©2005 by Austin Troy. All rights reserved Lecture 5: Introduction to GIS Legend Visualization...

©2005 by Austin Troy. All rights reserved

Lecture 5:

Introduction to GIS

Legend Visualization

Lecture by Austin Troy, University of Vermont


Visual Analysis• The most intuitive form of vector analysis is

visual analysis, where we code features with colors or symbols to deliver information

• Frequently, we code features by an attribute value and let the color or symbol express the attribute value

• Understanding legend editing and map classification is critical to making maps that effectively deliver information

Introduction to GIS


Mapping of Attribute Data

In GIS, each feature can have a number of attributes attached to it (e.g. land parcel>> property ID, assessed value, square footage)

We can map out these attribute values by their corresponding geography

Two basic approaches for classifying the data:

1. Quantities approach

2. Category approach

Introduction to GIS



Quantity approach: applies to numeric attributes that are ordinal (have order to them); this means one values is greater than or less than another; good for continuous data.

Category approach: applies to categorical data, where the categories can have, but don’t need to have, order. If they do have order, the category approach ignore that order

The same layer can have some quantitative and some categorical attributes

Introduction to GIS



Category approach, example: vegetation type

Introduction to GIS



Quantity approach, example: population

Introduction to GIS


Mapping Categories

This is the simplest type of mapping: we are simply assigning a different color or symbol to each feature with a given category value

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

To map categories in ArcGIS, we simply double click on the layer in the TOC and, in “layer properties,” click on the “symbology” tab

Generally,we will choose “Categories>> Unique values”

Introduction to GIS

The we choose our values field that contains the attribute and then click the “Add all values” button


Mapping Categories

The symbology in the last slide gives us conservation lands, categorized by type of ownership

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 CategoriesDo do this, we must group the “group values” function in the

symbology properties window

Introduction to GIS

We can then give that grouping a label

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

This classification is saved when I save my ArcMap Document


Quantity Mapping

This is more complex, because there are so many ways to map out quantities

Mapping options depends on the feature type:• For points, lines and polygons, we can darken or

lighten the color to express magnitude: this is called graduated color, or color ramping

• For lines and points we can increase symbol size to express greater magnitude: this is called graduated symbol; we can do this because points and lines have fewer than 2 dimensions

Introduction to GIS


Choropleth Mappinga thematic mapping technique that displays a quantitative attribute using ordinal classes applied as uniform symbolism

over a whole areal feature. Sometimes extended to include any thematic map based on symbolism applied to areal objects.-Nick Chrisman

A map that shows numerical data (but not simply "counts") for a group of regions by (i) classifying the data into classes and (ii) shading each class on the map. -Keith Clarke

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 ColorIn that case we used 5 classes. Changing the number of

classes changes the information delivered; more classes: more info, but harder to see differences

Introduction to GIS

3 classes for median value


Graduated ColorIn that case we used 5 classes. Changing the number of

classes changes the information delivered; more classes: more info, but harder to see differences

Introduction to GIS

15 classes for median value


Graduated ColorThe Classification

Method also affects how the mapped attributes will look. Arc GIS normally defaults to the Jenks, or natural breaks, method

Introduction to GIS

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

largesmall


Graduated ColorNow, here’s an

equal interval approach. Notice how all the breaks are evenly spaced. With a fairly normal distribution of data, this is usually OK

Introduction to GIS


Graduated ColorHere’s what the same

distribution looks like with only 5 equal intervals.

Introduction to GIS


Graduated Color

However, when the distribution is skewed, or there are significant outliers, then equal interval is problematic because most intervals have no data in them. Here’s an example, with number of vacant houses per tract—most have near none, but a very few have a lot

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

Introduction to GIS


Graduated Color

Introduction to GIS

This map, made with Natural Breaks, is more intelligible


Graduated Color

There is a similar approach to Natural Breaks called Quantile. This method sets class boundaries so each class has equal numbers of observations in it

Introduction to GIS


Graduated Color

This essentially sets the class boundaries so as to maximize the perceived variation in the map, as we see here

Natural Breaks is similar, but does not necessarily result in an equal number of data points in each class; rather it uses Jenks' Goodness of Variance Fit (GVF) statistic

Introduction to GIS


Graduated ColorGraduated color can also be applied to points.

Here are houses display by sales price

Introduction to GIS

Natural breaks Equal interval


Graduated SymbolSince points and lines are not dimensionally realistic, the symbols representing

them can also be graduated. Here the size of the dot represents the house price

Introduction to GIS


Graduated SymbolThe same thing can also be done with lines—for instance, the width of a line feature showing rivers

can be made to represent the flow of that river segment. For many line features, like streets, ArcGIS comes preloaded with symbol palettes that recognize the attribute codes and put the appropriate symbol

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


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


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


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

displayed as charts on the map

Introduction to GIS


NormalizationWith graduated color or symbol, we can also show 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. Otherwise we’re only tracking total number.

Introduction to GIS

numerator

denominator

©2005 by Austin Troy. All rights reserved Lecture 5: Introduction to GIS Legend Visualization...

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