describe the connection betweena spatial database and the corresponding attributes in the attribute database.

However, a GIS is not merely a means of storing and displaying information. Increasingly, itsstrengths lie in its processing or analytical capabilities. This might involve integrating (e.g.overlaying) data from different themes to find a solution to some form of planning problem e.g.integrating data from road, urban area and wildlife habitat themes to identify possible routes for aby-pass. Also, a GIS can incorporate the influence of the 4th dimension, time, very easily intothe analysis. This might involve comparing the same types of data for a particular area over aperiod of time to establish any trends e.g. how fast scrub is invading an area of grassland.

For this reason, GIS is becoming increasingly important to a growing number of activities andprofessions. These include people involved with any aspects of land use (e.g. planning,agriculture, forestry, nature conservation), human population studies (e.g. research into thegeographical distribution of particular diseases like so-called 'cancer clusters'), commerce,environmental monitoring and management etc. etc.

By now you should have some very general impressions of what a GIS is and what its potentialsare. Before passing on to more details about the specific characteristics of ArcView, this sectionconcludes with what is not exactly a precise definition of GIS but does emphasise its very wideappeal and potential!

"GIS is a sort of culture, born of a multi-disciplinary content, where every actor brings their skill,their point of view and their own GIS definition."

David Rhind (Director General of the Ordnance Survey)

Suggestion for further reading.

Chapter One of Principles of Geographical Information Systems for Land Resources Assessment,P. A. Burrough, Oxford University Press.

Unit 2:Introduction to ArcView and Simple display

The aim of this Unit is to introduce you to the basic components of ArcView, how they canbe used to display spatial data and how you can save and print your work.

2.0 Introduction - and running ArcView

ArcView is a Geographical Information System, GIS, which runs on a wide variety of differentcomputer systems, (often called 'platforms'). ArcView behaves in exactly the same way on eachof these platforms, so it does not matter which one you are using when working with this tutorial.There are some tools in ArcView which cannot operate in certain computing environments; thesehave been introduced as 'Extensions' from ArcView Release 3 (1996) and will only be met in themore advanced, optional, units of this tutorial.

Throughout the tutorial, terms specific to ArcView are printed using this font. When you arerequired to carry out a specific action on the computer the instructions are given in italics likethis. Where a specific text must be typed, for example to open a dataset, the text is printed likethis.

The ArcView GIS looks more or less identical on the screen no matter which platform it isrunning on, although the way the application is started, which we need to do next, may vary.

Now start up ArcView. As institutions differ in the way their computer systems are set up you willneed to consult Point 1 on the Addendum Sheet from your own institution for instructions on howto do this.

Once your screen has settled down it should look like Fig. 2.1

Fig. 2.1

ArcView is organised into a number of components which are displayed as 'windows'. At themoment your screen should show two windows, the Arcview window itself, within which allArcView is displayed, and inside this, a smaller window currently headed Untitled which isknown as the Project window (more of this in a moment!).

The basic ArcView window contains, from the top, a Menu bar, with named pull down menus,currently listing File, Project, Window and Help; a tool icon bar, with pictorial icons for thosetools available, currently the Save, and Help icons; a display area; and, at the very bottom, astatus line. When the mouse cursor is located over a tool icon descriptive text identifying thepurpose of that tool is displayed in the status line.

• Try this now by putting the cursor on the 'floppy disk' icon.

The collection of tool icons displayed in the tool bar will vary depending upon the type ofwindow that is currently active. Most windows provide a Help tool, which invokes the helpsystem in a context relevent to the specific window, in addition to the Help menu.

Now look at the Project window. On its left-hand side this lists the 5 main components ofArcView, each of which has its own type of window; those for displaying VIEWS of spatial data;examining database TABLES; preparing statistical graphs or CHARTS from selected data; forsetting out the LAYOUT of a printed page; and, for more advanced users, programming SCRIPTSfor new ArcView applications. When you want to save your ArcView session you do so as aproject which can contain any of these components.

Each window can be maximised to the extent of the main ArcView window, between the tool barand the status line. It can also be minimised to an icon, also within the ArcView window area.

Click in the top right-hand corner of the Project window to maximise it. Now click again toreturn it to its original size.

We will now concentrate on the View window in order to discover how to display spatialinformation. Cartography, commonly called mapmaking, is the technique of representing spatialdata in a graphical form. The advantage of doing this is that the human brain can process visualimages extremely quickly and therefore gain a rapid appreciation of relative distance anddirection. In ArcView maps are displayed as a VIEW but with distinct advantages over a papermap. The View window is more like an interactive drawing board where you can specify existinginformation to be drawn, control the way information is represented (symbology), sketchannotations onto displayed information and even modify the information presented.

Make sure the Views icon in the Project window is selected, as shown in Fig.2.1. Click on theother Project window icons in turn to see how selection is indicated before returning to Views.

The right-hand part of the Project window will list the different Views (or Tables etc.) currentlyavailable in the project. As this space is empty and the Open button is unavailable there are noviews at present and we shall have to create one. ArcView does allow you to have several viewsopen at once, but we shall work with a single View window for the time being.

With Views selected, click on the New button in the Project window.

Your screen should now look like Fig. 2.2

Fig. 2.2

Notice that as well as opening a View this has also changed the Menu Bar and icon bar to thosenecessary for a View. You will see that the View window is split into two portions - on the left isa scrollable region in which legends will be displayed, and on the right is the drawing area, onwhich the various pictorial elements will be displayed.

In ArcView terminology, a View comprises one or more Themes. Each theme is simply a distinctset of information - such as roads or rivers or county boundaries etc., rather like the differentlayers that can be used to build up a map as shown in Fig.1.2 of Unit 1. In addition, each Themehas its own legend, which is a list of the symbology used.

We now need to add a theme to the view.

Click the Add Theme icon. Alternatively, you can click View on the Menu Bar and then AddTheme from the drop-down menu displayed.

You should now see the Add Theme dialogue box as shown in Fig. 2.3, showing any files in yourdisk area which ArcView recognises as holding spatial information. These will be listed in theleft hand part of the window, although the files and directories listed will, of course, be thoseavailable from your institution.

Fig. 2.3

Navigate to the file called distrcts.shp by following the instructions in point 2 on theAddendum Sheet and then click OK.

Note how the name of the theme with its symbol appears in the View Legend area, together witha little selection box just to the left of the name.

Click in this selection box.

You should now see a tick in the selection box and the theme will be drawn using the symboldisplayed under the theme name, as shown in Fig. 2.4.

Fig. 2.4

You are now looking at a map showing some of the districts of the Eastern Midlands of England.The ways in which such features are represented vary depending partly upon the way theinformation is collected and partly on what is being represented. At this stage features will berepresented as one of three types:- points, polylines, or polygons (see Fig. 2.5).

Fig. 2.5

Point representation is used for individual, and discrete, items such as lamp posts, locationmarkers (e.g. Whitby Abbey - a named church on the map in Fig. 1.2), mountain summits, etc.When these are displayed, some symbol or other will be used to mark the point and, perhaps, todistinguish it according to what it represents.

Polyline representation is used for linear features that will be drawn as a line - like a road or arailway. It is called 'polyline' because, although if it is a straight line, it will be defined simply bytwo sets of coordinates (one at either end), a line representing a natural feature is rarely straightand will often need many sets of coordinates, one for each time it changes direction.

Polygon representation (which in reality is very similar to a polyline that has been closed in onitself to enclose an area and is defined by co-ordinates for each time it changes direction) is usedto represent areal information - for example the boundaries of a local authority, as in the districtsdisplayed on your screen at present, a census enumeration district, or the outline of a lake.

If you now look back at the map in Unit 1 (Fig. 1.2) you will see that the principle of points,polylines and polygons can also be applied to the way in which features are drawn on a papermap.

We can now add another theme to the view.

Click the Add Theme icon and navigate to the file called railf_gb.shp by following theinstructions in point 2 on the Addendum Sheet, click OK and then click on the small selection boxin the Legend column to display the data.

This new theme you have just added uses polylines to display its data. Each Theme in ArcViewcan only use one type of representation, in this case polylines.

Notice also that this new theme has been added at the top of the Legend column - and is nowdrawn last of all, on top of the earlier data.

Click on the railf_gb.shp theme name in the Theme Legend window (avoiding the selectionbox) and, with the mouse button depressed, drag the railway theme name to be below thedistricts theme name.

The screen will now be re-drawn, starting with the theme at the bottom of the list. If a themehigher up the list totally obscures an earlier one, that's life! It does illustrate, however, that evenat a simple display stage it is necessary to think about the order in which things are beingdisplayed.

Now return the railway data to the top of the Theme Legend list and click in the selection box tostop displaying it for the time being. Click on the distrcts.shp name in the Legend window tomake sure it is selected.

Several icons in the View window enable you to zoom in or zoom out from a View. For the timebeing we will introduce you to just two, the Zoom In tool (Insert + icon) and the Zoom Outtool(insert - icon).

Click the Zoom In tool on the tool bar and then move the cursor on to the View display area(notice what happens to the cursor).

Choose a particular district on the map, move the cursor to a point just above and to the left ofthe district and then, with the left mouse button depressed, drag out a rectangle which surroundsthe district.

The area within the rectangle should now be redrawn to occupy all of the View. Now try usingthe Zoom Out tool to reverse this situation! If things don't work out quite as you had hoped thendo the following:-

Go to the Menu Bar and click on View and then Full Extent or see if you can find the appropriatebutton on the tool bar and click on it.

To the right of the Zoom In and Zoom Out tools on the tool bar is the Pan tool.

Click on the Pan tool and then within the display area and with the left mouse button depressedmove the View around.

Practice using the Zoom and Pan tools for a short while, remembering that if you get lost you canalways go back to Full Extent.

So far the View shows the outlines of some local authority districts but tells us nothing else. We

shall now investigate what other information is available.

Click on the binoculars Find icon, type Mansfield in the resulting dialogue box and then clickOK.

You should now see the district of Mansfield selected in a different colour from the rest of thedistricts. To do this ArcView has searched the relevant part of the theme database for a particularstring of text, i.e. Mansfield. If there had been no match, an information window with the 'NoMatch' message would have been displayed. You do not need to know the details of the databaseto use this tool; ArcView will simply indicate whether or not it can find the particular string oftext you have asked it to search for.

However, it is important to learn more about the relationship between a theme and its database.

Click on the Table icon.

A new window is now displayed showing a table. The table shows part of the databaseassociated with this particular theme, in particular what is known as the descriptive attributes ofthe theme (other data associated with the theme e.g. the data necessary to draw the polygons isnot shown). You will see that this attribute table consists of columns, which contain fields(categories of data e.g. name, area, perimeter etc.), and rows which contain records.

The table is probably too large to fit on your screen, even when maximised, so you can use thehorizontal scroll bars, if necessary, to examine all the fields of a record. At the left end of thetable is a shape field containing the word 'polygon', indicating the category of feature drawn inthe view. To the right of this are other fields recording things like unit name, area and perimeter.In other tables different types of data may be recorded e.g. polyline features may have theirlength recorded; information on things like population statistics, construction materials, date andcontractor for the last maintenance event could be included - indeed any information that may berelevant!

Each feature in the theme, in this case districts, has its own record.

Find the Unit-name field (you will probably have to scroll to the right) and then scroll downuntil you have found Mansfield.

Notice that in this case the record for Mansfield, selected when you used the Find button, hasbeen emphasised with a yellow background. Note also that the other records displayed in thetable, around the selected record, may not be those for records of geographically adjacentobjects.

Now return to the View window by clicking the mouse pointer anywhere within its frame.

You can work the other way around, starting with the items displayed upon the screen. To do thisit is necessary to select a feature in the display and then to examine the record for the selectedobject.

Click on the Information icon, then click it on any location within the View.

An Identify Results window will be opened displaying the details of the object at the location

pointed to.

Click on another feature in the View.

Another line will be added to the Identify Results display. The detail shown in this window, onthe right hand side, is that for the selected record.

Click on the previous item in the scroll area to the left.

This provides a way of easily comparing information relating to the different locations.

Now close the Identify Results window by double clicking on the Bar tool in the top left handcorner of the window frame.

You can also measure distances across, or between, objects in the display.

Click on the Measure tool icon. Now locate the '+' part of the pointer icon at your chosen startingposition, click the left mouse button and move the mouse pointer across the View.

In the bottom right of the ArcView window, in the Status bar area, you will see displayedSegment Length and Length. These will change, as you move the pointer, until you click themouse button again. After you have clicked the button once you can continue to move thepointer - the Segment Length will have restarted from zero at the location at which you clickedthe mouse button but the Length value will continue to accumulate. This facility is useful, forexample, for measuring distances between locations, following the route of a road (or roads) or ariver, although there are also other ways of performing route finding which you will meet in alater unit.

Terminate your length measurement by double clicking the mouse button.

All objects within this theme, other than that selected with the Find tool, are currently displayedusing the same symbol; this is a little confusing.

Place the cursor over the symbol, in the View Legend, and double click on it.

ArcView will now display the Legend Editor window, shown in Fig. 2.6

Fig. 2.6

In the Legend Editor window the name of the selected theme is displayed, along with the currentLegend Type, which at the moment says Single Symbol.

Click on the scroll button (at the right hand end of the Legend type box) and then click on theUnique Value menu item.

The Legend Editor window will now change to display a table, several new buttons and threenew boxes with drop-down menus.

Click on the scroll button at the right-hand end of the Values Field box to bring up a menushowing all the available attribute fields in the theme's database. Select Unit-name.

The table below, with columns headed Symbol, Value, Label, Count will now display a differentcolour for each different value of the Name field.

Click the Apply button.

The View screen will now be re-drawn using different colours for the various areas displayed;that originally selected will remain highlighted. You may now decide that you want to changeone of the colours used.

Double click the colour you want to change in the Symbol column.

This will display ArcView's Symbol window. The title at the top of this window will dependupon which of the 6 buttons just below is depressed (see Fig. 2.7). You will need to make sure

that the Colour Palette (indicated by a paintbrush) is displayed.

Click on the Colour Palette button (if necessary) and then click on the colour of your choice. Ifthe Colour Palette window is obscuring the Legend Editor window you can move it by placing thecursor in the title bar at the top, keeping the left mouse button depressed and then dragging thewindow to one side. You can, of course, move the other windows around in the same way.

Fig 2.7

You should now see that the colour you wanted to change has been changed. Apply this changeto the View as you did for the change in Classification Field above.

Now close both the Symbol window and the Legend Editor window by, in each case, doubleclicking the Bar tool at the top left corner of the window.

2.1 Saving your work

At some stage you will need to save your work in order to have it available to work on at a latersession. You may remember from earlier in this Unit that ArcView saves work as Projects.

Make sure that the Project window is active and then click the Save Project icon. Alternatively,click File on the Menu bar and then Save As.

A Menu window will be offered with a possible name. You may choose the name offered, or typeto the File Name box a name of your own choosing. This name should not exceed eight

characters in length. You do not need to specify the file type (.apr); ArcView will add this fileextension automatically for you. You will also need to consider where your file is to be savedand it is probably a good idea to do this before you do any renaming.

Consult Point 3 on the Addendum Sheet from your own institution and then select theappropriate drive and directory.

Now select a name for your project, confirm by clicking on the OK button and then close theProject.

Before going on to the next section try the following self-assessment exercise on the materialcovered so far.

SAQ 2.1. Start a New Project and New View. Add the theme distrcts.shp and display. Find thedistrict of Great Grimsby and then maximise it in the window. What is its area? Repeat thisprocess for Derby and find out its perimeter.

2.2 Printing

For this part of the Unit we need to return to the project you saved earlier.

In the main ArcView window click File on the Menu bar and then Open Project. Navigate towhere your project has been saved, select it and then click OK.

It is possible to print the View, say, that you are working on just as it is on the screen by clickingon the Menu bar and then Print. However, you have to choose between the View's display or itstable of contents (i.e. legend) and a map without the latter is not a lot of use. The component ofArcView known as Layout enables you to put together a presentation suitable for printing whichcan include views, legends, tables, charts etc. However, before starting this it is necessary toensure that ArcView knows about the sort of units and the coordinate system, that the data isusing. Up to now these have simply been a stream of numbers.

To do this we must use the View Properties control window.

Click on View, on the Menu bar and then click on the Properties item in the menu list.

A View Properties window will be displayed, as shown in Fig. 2.8. Some of the options here aresimply descriptive, like the Name and Creation Date fields, and are provided to enable you tobetter organise your work. Around the middle of the window are two fields, Map Units andDistance Units (greyed out). Both are currently set to the value unknown.

Fig. 2.8

Click on the scroll button to the right of the Map Units box and select Meters, from near thebottom of the menu.

The Distance Units will remain set to 'unknown' but will no longer be greyed out. You may nowchoose a distance measurement system (e.g. kilometers) from the menu offered when you clickon the scroll button. After leaving this window, if you use the Measure tool again, the valuesreported will be in the units you have just selected. We need to set these units so that we caninclude a scale bar on the printed version of the display - without which a reader would be leftwith a pretty picture (perhaps!) but very little in terms of reference for size and distance.

Click OK to confirm the settings and to return to the main View window.

2.3 Simple Presentation

The ArcView Layout facility provides the tools required for the generation of the printed page, or'pages' which will later be incorporated into other documents, such as, for example, Word orWordPerfect reports or dissertations. Layout supports the gathering together of View displays, thepositioning of these displays on the page, the provision of scale bars, titles, frames, etc., such asare necessary for the neat presentation of your work. The components that this unit will teachyou about represent the bare minimum that will result in a comprehensible illustration. In a laterunit you will learn about more advanced techniques of presentation and page layout; for themoment we will keep things simple!

Double click on the Layouts icon in the Project window.

An empty page 'canvas' window will now be displayed. It will probably be a good idea at thispoint to 'maximise' this window, to the largest extent the ArcView window will contain, as youdid with the Project window earlier in this Unit.

A convenient way of thinking about the Layout window is in terms of an artist's canvas, or alarge, but empty, sheet of paper. On this the various elements that will make up the final page areplaced or moved around to obtain a better 'fit' or a more pleasing visual appearance. It is here thatdifferent Views may be combined - perhaps a general view of the whole of the United Kingdomin one view and a detailed view of a very much smaller part in another. Once these are placed onthe page, lines may be drawn to link them as for an 'exploded' view, and further explanatory text,figure numbers, or titles, etc., may then be added for completeness. See Fig 2.9.

Fig. 2.9

Now select Layout, from the Menu bar at the top of the ArcView window frame, and click onPage Setup.

This will open the page specification menu, from which the page size, orientation, margins, etc.,are controlled as shown in Fig 2.10. S/He is a wise user who sets up the page configuration firstthing, before defining any of the elements that it is desired to place on the page!

Fig. 2.10

Click the scroll button at the right hand side of the Page Size box and scroll down until youreach A4 which you should now select by clicking on it.

You will notice that Units are now set to Centimeters and the page width and height havevalues set. If the width is greater than the height then you have theOrientation set to Landscapemode and need to change it to Portrait mode (long edge vertical) which will swap over thevalues for width and height.

Make sure that the Portrait mode Orientation button is depressed by clicking on it.

By default, ArcView sets a page margin of a fraction over one half a centimeter. This marginwill permit printing to the extremes of the printable area for most printers but will not provide anadequate binding margin for inclusion in a dissertation, report, etc.

Choose appropriate values - perhaps 2.0 for the left margin and 1.0 for the top, bottom and rightmargins by highlighting in turn the numbers already there and then typing in your own.

There is no need to alter the setting of the Output Resolution box.

Now confirm these settings by clicking on the OK button.

Before attempting to locate any items on this page there is one further setup action that must beperformed. The Layout tool provides an alignment grid, which is not printed, but which makes iteasier to locate, and align, the various components of the page. The default unit size of this gridis almost certainly too coarse and you will need to change it.

Return to the Layout menu and select Properties.

It is the value settings for the horizontal and vertical grid spacing that you need to alter.

Set both to 0.1 (one millimeter) for the purposes of this exercise and then confirm using the OKbutton.

Now, having set up the various control parameters for the page, it is time to place something onit.

Click on the Zoom to Page icon in the Icon Bar when your blank page will now be centred in thewindow.

The two right hand icons in the second row of tool icons offer a further menu of tool icons whenclicked on. These provide the tools for drawing directly on the page (left icon) and for placingView Frame items on the page (right icon). See Fig. 2.11.

Select the View Frame icon and then position the '+' pointer icon at the place on your page atwhich you want the top left hand corner of your map view. Whilst holding the mouse buttondown, draw out a box with the tool to occupy that area of the page in which you want the map toappear. Release the mouse button at the bottom right hand corner of this area.

A View Frame menu window will now appear as shown in Fig. 2.11

Fig. 2.11

Select the View1 entry from the list of Views presented to you and confirm by clicking the OKbutton.

The View window display, which you created earlier, will now be drawn on your page. Thedisplay may not occupy the whole of the area you intially selected. This is because the ratio ofwidth and height (the Aspect Ratio) of the original display is preserved so as to avoid distortion.

If you are not quite happy with the size of the display you can modify it.

Click on the Selection arrow icon and then click on one of the corner boxes marked on thescreen and drag it to the desired size.

The Aspect Ratio of the original will still be preserved, no matter how you try to drag the frame.You may also want to reposition the display - perhaps to better centre it on the page.

Click on the Select arrow tool and then on the centre of the display object, such that the cornerlocation spots are displayed. Now hold the mouse button down, drag the whole unit around thepage to the position you desire. When you release the mouse button the display will be relocatedto the new position.

No map is much use without reference indications of scale and direction. These elements areconventionally provided by means of a scale bar, to indicate relative distance, and a North Arrow,to indicate orientation. There are tool icons to enable the generation of these elements in thesame vertical list of tool icons as the View Frame tool you have just used.

Select the View Frame tool icon. A dropdown menu of icons will appear. Keeping the mousebutton down select the North Arrow icon (usually 4th from the bottom) and then release themouse button. Position the '+'cursor in a suitable place on the page and draw out a box.

This box will determine the size of the North Arrow symbol that will be located at this point. ASymbol Menu window will be displayed showing a variety of conventional North Arrow symbols.

Click on the one you like best and then confirm by clicking the OK button.

Your selected symbol will now be drawn on the page, oriented in accordance with the orientationof the View Frame coordinates.

You may now decide that one of your existing page elements (view or north arrow) is not howyou would like it. Later on we will learn how to modify page elements after their initialdefinition but for the moment, if one does not work out the way that is desired, it may be deleted.Try this now.

Delete an element by using the Select arrow tool to produce the corner markers of the elementand then hitting the 'delete' key on the keyboard.

The selected element will be removed from the layout canvas. You will now need to repeat whatyou have done earlier to replace the element on your Layout. After this you will need to add aScale bar to your Layout.

Return to the dropdown palette of frame tools and find the Scale bar icon. Select it as for the

North Arrow icon above. Find an appropriate place to drag out a horizontal and narrow box -preferably just below your View display. Start to the left of your display and allow plenty ofwidth.

The text for the Scale bar units will be displayed to the right of the Scale bar itself. A Menuwindow will be displayed for you to select the parameters for your Scale bar as shown in Fig2.12.

Fig. 2.12

Firstly, select in the View Frame menu list the entry ViewFrame1: View1. [Note: if you omittedto set the map and distance units in the View Parameters menu, as discussed earlier, this may notbe an option; you will have to Cancel, return to the View window, and perform that step, beforecontinuing from here.] Select from the list of Scale bar styles by clicking on the scroll button onthe right of the Style box and repeating this for the Units box.

The values for the Scale bar Interval, the number of Intervals (right hand part of the scale bar)and the Left Divisions (the number of parts the Interval to the left of the zero marker will bedivided into) are calculated according to the style of scale bar selected and the size of the boxthat was drawn out for the scale bar to occupy. You may well wish to change these 'automatic'values but note that:

If you set the Interval value too high the number of Intervals may drop to zero. In this case,reduce the value you have chosen for Interval.

If you make the value for Interval too small, you may end up with an unreadable mess when

ArcView attempts to annotate the scale bar with distance values - you may need to experimenthere before you get the balance right.

Click OK to confirm these settings, or to examine the results of your selections.

A Legend, to explain the symbols used, is another necessary component. Layout can 'copy' theLegend window, from the View Frame window, and display it on the page. The symbolexplanations are those defined automatically or specified using the View Legend Editor tool.

Return to the dropdown palette of frame tools, select the View Legend tool and draw out a box ina convenient part of the page.

If you have a lot of symbols, you may need a long area for the box. A menu window, LegendFrame Properties will be displayed when you release the mouse button after dragging out thelegend area box.

In the View Frame menu part of the window select ViewFrame1: View1 and then confirm byclicking the OK button.

A copy of the View Legend will now be added to your page display.

The principle components are now in place. All that is really needed to finish things off is to givethe page a title and to place a neat line box around the page. Note that the box in the Layoutdisplay, within which you have been placing your page elements, will not itself be drawn: it ismerely a guide to indicate the area of the page on which you may work, within the limits set bythe page margins.

Click the T Text icon and position the Text Cursor icon at the point on the page at which youwish to place your title.

A Text Properties menu will be displayed as shown in Fig. 2.13. You will need to type the textyou desire into the scrollable region. If you want to start a new line press Return. You maychoose to use left, central or right justification of text; if you placed your text cursor at thecentral point about which you wish to title your map you may wish now to select centred, themiddle icon button for Horizontal Alignment.

Confirm your selection by clicking the OK button.

Now finish things off by drawing a box around your work.

Click and drag down the dropdown palette of drawing tools, immediately to the left of the frametools button. Select the Box tool by clicking it and then positioning the '+' pointer to the top leftcorner of your page and, keeping the mouse button depressed, draw out the box to the bottomright, enclosing all your map elements. Release the mouse button.

If you are able to print a hard copy of your Layout you can do so now. Alternatively you maywish to export it in a form suitable for inclusion in another document.

Click on the Printer tool, in the upper row of tool icons or from the File menu select Print.

The Print window will now be displayed as shown in Fig 2.14. (check this with AV3). If you

have more than one Layout window, the Print box will permit you to scroll through the layouts inorder to choose which to submit for printing. The 'Printer:' box allows you to select a printer,from those available to you on your system, or you may wish to type the name of a file into the'To File:' box, in which case a file of printing information will be generated rather than a printjob. The output format is specified by checking one of the items listed under 'Print Format'.'PostScript Enhanced' generates 'Level 2' PostScript, suitable for PostScript Level 2 printers only,whilst 'PostScript Basic' generates 'Level 1' code, suitable for all PostScript printers. ArcView 3also has a 'Print to File' button, in addition to the file name box; both must be activated in orderto print to a file.

Consult Point 4 on your Addendum sheet for details of the printer(s) available in your institutionand set the options in the Print window accordingly. Click OK if you can go ahead and print orprint to a specified file. Otherwise if you cannot proceed any further click Cancel.

If you are able to print, ArcView will now use your system printing tools to submit the page tothe printer you have chosen, or will write the printer commands into the specified file for you to'include' in Word, WordPerfect, etc.

Incidently, if you were to return to the View window and modify the content of the display, thosemodifications will automatically be reflected in the content of the Layout display.

This concludes Unit 2. The original Project you were working with and saved has now beenmodified by the inclusion of a Layout. If you want to save this modified project do the following:

Double click the Bar tool (-) in the top left corner of the project window and when asked if youwant to save the changes, click OK.

SAQ 2.2. Re-open the project you have been working with and start a New View. Add the themescalled counties.shp and railf-gb.shp and then display them so that you can see them both inthe view. Set the map and distance units for the View.

a) Select the county of South Yorkshire and then find out its perimeter from the attribute table.

b) A railway line runs from just south of the centre of Lincolnshire to its east coast. Use themeasuring tool to find out its length in kilometers.

If you now need to exit from ArcView:-

Click on the File menu, on the left hand end of the list of menus in the Menu bar, and select Exit.

Suggestions for further reading:

Chapter One of Principles of Geographical Information Systems for Land Resources Assessment,P. A. Burrough, 1986, Oxford University Press.

Geographic Information Systems: an overview, Duane F. Marble, 1990, in Introductory readingsin Geographic Information Systems, Donna J. Peuquet and Duane F. Marble (eds), Taylor andFrancis.

Maps and Diagrams, F. J. Monkhouse & H. R. Wilkinson, 1963, Methuen & Co. (This book

concentrates on paper based techniques; despite this the principles of communicating spatialinformation remain well presented.)

Unit 3:Introduction to Analysis

The aim of this unit is to introduce the more powerful characteristics of GIS, such asstatistical and overlay analysis, which collectively, in the context of a GIS are often termedSpatial Analysis. These techniques go beyond those introduced in UNIT 2, which are littlemore than techniques of display and presentation. This then presents you with anIntroductory Analysis Assignment.

3.0 What is Analysis?

Analysis is about selecting those parts of your data which are relevant to the task in hand, andusing them to attempt to find answers to questions. Analysis with a GIS, often termed spatialanalysis, unlike other Information Systems, has the added dimension of 'space' or geography.This combination of descriptive attributes on various phenomena, e.g. a person's age, the type ofroad etc, together with information on e.g. where a person lives or the location of a road, permitsa variety of locational questions to be asked such as where are ..., show me where ..., how far is..., what is next to ..., what is this ..., are there any ... near this ..., ... In addition, whereinformation relating to time is also available, these may also include such questions as how longwill it take to get from ... to ... and how long before ... reaches ...

It is this ability to perform spatial analysis which marks GIS as being different from drawingtools, Computer Aided Design (CAD) tools, and mapping systems. Although GIS can be used toproduce maps - and often to a standard of quality equal to any 'mapping' tool - it is the verycombination of analysis with display, and data input and management, capability that gives GISits particular distinctive properties. Put succinctly, in a GIS, display is usually regarded as ameans to an end rather than the end in itself.

Using a GIS, the same information may be investigated in more than one way, using differentspatial areas. For example, census returns might be analysed for the whole of the UK in order todetermine the overall age: sex ratio of a population. Alternatively, the data may be dis-aggregated into separate counties, or even smaller areas such as Wards and EnumerationDistricts perhaps to determine whether adequate school capacity is provided for the numbers ofchildren in different areas, or to target the budget for community support services tocommunities comprising a high proportion of elderly people. Spatial Analysis, therefore, is aboutaddressing the where component of information in addition to the what, when, and how much,which are identified as attributes of the 'where'..

3.1 Statistical Analysis

Spatial analysis provides facilities for calculating a range of statistics for any of the attributes,e.g. area of polygons, length of arcs or perimeter of polygons as well as any of the e.g. censusvariables that you may have associated with e.g. a suite of polygons. You can also query yourdata to e.g. select and display only that data where roads represent motorways or census areas

have 20% of the households owning two cars.

3.2 Overlay Analysis

One of the particular distinctive characteristics of a GIS is the ability to overlay areas, rather likedrawing areas on transparent paper and placing one on top of the other on an OHP to visuallydetermine where phenomena on each of the transparencies overlap. For example, one layer maycontain information of soils and soil boundaries, the other on crops and field boundaries. Byoverlaying the two maps, it is possible to observe the coincidence of crops with given soil types.One particularly useful overlay analysis available in ArcView is known as intersect. This makesit possible to identify where phenomena meet or cross. You will use this technique in theassignment later in this Unit.

Applications of spatial analysis, using GIS, include route planning (for example, the AutoRouteproduct or the AA's Routefinder), market analysis, insurance risk assessment, environmentalimpact assessments, emergency service dispatch and control ... and GIS is used by workers indisciplines as diverse as Social Policy and Ecology, Civil Engineering and Anthropology,Marketing and Politics!

ArcView provides the means to undertake a range of overlay analytical tasks. It should be noted,however, that the software does not attempt to fully replicate the functionality GIS software suchas Arc/Info, which is also produced by ESRI, the additional functionality can be added toArcView using Avenue, which is an additional product, which can be used to provide customisedextensions.

3.3 Analytically creating new Themes

Using a GIS, not only can the result of an overlay analysis be visually produced, but the resultmay itself be saved as a completely new dataset - itself then available to be used as a componentof further analysis. For example, the results of one analysis may generate a zone of influence -the area affected by the release of a toxic gas perhaps from a factory chimney. This could besaved as a new theme and overlaid with e.g. the area of a housing estate to identify what areas,given various weather conditions, might be adversely affected, and so on. Finally, a spatial,statistical query could be constructed to identify the names of the residents that are affected, forensuring that all have been safely evacuated, and to assist in establishing the provision ofemergency accommodation pending the dispersion of the pollutant. There are no real limits tothe extent of this process of deriving new datasets from the combination or interaction of onespatial dataset upon another, nor need the results necessarily be graphics, despite the use ofgeographic information during the computation process.

3.4 Modelling

As noted in Unit 1 the real world is infinitely complex and we therefore need to use a muchsimpler representation or abstraction of reality. This abstraction is, in traditional cartographicterms, often seen as modelling reality either as points, lines or areas. It could therefore be arguedthat working with a GIS is always a modelling activity. For the purposes of this tutorial,however, the term model will be confined to refer to the active representations of processeswhich are intended to facilitate the understanding of that process and / or its interactions. This

modelling concept is illustrated in Fig 1.

FIG 1

3.5 Topology and Spatial Relationships

We have been displaying geographical information in terms of lines and areas, polygons. Someof these have, of course, a relationship with similar objects, with which they share a commonboundary or maybe overlap. This spatial relationship between objects is termed the topology orthe spatial topology. Some GIS make use of these relationships internally and store appropriateinformation that supports rapid access to information using these relationships - and also to avoidstoring shared information, such as the co-ordinates that define a boundary between two adjacentpolygons, more than once. GIS which build and maintain topology in this way are termedTopological GIS. It is this topological structure than enables many analyses to be performed. Ifthe primary objective was merely to produce maps, then such a detailed structure would not benecessary. The term spaghetti structure is sometimes used to imply a minimalist ordering of thespatial data when only map production is required. ArcView does not, in fact use topology, but itcan readily import data from Arc/Info which is topologically structured, as will be explained inthe assignment later.

3.6 Reliability of results

All analysis should also include an assessment of the reliability of solutions provided in answerto the question - the error rate. Some errors may be inherent in the data, due to sample size ordistribution, accuracy of recording, frequency of re-processing, etc. Errors may also result fromthe application of a particular technique upon data that does not meet some minimum standard ofquality. There may have been assumptions made about information not collected, or deemed tohave little impact on the outcome of the investigation. It has been known for bad answers to havebeen produced as a result of using the wrong data! Some of these implications of data quality and

fitness for purpose will be discussed in a later unit of this tutorial. Fig 2 offers a visual summaryof these various inter-relationships of Analysis.

FIG 2

3.7 Visualisation

The inherent display facilities of a GIS, coupled with the speed of modern computers, make it anattractive tool for exploration forms of analysis, using the graphical display as a visualisationtool; a means to illustrate the results of the analysis or exploration. In this context, the termvisualisation is used to describe not just the visual display but also the suite of mental processesyou are using to achieve that display. For example, your analysis is frequently used to explore anumber of what if ... scenarios potential alternative actions, or to visualise how a processoperates in order to better understand side effects or components. An analogy could be the use ofsimulators used by trainee aircraft pilots. Similarly a GIS could be used by e.g. a town planner toexplore the consequences of a proposed new road or housing estate.

3.8 Data Quality

The relative ease with visual images can now be displayed on modern computers, with easy-to-use software, can seduce the unwary into a false reliance upon material of a dubious nature. Incomputing circles there is a catch phrase, "garbage in, garbage out" - GIGO for short. With theuse of GIS a very pretty, and appealing, map may readily be produced ... but which may haveless meaning than many of the statistics bandied around by politicians seeking (re-)election. It isessential that every GIS user remain alert to the quality limitations of the data being used and theconsequent implications for the quality of any analytic results, otherwise you may find yourself

victim to "garbage in - pretty (meaningless) map out!"

3.9 Introductory analysis assignment

There are many more techniques of spatial analysis than can be introduced in this tutorial course.Since some, perhaps many, of these may represent very new ways of thinking, this unit willgently explore some basic techniques and later modules will take these further, together withsome more advanced procedures, and will also provide some ideas for further reading, for thosewho desire to learn more.

3.9.0 Summary of overall objectives:

a) to quantify the ratio of urban to rural area in the county of Nottinghamshire,b) to identify those canals within Nottinghamshire which pass through urban areas, andc) to determine the total length of these selected canal segments.

You will be using data, generously provided by Bartholomew, which will be used to createthemes representing urban areas, rivers and administrative boundaries for Nottinghamshire andthe surrounding area.

3.9.1 Reading the County Boundary data

• If you do not have a new (empty) view open up a new View now.

The particular details you need to locate the data on your computer will be given to youby your course tutor. But once you are in the correct location, select 'counties.shp'.

• Click OK and click the Theme Legend box to cause ArcView to display the theme.

3.9.2 Performing a query on a single theme (for counties), to selectNottinghamshire

• Select Properties from the Theme menu in the Menu Bar and then click on the Hammericon under Definition. This will invoke the Query Builder form; (see Fields to find'Unit_name' and double-click on it. Unit_name will appear within the round brackets inthe lower left area of the form. Now single-click the = button, from the tools in the centreof the form (top left) - it too is added to the expression in brackets in the lower left area.

The list labelled Values now contains the various County Names.

• Scroll down this list to find Nottinghamshire and double click.

You should now have something of the form

( [Unit_name] = "NOTTINGHAMSHIRE")

in the area to the bottom left. This is the query request and should be similar to Fig 4.

Query Builder form and you will be returned to the Theme Properties form.

Note that the query defined now appears in the Definition part of the Theme Propertiesform, which should now resemble Fig 5.

ArcView will use this query and search for that part of the database which matches yourrequirement. This will now be the only part of the database that will be available fordisplay or computation for this Theme.

• Now click OK to complete the Theme Properties form. It is at this point that the querywill be applied and ArcView will display a Status bar indicating that it is performing therequested query.

3.9.3 Getting your display for Nottinghamshire to fill up the View window

Note that, when you enable display of this data, it will appear as a small 'island' in a 'sea ofnothing'. This is because the viewing area currently remains for the whole of the boundariesarea but only a small part of this has been selected.

• Click on View in the Menu bar then Zoom to Themes. Your display for the county ofNottinghamshire will be enlarged to fill the size of your View window.

3.9.4 Reading the rivers data and selecting just the canals

This theme contains details of rivers, major and minor, small streams and drainage channels,aqueducts and reservoir dams, in addition to canals. There are various classifications of eachof these major features in addition. We only require the canals so it will be necessary,therefore, to use the Query Builder to refine the dataset for this exercise in order to have atheme which simply represents canals.

• Select 'riversgb.shp'. Click OK.

Now display the theme. You will observe quite a large number of drainage features.

• Click on the name of the rivers theme in the Theme Legend list. This will ensure that thisis the currently active theme. Select Properties from the Theme menu in the Menu bar.

• Now click on the Hammer icon under Definition. This will invoke the Query Builder form.

As before, ensure that the update box has a cross in it.

You now need to extract the canals from the Bartholomew drainage data. The differentdrainage features are classified using a field called Obs_acc_no. , Further, the 'canal' datarequired is divided up into a range of discrete features so that several values ofobs_acc_no will be required. Only those categories likely to appear within the area ofinterest (Nottinghamshire) will be selected. These are (together with their Bartholomewexplanations):

Obs_acc_no value Represents

135641 Canal class 'A'

135642 Canal class 'B'

135643 Canal class 'C'

138682 Canal tunnel

• Select Properties from the Theme menu. From the list of fields, double-click onObs_acc_no; it will be placed in the query area. Now single-click the 'greater than orequal' symbol, >=, , and then double-click the value 135641 in the Values list.

• It is a range of values, plus one, that is required, so now single-click the 'and' button,followed by a double-click on Obs_acc_no from the Fields list, then single-click on the'less than or equal' button, and then double click on 135643 from the Values list.

This will select those records meeting the range of values specified.

• To obtain those records matching the single value of 138682, single-click the 'or' button,double-click on Obs_acc_no from the Fields list, then the '=' button and finally double-click 138682 from the Values list.

You should now have an expression of the form:

([Obs_acc_no] >= 135641) and ([Obs_acc_no] <= 135643) or ([Obs_acc_no]="138682)"

• Click OK to the Query Builder form and you will be returned to the Theme Propertiesform. Note that the query defined now appears in the Definition part of the ThemeProperties form. Now click OK to complete the Theme Properties form. The query is nowapplied.

You will notice that, although Nottinghamshire is the only county drawn, the canals datafor the surrounds in the View window are still displayed. This is fine for the moment ...

3.9.5 Reading the urban areas data

• Select 'urban_gb'. Click OK.

Now display the urban areas. You will notice that, although Nottinghamshire is the onlycounty drawn, the urban area data for the surrounds in the View window are stilldisplayed. Again, this is fine for the moment ...

3.9.6 Determining the urban : rural ratio for Nottinghamshire

This requires:

a) the area of the whole county of Nottinghamshire be determined, thenb) the sum of the areas of the urban parts of the county must be found, and finallyc) the urban value be subtracted from the whole.

The display we now have, with the urban areas overlaying the county outline, gives animmediate visual impression, but not a quantitative evaluation.

Although ArcView, in itself, does not build spatial topology, it can make use of spatialtopology that exists in feature source data imported from topology-based systems. TheBartholomew data is such as this, having been created by, and stored within Arc/Info, yetaccessed directly with ArcView. Using Arc/Info, the area and perimeter of polygon data(areas) are automatically calculated and stored for every polygon, in the feature attributetable. This means that it is not necessary to calculate the area of the county ofNottinghamshire, rather, merely to examine the relevant field in the Nottinghamshiretable.

• To obtain this, click on the name of the Counties theme in the View Legend area so that itbecomes highlighted - it should appear almost like a raised button in the Legend list.Now click on the Table icon in the Tool bar.

A window displaying the attributes of the County Boundaries dataset will be displayed -with a single record, that for Nottinghamshire. The values are in map units - in this case,because they are derived from the UK National Grid co-ordinate system which uses unitsof one metre, the units are in square metres for Area (and metres for Perimeter ...).Inevitably, for a large administrative area such as Nottinghamshire, reporting this value in

metres gives a very large number.

Question 1: What is the area of Nottinghamshire? Note it down.

• Now double click the button in the top left-hand corner of the frame in order to close it.

3.9.7 Selecting just the urban areas within Nottinghamshire

The Urban Areas Theme, as currently displayed, represents all the urban areas, not just thosein Nottinghamshire! In order to extract just those areas of interest, a spatial overlay selectionmust be performed. The tool that will be used here, Intersect, will use the outline boundary ofNottinghamshire to select those objects in the Urban Areas Theme which lie within, or cross,this boundary. Clearly, doing it this way may introduce an over-estimation of the sum of theurban areas since it may include some space outside the Nottinghamshire boundary. Thisover-estimation will, in fact, be less than the under-estimation which would result from usingonly those areas contained wholly within the boundary - and excluding those which cross. Inthe interests of simplicity this error will be deemed acceptable at this time. A more accurate,but more complex, procedure will be introduced in a later unit.

• Click on the Urban Areas name, in the Theme Legend list, to highlight it. Now click onTheme, in the Menu bar, and select Select By Theme. A window will be displayed, similarto Fig 6:

Intersect in the top left box, using the scroll arrow on the right of the box if necessary,and from the selected features of box to the bottom left, scroll to select the Countiesdataset, i.e. counties.shp. Click the New Set button on the top right. The selectedurban areas will now be displayed in yellow.

• Ensure that the Urban Theme is the one currently active. Now click on the Table icon.

You will find, if you scroll down the table, that there are a number of records highlightedwith a yellow background. These comprise the currently selected set of records andrepresent those which are located either completely or partially within the area ofNottinghamshire.

• To obtain the sum of the areas of the selected records, click on the Area field in the Tablewindow for the urban areas features attribute table. Now, from the Field menu, in theMenu bar, select Statistics (at the bottom of the list). A window will be displayed withvarious summary statistics for the selected records - the top item being the sum of theareas.

Question 2: What is the total of the urban areas totally or partially intersects with thecounty of Nottinghamshire? Note it down.

• Click OK to the Summary Statistics window, close the Table window and thereby returnto the View window.

The ratio of Urban Nottinghamshire to Rural Nottinghamshire is:

Urban Area : (County Area - Urban Area)

Question 3: : What is the ratio, expressed arithmetically? Note it down.

3.9.8 Identifying those canals which pass through urban areas

To achieve this it is necessary to perform similar steps as above, but this time using allthree datasets. Given that the urban areas of Nottinghamshire are a currently selectedsubset, use can be made of this by extracting this into a new theme, comprising just theserecords. So a completely new theme will be created as a result of performing a spatialoverlay operation on two existing themes.

• Firstly, turn off visually all but the Rivers Theme. Ensure that it is the currentlyhighlighted theme, in the Legend list. Now click on the Theme menu and Select ByTheme.

The idea is to select those parts of the current selection set (canals) which intersect withselected urban areas (those in Nottinghamshire).

<liSelect Intersect and Urban_gb.shp from the Window menu and click the New Setbutton.

Just a few of the canals will turn yellow, to indicate that they are selected.

3.9.9 Creating a new theme, based upon a previous overlay query

• Select the Convert to Shapefile option from the Theme menu. This will save the selectedfeatures only into a new feature dataset.

Be sure to have only the rivers dataset displayed when you do this! The default name willbe something like theme1.

• Give your new dataset the name canals.shp. Then click OK. You should now respond'yes' to the following panel containing the question 'add shapefile as theme to the view?'.

A new item will appear in the Theme Legend list, called canals.shp. It will now besaved on your computer for future use. Your tutor may need to advise you as to the exactlocation for saving this file.

(Note: Shapefiles are Arcview's internal method of storing spatial data.)

• We have now finished with the Rivers Theme so, from the Edit menu select DeleteThemes. The Rivers Theme, once you have confirmed your request, will be removed fromthe Theme Legend list. Now click the display box on the Canals Theme to display just thepreviously selected canals; click on the Theme name to make this the highlighted theme.

Note that as with the urban intersecting with the county boundary, if it were required toextract only those parts of each canal which lay within an area designated urban it wouldbe necessary to perform further processing in order to split the line segmentsaccordingly.

3.9.10 Calculating the total length of the canals that pass through theurban areas

This requires you to use the summary statistics procedure, as used above to determine thesum of the urban areas of Nottinghamshire: this would be much easier (and moreaccurate) than repeating the earlier measurement procedure, from Unit 2, to measure thelengths of canal within the urban area boundaries.

• Ensure that Canals is the highlighted theme. Currently, there are no selected records inthe Canals Theme. The easiest way to select all records and calculate the total length ofthe canals totally or partially within Nottinghamshire is to click on the Table icon, thenSelect All, from the Edit menu, then select the Length field in the Table window for thecanals feature attributes. It may be necessary to use the scroll bar at the bottom of thetable to move to the right across the table to locate the length field. Finally, select theStatistics option of the Field menu.

Question 4: What is the total length of the canals selected and saved as the Themecanals? Note it down.

• Click OK to down the window containing the statistics and then close the Table window.

3.9.11 And finally ......

The analytical tasks are now complete. Finish off by using the presentation skills learned inUnit 2, to create a layout, suitable for printing, which displays the county of Nottinghamshirewith the Urban areas and the canals overlaid. The canals selected as passing through theurban areas should be distinguishable from the rest of the canal system.

• Now save your work as a new Project, using the Save As item in the File menu and givingthis project the name Unit3.apr.

3.10 Summary

With the completion of this unit, you should now be familiar with the following techniques:

• Overlay display

• Investigation of attribute (table field) values of data selected spatially

• Spatial Selection, using intersection

• Measurement

• The creation of new spatial datasets resulting from a spatial query

3.11 Answers to Questions

Question 1: The area of the county of Nottinghamshire is 2,166,049,536.0 sq metres.

Question 2: The total of the urban areas that fall partially or totally withinNottinghamshire is 220,085,810 sq metres.

Question 3: The ratio of urban to rural Nottinghamshire is 2166049536 : 220085810.

Question 4: The length of canal selected is 74278.232 metres.

Suggestions for further reading

Chapters Five and Six of Principles of Geographical Information Systems for LandResources Assessment, P. A. Burrough, 1986, Oxford University Press.

A classification of software components commonly used in geographic informationsystems, Jack Dangermond, 1990, in Introductory readings in Geographic InformationSystems, Donna J. Peuquet and Duane F. Marble (eds), Taylor and Francis.

GIS versus CAD versus DBMS: what are the differences?, David J. Cowen, 1990, inIntroductory readings in Geographic Information Systems, Donna J. Peuquet and DuaneF. Marble (eds), Taylor and Francis.

Unit 4:Data - input, conversion, integration and

managementThe aim of this unit is to introduce you to thinking about the data that you are working withand about the processes that are necessary to make effective use of this information. You willhave the opportunity to create some new information.

4.0 Introduction

Data for use in a GIS comes in a wide variety of forms. Fig. 4.1a shows an example of spatialdata but associated with this is non-spatial data (Fig.4.1b), in this case statistical returns forspecific towns shown on the map. (See section 1.2). These latter are examples of quantitativedata with which one can carry out calculations (e.g. mean number of banks in the three towns).But the data could also be merely qualitative, e.g. the different categories of land use shown inFig. 4.1a..

Although we may initially think of maps and their associated attribute data when consideringGIS, Arcview (and other GISs) can also deal with many other types of data, so long as there issome form of locational key. Examples of data sources, other than maps, include census returns,customer address lists, (aerial) photographs and satellite images, GPS (satellite navigationsystem) position reports, etc.

FIG 4.1.a

Town Population Males Females Under 18's Over 65's

Carlton inLindrick 6726 3161 3565 663 1212

Retford 19380 9108 10272 1912 3492

Worksop 34993 16446 18547 3453 6305

Market Warsop 1031 4848 5468 1018 1859

Ollerton 7395 3475 3920 729 1332

MansfieldWoodhouse 17609 8276 9333 1737 3173

Mansfield 72108 33890 38218 7116 12994

Sutton inAshfield 39622 18622 21000 3910 7140

Rainworth 7078 3326 3752 698 1275

Kirkby inAshfield 26127 12279 13848 2578 4708

Ravenshead 5250 2467 2783 518 946

Newark-on-Trent 33390 15693 17697 3295 6016

Southwell 6404 3009 3395 631 1154

Balderton 8573 4029 4544 846 1544

Calverton 6997 3288 3709 690 1261

Hucknall 27506 12927 14579 2714 4956

Eastwood 18112 8512 9600 1787 3264

Arnold 37765 17749 20016 3727 6805

Kimberley 9855 4631 5224 972 1776

Carlton 46119 21675 24444 4551 8310

Nottingham 277203 130285 146918 27359 49952

Bingham 6155 2892 3263 607 1109

Radcliffe onTrent 7515 3532 3983 741 1354

West Bridgford 27506 12927 14579 2714 4956

Stapleford 32589 15316 17273 3216 5872

Beeston 32589 15316 17273 3216 5872

Cotgrave 7229 3397 3832 713 1302

Ruddington 6498 3054 3444 641 1170

Keyworth 8327 3913 4414 821 1500

East Leake 5613 2638 2975 553 1011

Fig 4.1.b

All forms of photography are handled, initially at least, as an image. An image may just be apicture, which could be regarded as representing the attributes in visual form, e.g. a photographof a building offered for sale whose location is shown on a map in a particular Arcview theme.Alternatively the image could contain spatial information itself e.g. an aerial photograph or asatellite image of an area of land could be imported into an Arcview theme and used as abaseplan for the construction of additional themes (Fig. 4.2).

4.1 Data Input

In the previous units you have been using data that has been collected and input into the GISready for you to use. Eventually you are almost certain to have to collect some data for yourself.There are a number of issues that need consideration even before starting to collect thisinformation.

4.1.1 Collection of data

Data obviously has to be collected. Primary data is that obtained directly from a field survey.However, more often than not you will be obtaining data from someone else, who has eithercollected the data themselves or who is some type of 'middleman', or distributor, acting as amarketing agent between the collector and the GIS user.

Whatever the potential source of the data might be, some questions need answering beforehand:-

What data are required?

Boundary or location details?Areal coverage?Population samples?SAMPLE SIZE can be critical. If the sample size is too small, the rarer members of apopulation may be under-represented - or not even recorded - and give a false representation

of the population.

What sort of resolution is required of the data?

RESOLUTION is the measure of the smallest discernable unit that the representationcontains and is analogous to the 'grain size' of a photographic film. This is especiallyimportant in relation to aerial photography, satellite imagery, etc., as well as in determiningthe sample size for a population census. If the objects of interest are too small to distinguish,the data may be of little use. For example, a Landsat satellite image, with a groundresolution of approximately 30m2, will not yield data from which you can count the numberof cars parked in a car park.

How accurate / precise does the data need to be?

PRECISION is a measure of how close the representation of a real world object is to thelocation of the original e.g. to within 100m; to within 10m etc.ACCURACY refers to the correctness of the representation.

Basic questions such as these must be asked throughout any project. If data are selected whichcannot provide the required information, then the project will be a failure from the start.

What data are available?

From previous workers?From public sources?From published sources?

These may include base maps, upon which you may wish to add your own information, such asOrdnance Survey maps for the UK.

What form are the data in?

Paper maps, printed tables of values, or reports?Photographs?Hand sketches?Digital source files for maps or value tables?

What form do you need the data in?

To merge with information from other sources?To use in particular tools (e.g. ArcView, SPPS, etc.)?

Thus the following questions must be asked about each potential dataset:

What needs to be done to this data in order that it may be used?

Does it need to be digitised?Does it need to be converted in some way?

What is the precision of this data?

What is the accuracy of this data?

Have you Copyright permission to use this data?

Some copyright owners insist in a share of the intellectual property rights of any newinformation that is based upon their data. This may mean that you do not own the full rightsto any results of your work using this data and that ongoing royalties may be payable.

Is it Ethical to use this data for this purpose?

This issue is even more sensitive than the copyright issue. It may be that data, collected forone purpose, may appear suitable for use for something completely different. If it would notbe acceptable to (re)collect that data for the new purpose, it may not be acceptable to usethat previously collected data either.

In the end it is necessary to decide whether the available data is acceptable for the purpose forwhich it is required. By such a definition, there is no such concept as 'good' or 'bad' quality datain a theoretical sense, but only in the sense of the purpose for which you wish to use the data.Such an idea, is, however, compromised by cost. So a more pragmatic definition could berepresented by the equation:-

Acceptability = Available accuracy and/or precisionCost of improvement

In other words, it usually comes down to economics and how much one is prepared to pay for thedata!

4.1.2 Types of data

Data to be imported into a GIS can exist in a number of different models e.g. tables of numericdata, alphabetic text, paper plans, photographs, digital files.

Obviously, of these models of data listed above, the digital data (whether on a floppy disk, harddisk, CD Rom, network server etc.) may simply be able to be loaded into the GIS as you woulddo with any other computer application like word-processing etc.

The other models will certainly require some form of preparation before they can be used in theGIS. Information obtained from printed tables of numeric data and alphabetic text can be typedin to Arcview, or a text editor, to annotate spatial information, boundary polygons for example,or to provide a database or file of information.

Since computers can only work with digital information, any non-digital (called analogue) dataneeds to be converted to digital form before it can be used in a GIS. Spatial information, fromprinted or drawn maps or plans, must be digitised before it is available for use; photographs,etc., must be scanned to obtain a digital representation.

The method of digitising that needs to be adopted depends not only upon the nature of the sourceinformation but also upon the purpose to which the information is to be put. For example, aphotograph of a building offered for sale may need only to be linked with the property addressbut an aerial photograph of the extent of flood waters may also need to be geo-referenced in

order that the area flooded may be aligned with other mapped data.

4.1.3 Different ways of representing spatial data

In the previous Units you have been working with information which is, essentially, representedby lines. The lineation of roads, rivers, and canals, the boundaries of responsibility for units oflocal government, etc. Locations for these have been found in terms of Grid References, or co-ordinate pairs. This, vector model is one data model for representing features of the real world:

VECTOR - this is where the data is recorded as a series of co-ordinate points. A linear feature,such as a road, is recorded by means of two, or more, pairs of co-ordinates; a point feature, suchas a specific building, might be recorded by means of a single pair of co-ordinate values. For aline which changes direction co-ordinates are also required for each point where the linechanges direction. The linear representation is termed a polyline, whilst the specific location istermed a point. A bounded area, such as a lake or an administrative unit is represented by aseries of connecting, and closing, lines, which is termed a polygon.

Points are very useful for representing features which have location but no areal extent, orwhich at the scale at which the data is collected have no effective area. Examples might includethe locations of bus stops, Post Offices, Information Points or car parks.

Polylines are ideal for representing linear features, such as roads and other transportationfeatures, rivers and canals, pipeline or cable services, etc. Again, features represented in thisform have no effective area.

Polygons are used for features which have area, and thus clear boundaries. Examples includethe administrative areas, discussed earlier, lakes, property boundaries, planning regions, etc.

RASTER - represents continuous areal information, sampled and recorded as a framework ofrows and columns where a single piece of information is stored in each grid cell or pixel. Eachcell or pixel represents a value for an area of reality and have the same size and shape. MostGIS deal with square cells or pixels, though some can handle rectangular or even hexagonalcells.

The raster is ideal for representing information which is continuously varying, such as altitude,temperature, land use, etc., but has the disadvantage that all cells or pixels for an area must bepresent, even if there is no useful information conveyed by the values recorded for a particularcell or pixel.

Vector or Raster? The choice can depend in part upon the source of the information but ismainly dependent upon the use for which the information is required. The co-ordinates for thevector representation may be collected with a high degree of precision - surveyed to the nearestmillimetre, for example, and only those points at which there is a change of direction need becollected for a linear feature. This can produce data which is very compact yet very precise. Incontrast, for the raster model, all cells must be collected for the area of the dataset, and all cellsmust have the same size. This makes for large datasets - potentially containing very littleinformation. Halving the cell size results in squaring the number of cells. There is another effectof the use of a regular framework of cells or pixels: this is that the only directions that can berepresented accurately are those in the direction of a face of the cell. This limits the suitability of

the raster model for the representation of linear features, especially those where the delineationmust be accurate - such as a property boundary.

4.1.4 Digitising data

Where spatial information is to be digitised, choice of digitising technique involves not onlyconsideration of the form and format of the data but it is also necessary to consider what thedata is to be used for. For example, if distances along roads are to be measured then sphericalco-ordinates (degrees of latitude and longitude) are not too helpful. Also, the size of the area willdetermine whether curvature of the Earth has to be taken into account and hence the type of mapprojection used.

A plan or image can be scanned, in which case it will be imported in a raster format. With thisprocess paper-based data is captured using a xerographic type technique to determine thepresence or absence of a mark at a point on the page. This process is ideal for the capture ofinformation such as the photograph of property offered for sale. Unless you have the SpatialAnalyst extension available with ArcView 3 you cannot analyse this data but can use it as a baseplan for creating new themes.

Once the source is scanned, the additional techniques of mark capture or OCR may be used toextract digital numeric, or character, information from the scanned image.

Mark capture is a similar type of process involving photo-recognition of marks on paper, likethe pencil 'bar' marks on a multiple choice answer form. This technique can be useful for thecapture of field measurements, using a technique similar to that also sometimes used for marketresearch questionnaires.

Optical Character Recognition (OCR) is similar to these previous two processes but specificallyinvolves machine recognition of a printed page into ASCII characters in a file which can thenedited etc. like text typed in.

Remote sensing (e.g. using satellites) can automatically record spatial data as raster-baseddigital images. These are then typically processed using tools such as the ArcView SpatialAnalyst extension, or specialist image processing software. Data such as aerial photographs,after scanning, may often be processed in a similar way.

Spatial data from a paper map, say, can also be digitised by using a digitising tablet or by usingthe mouse. This method, which mimics the actions of a draughtsman by tracing the requiredinformation is the traditional means of capturing spatial information. ArcView includes, as anExtension, a digitising tool. This tool, which currently only runs on Windows platforms, requiresa digitising tablet or table to be connected to your computer. At this stage using the mouse isprobably the most useful and straightforward example of data input to actually try out:-

Exercise

In this exercise you will learn how to convert information that is already in a digital form, butcollected for some other purpose, and add to it other information so as to create a new spatialdataset.

Fig 4.3 shows a crop plan for a mixed farm, together with the hedgerows that divide the field. AnOS-type base plan Fig 4.1.a (which cannot be edited itself) is available and you are required toenter data concerning the crops grown, their area and the types and lengths of hedges.

In ArcView open a new Project and new View before adding the theme with the farm baseplan(give file name here).

You now need to create a new Theme for some of the data to be entered. Click View on the MenuBar and then New Theme from the pulldown menu. In the subsequent dialogue box choose theappropriate Feature Type (polygon, line or point). For crop types these would be polygons andas each Theme can only be of one feature type you will need to create a second new Theme forthe hedgerow.

Before creating anything in the new Theme make sure that the Map Units and Distance Units areset in the View Properties box (as described in Unit 2).

Now you will trace around the boundary of a field to create a feature for your new Theme. ClickTheme on menu bar and then Start editing. Ensure that the shape you are tracing over ismaximised in the window by using the Zoom In icon (the one with a + on it) to drag out arectangle which just encloses the extremities of the shape. You can then maximise the accuracywith which you draw the polygon.

Now move the cursor over the Draw button and pull down until the cursor is on the polygonsymbol and then release. Trace around the field, clicking once every time you need to changedirection. When the polygon is completed click twice when the polygon will appear selected.

Continue with the other fields until you have traced them all. Move cursor to Select button andpress to switch off Draw tool before you end up starting another unwanted polygon! If no furtherpolygons or editing are required click Theme on the menu bar and then Stop editing.

If the shape of a polygon requires editing first make sure that it is still in editing mode (has adotted line around the check box). If not then click Theme on menu bar and then Start editing.Click Select button and then click inside polygon to select. Click again to show vertices ofpolygon. When in a vertex the cursor changes to a cross-hair type and can be used to drag thevertex to a new position. When complete, click outside the polygon to deselect and then clickTheme on menu bar and then Stop editing.

When you have finished drawing all the polygons open the theme's Attribute Table. You will seethat the number of records corresponds to the number of polygons you have drawn but the onlyfield will be shape. You will now need to create new fields for area and crop type. It's also agood idea to have an ID field so you can give each feature a unique identity. Click Table onmenu bar and then Start editing (or click Edit tool button). You can now add additional fields byclicking Edit on menu bar and then Add Field. Don't forget to specify the data type (number forarea and string for crop type) and to give the field a name in the Field Definition box.

When you have created the area field you can fill in the areas of all the polygons by first makingsure that field name is selected and then clicking the Field Calculator button. In the FieldCalculator dialogue box double click shape in the list of fields on the left and you should then see[shape] appear in the box in the bottom left hand corner. Now immediately after this type.ReturnArea so that the bottom left hand box should now contain the expression [shape].ReturnArea. Click OK and areas should be filled in for all your polygons.

To do add crop type or ID number to the other fields click the Change cell value icon. Add theappropriate value and then repeat for the other cells. When you have entered all the data clickTable on the menu bar and then Stop editing before returning to the View.

To show the fields by crop type in the View, double click the Field Theme in the View Table ofContents to call up the Legend Editor. Click Field pull down arrow and select Crop Type. Doubleclick the symbol of the first crop type to both select it and call up Symbol Palette. If you want touse colour for that particular class of feature ensure that the Fill button on Symbol Palette ispressed and that choice is anything but white (and that it is set for foreground in case ofpolygons). Then click Colour button and choose colour. Repeat for the other crop types. CloseSymbol Palette and Legend Editor when finished.

Each time Legend Editor is changed/closed there is a tendency for it to select new coloursrandomly when it is re-opened. One way of overcoming this is to create the legend you want andthen save it as a named file. It can then be loaded when required for use. When doing this it is agood idea to set the full range of feature categories first and then save the legend. Otherwise, ifyou have to amend the legend you'll probably have to reset the whole range of colours again!

Now try repeating this exercise with the hedgerows. Don't forget that the new theme you willneed to be a polyline theme.

You may now wish to repeat parts of the exercises in Unit 2 in order to produce a hardcopyversion of your work in this exercise.

4.2 Data Conversion

Data, when it first arrives, will almost always need some form of conversion in order that it canbe used in your work. This may involve converting a file into a form acceptable for yourcomputer, or translating location, coordinate, values to the same system as you are using forother information.

4.2.1 Geo-referencing and co-ordinate conversion

The spatial data captured above will be recorded using the coordinates of the source material,or the scanner. In the case of exercise 4.1, the crop plan, this data is already in map coordinates- those of the British National Grid. Where the source data has been scanned, the coordinatesare those of the scanner; in this case they must be converted to reflect their position in the realworld. This is called geo-referencing. ArcView has but limited built-in facilities for geo-referencing, and to perform this operation on (scanned) images requires the ArcView SpatialAnalyst optional extension. Images will not, therefore, be discussed further in this unit.

Sometimes digital data is available, but is recorded using a co-ordinate system which is differentto that being used for other data, and the co-ordinate systems must be aligned so that the datawill match up. This change of co-ordinate systems is called re-projection.

In order explain the next steps, it is necessary for us to think a little more about co-ordinates.

In Unit 1 the concepts of co-ordinates and the National Grid were introduced. Brief mention wasalso made of latitude and longitude which are measured in degrees. The reason for this is thatthe Earth is a sphere and not a simple flat surface as maps, at first sight, would suggest!Therefore, the latitude and longitude of a particular feature on the Earth's surface give theangles relative to the centre of the Earth's sphere which define its location (see Fig. 4.4 cut-openEarth to show angles of lat & long for part. location). Angles have the advantage of beingindependent of surface shape and so do not depend upon a flat surface like measurements on apiece of paper but are not convenient for the measurement of distance between locations.

Since it is not possible to accurately represent the curved surface of the Earth on to a flatmedium, such as paper, some approximations must be made. The extent and nature of theseapproximations will depend upon the proportion of the earth's surface that is to be representedand the purpose for which the product will be used. This process of approximating the curvedsurface on to the flat one is termed Projection and in the specific case of geographicrepresentations, Map projections.

ArcView provides facilities for different map projections, and for georeferencing image data,such as satellite imagery. There are no built-in facilities, other than within the DigitiserExtension for controlling a digitising tablet or table, which support georeferencing of torinformation, nor for advanced correction of photography (ortho-rectification). This is an areawhich often requires the capabilities of the more advanced GIS, such as Arc/Info.

The issue of converting data supplied in one map projection to another is considered in moredetail in section 4.3, Data Integration.

4.3 Data Integration

In almost every real life project it is necessary to bring together information from more than onesource. The moment this is done there are problems to resolve that result from this informationhaving been collected at different times, or by different people, or using different units ofmeasurement. The process of bringing different datasets together, of resolving so far as ispossible these problems, is called data integration. Perhaps the classic example of a problem ofthis nature is that of trying to work with data from different censuses, where the enumerationarea - the basic unit of aggregation - may represent a different group of households from censusto census. In this instance the problem lies between different instances of what is apparently thesame dataset. The problem may get more significant when different data sources are involved!

Both spatial and non-spatial data can also vary considerably in scale. For example, the map inFig. 4.1 is of a small scale and the data associated with it is fairly general in nature. In contrast,Fig. 4.3 is a much larger scale map and the data associated with it ( information about theindividual fields and crops grown) is much more intricate in detail.

The concepts of scale, resolution, precision and accuracy (section 4.1.1) apply equally to spatialand non-spatial data. For example, non-spatial data on population sizes of cities may be given intens of thousands (relatively coarse resolution) or in hundreds (relatively fine resolution). Thiswould then determine the precision of the data (e.g. to the nearest hundred etc.). The accuracy ofthe data would be affected, for example, if significant numbers of people had moved out of thecity since the population size had last been estimated.

One important principle to remember is that processing in a GIS (e.g. overlaying themes derivedfrom differently derived sources and observing any relationships between them), may exacerbateany errors between such sets of data so it is important to know, as far as possible, about thequality (i.e. resolution, precision and accuracy) of the original data before it is processed. Forexample, spatial data intended for display at a scale of 1 : 250000 will most likely containgeneralisations of detail; a line 1 mm wide would represent a feature 250m wide, so widths ofroads and most rivers cannot be depicted. If such data is then combined with a dataset intendedfor display at a scale of 1 : 2500, it would be unlikely that the representations of the variousfeatures would align. With non-spatial data the size and delineation of the collection unitbecomes important.

4.3.1 Integration of datasets

Where spatial data, to be used within ArcView, is stored in geographic coordinates, ArcViewwill automatically reproject to the projection and scale defined for a View, in the ViewProperties. ArcView cannot integrate spatial data which is stored in other coordinate systems,although it can work with multiple datasets stored in the same coordinate system.

Where data is collected to different spatial aggregations, ArcView can be used to combinesmaller units of information in order to standardise at the coarser resolution. It is not possible tounpick generalisation, so it is always the coarser resolution which wins.

ArcView can also integrate spatial data in different formats. For example, some of the data used

in this tutorial have been in ArcView's shape format, some others may have been in Arc/Infointernal format. ArcView will read Arc/Info format data but you cannot change any of thecontents. ArcView can also read datasets created in CAD software, especially when in AutoCADdxf format.

Because ArcView uses dBase format database files, it is easy to share tabular, non-spatial,information between ArcView and spreadsheet software and database systems, such as QuattroPro or Microsoft Access. Attention will have to be given to ensuring that one of the databasefields is a valid index, or key, to your spatial information if you wish to use this information on amap display or in analysis. A common key, for such purposes, is location given as coordinates. Itmay seem obvious, but it is essential that coordinates used in this way are expressed in the sameway as those being used within the map data. For example, spatial, map, data which uses UKNational Grid coordinates will use these as numeric, metre, references. The coordinates in adatabase table to be related to such coordinates must also be in the full, numeric, form.

4.4 Data Management

All this data, spatial and otherwise, presents the possibility of confusion between many differentdata objects, each of which may occupy one or more computer files. Spatial data tends to bevoluminous, so the amount of computer disk space required may also be large. This sectionexplains something about the various files that are usually associated with ArcView.

4.4.1 Managing ArcView files

The files used by ArcView comprise a database of the spatial component, the .shp file, and theattribute data, the .dbf file. These are very much like those used by other database andspreadsheet tools, indeed the .dbf file may be read with any software which understands the PCdBase database file format. In addition to these data files, ArcView also a file to maintain theindex between the spatial and attribute data, the .shx file - but this file is managed by ArcViewand there is never normally any need to touch it directly.

In addition to the data files, ArcView also creates project files, .apr, in which you can save acomplete working environment, together with any Avenue scripts or customisations you mayhave created.

Because spatial data tends to be large - a typical OS digital map may occupy anything between8MB and 20MB for a 0.5km square area - existing data files (e.g. digital maps with associatedattribute tables) will probably be kept on a hard disk or network server. You have seen how sucha file can be brought into ArcView in the form of a theme. If you then carry out some processingof the data (e.g. as you did by selecting a particular set of canals in Unit 3) and want to savewhat you have done you do so in the form of an ArcView PROJECT. However, the project files donot contain the data but remember where it was located and what had been done to it for thatparticular project. Therefore, when you open a project you have saved previously, ArcView willautomatically seek out the relevant data, import it and manipulate it for that project.

Another reason for mounting data on a server may be to comply with the conditions of copyrightlicenses. Virtually all spatial data available in the United Kingdom, even throughout Europe, isprotected under copyright law and its usage is licensed. It is a legal duty of both institution and

data user to ensure compliance with such licenses. The Bartholomew and Census Boundary datayou used in the previous units is copyright in this way, and may not be used other than as a partof this tutorial.

However, there may be times when you have created your own theme(s) in the form of a shapefile, e.g. as explained above, and, for your own piece of mind, you may want to save such atheme on a floppy disk. You need to know that although when you 'add theme' to a project, or use'convert to shapefile' from the Theme menu of a View, it appears as a single .shp file in the 'addtheme' dialogue box, it does in fact consist of three different files, all with the same name butwith different file types. These are:-.shp .dbf .shx

If you wish to remove an ArcView shape file, it is necessary to delete all three of the files.ArcView does not provide a tool to delete files.

You may move files from one disk directory to another (remember to move all three together!)but if you do and you have referenced them in a saved project (.apr) file, ArcView will no longerbe able to find the files.

Therefore, if you are copying on to a floppy disk remember to copy all three files for a particulartheme. Similarly, if you wish to delete a shape file it is necessary to delete all three files of theset.

4.5 Summary

In this unit you have learned how GIS works with different forms of data and about the differentways that real world objects may be represented in ArcView Themes. You have learned that it isnecessary to investigate information about data that you wish to use, such as sample size,resolution, precision and accuracy. You have learned that it is sometimes necessary to make useof information which may not be ideal because of the cost, or unavailability of betteralternatives. You have also learned about some of the issues involved in bringing differentdatasets together, and about managing an ArcView workspace. You have experienced some ofthe pain of digitising spatial data.

You should now be equipped with the basic knowledge of how to use ArcView, of how to getinformation into the GIS, how to use the GIS to analyse the information and how to display, tocommunicate, your findings. The next stage is practise! To get more familar with the GIS and touse it for something for real before you attempt more advanced activities, such as are providedin the various ArcView Extensions, such as Spatial Analyst.

Suggestions for further reading

Chapters 4 and 6 of Principles of Geographical Information Systems for Land ResourcesAssessment, P. A. Burrough, 1986, Oxford University Press.