CS 128/ES 228 - Lecture 2b 1

Attribute Data and Map Types

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What kinds of data are in a GIS?

Spatial data

Non-spatial data (also known as attribute data)

“A GIS with no attribute data is a mapmaking system, not a GIS!”

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What is Attribute Data?

Attribute data is data about objects stored in a GIS that refers to non-spatial properties of the object.

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Examples of Attribute Data Date of construction of a building Purpose of a building Name of a stream Population of a city Breed of dog that lives at a house Photo of a fire hydrant

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How is Attribute Data kept in a GIS?

Attribute data is generally stored in database tables.

CampusID Name Type Floors Footprint

6 Murphy Academic 2 2001

9 Hopkins Support 2 946

12 Maintenance Support 1 1848

15 Hickey Support 2 2367

17 Shay-Loughlen Dorm 3 1298

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How is Attribute Data extracted from a GIS?

GIS’s have two main types of output Reports Maps

As always, these can be combined

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Reports A Geographic Information System is,

at its core, a database.

Good database software always has a report generator. (We have something called Crystal Reports.)

Ergo, one can produce reports from a GIS.

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Maps A picture is

worth 1000 words

What attribute data is being shown?

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How is the Data Shown? Symbolization

Issues:

Realistic vs. abstract symbols

Size, texture, and density

Use of color

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Visual Display of Attribute Data

Easy for discrete features

There are many ways to represent continuous features on a map

Beware of the boundaries between classifications – they’re not usually very meaningful

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How do we distinguish among our data values?Dichotomous scale

(i.e. two classes)

Each class quite heterogeneous

Placement of boundaries is extremely sensitive to data density & quality

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How many classes to use?Multiple classes:

Classes more homogeneous

Large number of classes hard to interpret

Note: density of symbols should match the quantitative order of the classes, i.e. greater density => greater value

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How shall we determine the class limits?1. Intervals of constant size

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How to set class limits?2. Intervals that have equal numbers of cells

(equal class size)

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How to set class limits?3. Natural breaks in distribution

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A GIS Riddle Q: When is a map not a map?

A: When we call is something else.

Q: Why would we do this?

A: Because we do…

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Thinking about the data in a mapHow “processed” is the data?

Not at all Some Lots

Image Maps Line Maps Cartograms (Choropleths)

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Image maps (unprocessed data)

Composed of images of the area under study (usually aerial photos)

Often pieced together to make “mosaics”

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Advantages of image maps

What you see is what is there (assuming the photo is current)

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Problems with Image Maps Interpretation

Details can be tricky – perspective is unusual (see math at right)

Distortion Especially near

edges and seams No annotation

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Line Maps (Somewhat processed data?) Reality is replaced

by “reality-based” renderings

“Raw” data is replaced by a representation of that data

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Advantages of Line Maps Can concentrate

on information “of interest”

“Easy” to understand

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Disadvantages of Line Maps Data is not as

accurate due to: Incompleteness Representation

(especially scaling) Deliberate

“editorial” changes (see Exaggeration from previous lecture)

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Cartograms (Choropleths)

Similar to line maps, but geographic data is deliberately distorted to make some other point

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Utility of Cartograms Strengths

Highlight exactly what is desired

Strong visual imagery

Weaknesses

Not useful outside initially intended domain

Relatively difficult to produce Lots of information is lost

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Question???

Is a cartogram a map?

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Other types It is not uncommon to combine

some of these types

Cartographically Enhanced Image Maps are particularly common For example, the map we use in lab

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Map Forms

Historically, maps have been static, e.g. on sheets of paper

Computer technology has rendered maps dynamic and/or interactive

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Two “dynamic” maps

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One last issue

When do we compute using the attribute data?

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Early Processing… Compute your answers early and

then reveal them when asked. Commonly done for systems such as

search engines

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Late Processing… Store only your data; compute

answers as needed

MapQuest does this, as requests can’t be known in advance

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Hybrid Do some processing early, do some late

It is usually hard to detect that this is happening

“Caching” is one (not so good) example of this approach.• (Not so good because caching isn’t really

processing, per se)

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Conclusions A map is interesting, but a map that

highlights attribute data is useful

There are tradeoffs between completeness of information and ease of user processing

Caveat user!