introduction to gis ©2008 by austin troy. all rights reserved lecture 5: symbology lecture by...

©2008 by Austin Troy. All rights reserved

Introduction to GIS

Lecture 5:

Symbology

Lecture by Austin Troy, University of Vermont


Introduction to GIS

Mapping of Attribute Data

Two basic approaches for visually displaying attribute data:

1. Quantities approach

2. Category approach


Introduction to GIS


Quantity approach: applies to numeric >> ordinal

Category approach: text values; order is irrelevant


Introduction to GIS


Category approach, example: vegetation type


Introduction to GIS


Quantity approach, example: population


Introduction to GIS

Categorical Examples

Vegetation types, land use, soil types, geology types, forest types, party voting maps, land management agency, recategorizations of numeric data (“bad, good, best” or “low, medium, high’).

Can you think of any others?


Introduction to GIS

Mapping Categories

TOC >> layer properties>>symbology tab>>Categories>> Unique values

Choose values field that contains the attribute

Click the “Add all values” button


Introduction to GIS

Mapping Categories


Introduction to GIS

Mapping CategoriesOften categories must be aggregated and redefined: this land use

map had over 110 categories that were condensed to 12


Introduction to GIS

Mapping Categories--grouping

In this case 1262, 1263, 1264, 1265, etc. refers to different subcategories of commercial land use

Can then save symbology as .lyr or in .mxd


Introduction to GIS

Quantity Mapping

Also known as “choropleth mapping”

• For points, lines and polygons: graduated color, or color ramping

• For lines and points can also do graduated symbol


Introduction to GIS

Graduated Color

In Arc GIS layer properties>>symbology, we choose Quantities>>graduated color

We then choose a value to representIn this case we choose

median house value

It automatically choosesfive classes for the data


Introduction to GIS

Graduated Color

The resulting map shows high housing value areas with dark colors and low with light


Introduction to GIS

Graduated Color

Now with 3 classes for same variable


Introduction to GIS

Graduated Color

…and 15 classes


Introduction to GIS

Graduated Color Classification

interfaceThese are the breaks it makes, based on the distribution of the data

largesmall

Classification method (default= Jenks)


Introduction to GIS

Graduated ColorEqual interval

method

What kind of data is does this work for?


Introduction to GIS

Graduated ColorHere’s what the same

distribution looks like with only 5 equal intervals.


Introduction to GIS

Graduated Color

Problematic example: skewed data (# of vacant structures)


Introduction to GIS

Graduated Color

This map of vacant properties tells us almost nothing, because almost all the records fall into the first class


Introduction to GIS

Graduated Color

Notice how with natural breaks there are now more classes on the left side, where most of the data are. Natural breaks uses Jenks Goodness of Fit statistic to adjust class boundaries


Introduction to GIS

Graduated Color

This map, made with Natural Breaks, is more intelligible


Introduction to GIS

Graduated Color

Quantile method: sets the class boundaries so as to maximize the perceived variation in the map; equal number of data points in each class


Introduction to GIS

Graduated Color

Quantile output


Introduction to GIS

Graduated ColorGraduated color can also be applied to points.

Here are houses display by sales price

Natural breaks Equal interval


Introduction to GIS

Graduated SymbolIn this case housing price is expressed by symbol size


Introduction to GIS

Graduated SymbolThe same thing can also be

done with lines—for instance, traffic volumes


Introduction to GIS

Symbol StylesWe can also choose to “match to symbols in a palette” and then apply the

“transportation.style” palette to the CFCC, or roadcategory, attribute in our roads layer

Introduction to GIS

Results in this map

Must click here to match

Choose your style palette here


Introduction to GIS

Symbol Styles


Introduction to GIS

Symbol StylesOne could also manually create symbol styles for each street type. Clicking on each

symbol in either the TOC or properties windows brings up a manual symbol selector. You can assign a separate one to each category.

Introduction to GIS

Includes many more classes of symbols that are industry standar


Introduction to GIS

Symbol StylesThere are also a huge variety of industry-specific point symbols

that can be either assigned through matching symbols to a predefined style or manually assigning those symbols

Introduction to GIS


Introduction to GIS

Charts displayed geographicallyAttributes for point, line or polygon features can also be

displayed as charts on the map

Introduction to GIS


Introduction to GIS

NormalizationShow an attribute normalized by another attribute or expressed as a percentage of

total. Here we have number of vacancies per tract as a percentage of total households.

Introduction to GIS

numerator

denominator

introduction to gis ©2008 by austin troy. all rights reserved lecture 5: symbology lecture by...

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