Prac - Choropleth11

8/12/2019 Prac - Choropleth11

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University of Portsmouth

Department of Geography

Introduction to GIS

Level 1 Semester 1

Mapping Population and Deprivation using the Choropleth Map

by

Alastair Pearson and Liz Twigg

For submission through the Office by the date stated on the Unit Outline


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Introduction

The International Cartographic Association define a choropleth map as a"method of cartographic representation which employs distinctive colouror shading applied to areas other than those bounded by isolines. Theseare usually statistical or administrative areas."

According to Dent (1985), there are three ways in which map readers usechoropleth maps:

i) To ascertain an actual value associated with a geographical area (itis probably better to consult a table of values).ii) To obtain a sense of overall geographical pattern of the mappedvariable with attention to individual values.iii) To compare one choropleth map's pattern to another's.

When to employ choropleth mapping

This technique can be used

i) When the cartographer is required to portray a geographical themewhose data are discrete and occurs within clearly defined administrativeor enumeration units.ii) When the assumptions of this simple technique are acceptable to both

cartographer and reader

This technique should notbe employed for

i) Mapping continuous geographical phenomena such as airtemperatures, barometric pressure and rainfall.ii) If the data cannot be dealt with as ratios or proportionsiii) To show precise, actual values within enumeration units

Part 1. Mapping Population Density from the 1991 Census

1. Selecting the base map and worksheet

We will use a UK base map and population data.

Copy the folder K:\Student\Science\Geography General\CourseSupport Materials\Introduction to GIS\choropleth to your N:\drive


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Open Mapviewer 5 and once you have opened it, select Close fromthe File menuNow select Open and select the Ukpop81.gsm fileSelect Fit to windowfrom the View menu. The counties of the UKshould now appear on screen.

Check the Preferences making sure that the default path is set toN:\choropleth

Select the Load data option from the File menu and select theukpop81.dat worksheet file from your Choropleth directory.

2. Viewing the worksheet and creating ratio data

Select the Worksheet option from the File menu.

The Worksheet window displays the data file of the currently active map.This window makes it possible to view, alter, and add data to the datafile. The worksheet also contains the Primary ID (text string) that linksdata to specific areas on the map. The Primary ID is always contained incolumn A, and the associated data is in the corresponding row. The nextcolumns contain the county name, area in km2 and total population. Wewill now calculate the population per km2 in column E.

Choose the Transformcommand from the Computemenu and theTransformdialog box appears

Type a formula in the Transform Equation box in order to calculate thepopulation per km2 in column E. The letters in the formula correspond tothe column letters of the worksheet.

Type in the values 2 and 66 in the Transform rows edit boxes, andclick on OK. (Check that all the rows are transformed)


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This creates a new column of data in column E that represents the ratio ofpopulation per km2 as a basis for producing the choropleth map. Checkthat the formula has worked and has been applied to the whole of columnE.

Click on cell E1 (column E, row 1) and type 'Pop. Per Km' as thevariable name of this column.Click on Column E in order to highlight the whole column thenselect Cell Properties from the Format menu

Select Fixed as the Typethen set the Decimal digits to 0

3. Creating the choropleth map

Move back to the map of the UK. We will now create a palette of tones toassign to our classes of data then test the methods of classificationavailable in Mapviewer before producing our final map.

Select Map then Thematic Map then Hatch Map. The hatch mapdialog box should appear.

Make sure that the PID is set to Column Aand that the Variable isset to Column E. Click on OK.

Click on OK in the Hatch Map dialog box to see the choropleth mapusing the default settings.

Choose the Legend command from the Map menu. The legendautomatically reflects the hatch patterns and data ranges specifiedin the hatch map dialog box.

The default settings provide a quick map. Let's experiment with theclassification of the map in order to demonstrate the effect differentmethods of classification have on choropleth maps.


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4. Selecting a class interval

A list of the major class interval methods is given below. The first tworange types. Read but do not attempt to implement the followingclassification methods until later.

Equal number Also known as quantiles. Ensures that there is an equalnumber of observations in each class. If the enumeration districts varygreatly in size, this can be a misleading method.

Equal intervals Particularly useful when the histogram of the data has arectangular shape (rare in geographical phenomena) and whenenumeration districts are evenly sized.In such cases, this method

produces an orderly map.

Standard deviation Should only be used when the data arrayapproximates to a normal distribution. The classes formed provideinformation about frequencies in each class. Particularly useful forshowing deviation around the mean of the data. Usually limited to 6classes.

Jenks' natural breaks data classification method identifies naturalbreaks within the data set. Typically these natural breaks are ideal

beginnings and endings for data classes, but finding the natural breaksand defining each natural break class can be difficult and time consuming.The Jenks' Natural breaks data classification method makes dataclassification based on natural breaks quick and easy.

User Defined allows you to create your own class definitions.Overlapping or discontinuous classes are not allowed. Double-click onthe minimum and maximum values in the Objects in classeslist to definethe classes after you define the Number of classes (below).


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Task 1

1. Produce 4 maps that show the population density using EqualNumber, Equal Interval, Standard Deviation and Jenks Naturalbreaks and using the same colour scheme the colour schememust be suitable for single variable mapping

2. For each map, you must include the title e.g.Population density ofthe United Kingdom using Equal Intervals and a legend headedPopulation Density. Make sure that the figures in the legend haveno more than 2 decimal digits by adjusting the numeric format inthe legend dialog box.

3. All 4 maps should be imported into MS Word and printed in colouron one A4 sheet. The easiest way to do this is to use Select alllayers from the Edit menu, then Copy. Paste the copied map intoWord. Click on the map when it appears in Word and then adjustits position and size appropriately. Use a right mouse click thenFormat object to make further adjustments.

4. Print out to an A3 colour printer. You will have to pay first beforethe printing can be carried out.

Task 2If we wished to group county populations of similar densities, a more

representative classification can be created by determining natural breaksand classifying the data manually. The aim is to form data ranges thatare internally homogeneous (uniform nature) whilst preservingheterogeneity (uniqueness) between classes.

Produce 1 x A4 map using your own user defined classification byreferring to the histogram below. This map should be properly annotatedwith some counties named. Include the title and select an appropriatecolour sequence.


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write down a classification system that uses at least 5 classes (e.g.0 to 250; 250 to 500; 500 to 1000 etc). The classes need not beequal in range.

Tips

The Hatch Map dialog box contains a List box that displays theparameters of each of the data ranges. This information includes the

>=Minimum (minimum value of the data range),


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Part 2: Mapping Deprivation using the 2001 Census

The aim of this part of the practical is to construct a deprivation index forthe output areas across the Portsmouth Unitary Authority using 2001Census Data, map the results and provide a commentary on these.

We are going to calculate an index similar to that constructed by Carstairs(Morris and Carstairs, 1991) known as the Carstairs Index of MaterialDeprivation. This is based on four census variables. NB These have beenamended slightly for ease of use in this practical and to reflect 2001census definitions:-

1. Unemployment unemployed male residents (16-74) as aproportion of all economically active males residents aged 16-74

2. Overcrowdingnumber of households with 1 or more personsper room as a proportion of all households.

3. Non-car ownership resident (aged 16-74) in householdswithout access to a car or van as a proportion of all residents inhouseholds (aged 16-74)

4. Low social classResidents aged 16-74 NS-SeC 7 and 8 as aproportion of all economically active people

There are two files in the folder that you copied from the K drive earlier Port_outareas.gsmand Port_outareas.dat. Open the .gsm file from

within Mapviewer using the Open command from the Filemenu andLoad the .datalso from the Filemenu. Select Worksheetfrom the Filemenu to view the .dat file.

The data for 630 output areas have been extracted from the 2001 censusvia CASWEB. The names and contents of the columns are as follows (andalso follow the definitions above):-

COLUMN ZONE Output Area code

D UNEPMAL Total number of unemployed males

E ECONMALE Total number of economically active malesF OVCROWD Total number of overcrowded householdsG TOTHHLDS Total number of householdsH NOCARVAN Total number of residents without access to

a car or vanI TOTRESID Total number of residentsJ CLASS78 Total number of residents classed as low

working class

K TOTECONA Total number of residentseconomically active


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Calculating the Index:-

Four proportions need to be calculated as given in 1-4 above and storedin new columns. This involves dividing unepmal by econmale; ovcrowd bytothlds; nocarvan/totresid; class78 by totecona. To do this:

Insert a new column after column E to enter the calculatedunemployed males as a proportion of economically active males.To do this - Select the top of column Fso that the whole columnis highlighted then select Insert from the Edit menu. The blankcolumn F should appear.Enter an appropriate title for the columnNow calculate the proportion using the Transform command tocalculate the number of unemployed males as a proportion ofeconomically active males for rows 2 to 632.

The proportions need to be converted to a standard score. To do this weneed to derive the standard deviation and mean of each proportion to usethe standard score equation:-

Nscore = (x mean(x))

x

This is quite straightforward. Lets calculate the mean and standarddeviation values for our new column of data in Column F.

Select the column by clicking on the letter F. The whole columnshould now be highlighted.Select Statistics from the Data menu and select those itemsthat we wish to compute (ie mean and standard deviation). Opt


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to show the results in a window and select the Population checkbox.

The results should be as follows:

We can now calculate the Z-score by substituting the values inthe formula. But first we need to create a new column next tothe Proportion Unemployed (do this as instructed above).Calculate the Z-score using the following transformation:G = (F - 0.05634)/0.05217

The results should be as follows:


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Repeat this process for unepmal by econmale; ovcrowd bytothlds; nocarvan by totresid; class78 by totecona.The overall index is calculated by adding up each of the fourcolumns of standard z-scores (T = G + K + O + S). A highnegative score indicates that the area is not deprived and high

positive scores represent high levels of deprivation. The finalIndex values for the first 8 rows are shown below as a check.

When mapping this index we can use the 95 and 99% standardcut off points +/1.96 and +/-2.58 to classify the index valuesfor producing a choropleth map showing significantly high andlow areas of deprivation. But first we need to calculate the z-score of the index value:Calculate the z-score for column T (Index) and place theresults in column U.


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Task 3

Creating the choropleth map

1. Classify the data.

Employ a classification of 6 categories using the Hatch Map option, usingthe user-defined option.

Minimum to 2.58 Deep Blue-2.58 to 1.96 Blue-1.96 to 0 Light blue0 to 1.96 Light red

1.96 to 2.58 Red2.58 to maximum Bright red

2. Set appropriate colours.Select the fill by double clicking on the actual fill in the Hatch Map dialogbox. Set the colours to conform to a double ended colour plan as above.You need not use red and blue select something appropriate.

3.Add colour fills to land and sea areas.The final map should include colours for the land and sea areas that areheld in other layers. Go to the Layers option from the Map menu andedit the relevant layer. Making sure that the correct layer is active,

double click on the land to select Object Properties. Use the FillProperties to assign a suitable colour to your land and sea areas. NB donot use strong colours for these background areas greys may be moresuitable than blues and greens.

4. Add a scale barSelect Scale Bar from the Map menu. Make sure that your units arealready set appropriately in the Preferences. Experiment with thedesign and options.

5. Add textAdd text to identify major neighbourhoods within the map area particularly where there are significantly highor low deprivation values.

The marking criteria are provided below


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Text

Positive 1st 2:1 2:2 3rd Fail Negative

Appropriateuse of typefont

10 9 8 7 6 5 4 3 2 1 Inappropriateuse of typefont

Appropriatefont sizes

10 9 8 7 6 5 4 3 2 1 Inappropriatefont sizes

Clear andlogicalhierarchy

10 9 8 7 6 5 4 3 2 1 Unclearhierarchy

Good overalllegibility

10 9 8 7 6 5 4 3 2 1 Poor overalllegibility

Accuratepoint

locations

10 9 8 7 6 5 4 3 2 1 Inaccuratepoint

locationsSuitablenameplacement

10 9 8 7 6 5 4 3 2 1 Poor nameplacement

Good overallimpression

10 9 8 7 6 5 4 3 2 1 Poor overallimpression

ThematicPositive 1st 2:1 2:2 3rd Fail Negative

Appropriatecoloursequence

10 9 8 7 6 5 4 3 2 1 Inappropriatecoloursequence

Appropriateland/sea

10 9 8 7 6 5 4 3 2 1 Inappropriateland/sea

Classescorrect

10 9 8 7 6 5 4 3 2 1 Classesincorrect

Classes

properlylabelled

10 9 8 7 6 5 4 3 2 1 Classes

improperlylabelled

Marginaliaappropriateandcomplete

10 9 8 7 6 5 4 3 2 1 Incompletemarginalinformation

Good overallimpression

10 9 8 7 6 5 4 3 2 1 Poor overallimpression


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Task 4

In no more than 150 words describe and attempt to explain the pattern ofdeprivation shown on the map. Make specific reference to such issues as:

Methodological issues

1. Using aggregate data2. Choice of variables used to create the index

Apparent geography of deprivation

1. Factors specific to Portsmouth2. Factors common to other cities in Britain

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