Introduction to GIS ©2008 by Austin Troy. All rights reserved Lecture 5: Symbology Lecture by Austin Troy, University of Vermont

Introduction to GIS ©2008 by Austin Troy. All rights reserved Mapping of Attribute Data Two basic approaches for visually displaying attribute data: 1.Quantities approach 2.Category approach

Introduction to GIS ©2008 by Austin Troy. All rights reserved Mapping of Attribute Data Quantity approach: applies to numeric >> ordinal Category approach: text values; order is irrelevant

Introduction to GIS ©2008 by Austin Troy. All rights reserved Mapping of Attribute Data Category approach, example: vegetation type

Introduction to GIS ©2008 by Austin Troy. All rights reserved Mapping of Attribute Data Quantity approach, example: population

Introduction to GIS ©2008 by Austin Troy. All rights reserved 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 ©2008 by Austin Troy. All rights reserved 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 ©2008 by Austin Troy. All rights reserved Mapping Categories

Introduction to GIS ©2008 by Austin Troy. All rights reserved Mapping Categories Often categories must be aggregated and redefined: this land use map had over 110 categories that were condensed to 12

Introduction to GIS ©2008 by Austin Troy. All rights reserved 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 ©2008 by Austin Troy. All rights reserved 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 ©2008 by Austin Troy. All rights reserved Graduated Color In Arc GIS layer properties>>symbology, we choose Quantities>>graduated color We then choose a value to represent In this case we choose median house value It automatically chooses five classes for the data

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color The resulting map shows high housing value areas with dark colors and low with light

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color Now with 3 classes for same variable

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color …and 15 classes

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color Classification interface These are the breaks it makes, based on the distribution of the data largesmall Classification method (default= Jenks)

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color Equal interval method What kind of data is does this work for?

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color Here’s what the same distribution looks like with only 5 equal intervals.

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color Problematic example: skewed data (# of vacant structures)

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color This map of vacant properties tells us almost nothing, because almost all the records fall into the first class

Introduction to GIS ©2008 by Austin Troy. All rights reserved 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 ©2008 by Austin Troy. All rights reserved Graduated Color This map, made with Natural Breaks, is more intelligible

Introduction to GIS ©2008 by Austin Troy. All rights reserved 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 ©2008 by Austin Troy. All rights reserved Graduated Color Quantile output

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Color Graduated color can also be applied to points. Here are houses display by sales price Natural breaks Equal interval

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Symbol In this case housing price is expressed by symbol size

Introduction to GIS ©2008 by Austin Troy. All rights reserved Graduated Symbol The same thing can also be done with lines—for instance, traffic volumes

Introduction to GIS ©2008 by Austin Troy. All rights reserved Symbol Styles We can also choose to “match to symbols in a palette” and then apply the “transportation.style” palette to the CFCC, or road category, 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 ©2008 by Austin Troy. All rights reserved Symbol Styles

Introduction to GIS ©2008 by Austin Troy. All rights reserved Symbol Styles One 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 ©2008 by Austin Troy. All rights reserved Symbol Styles There 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

©2008 by Austin Troy. All rights reserved Charts displayed geographically Attributes for point, line or polygon features can also be displayed as charts on the map Introduction to GIS

©2008 by Austin Troy. All rights reserved Normalization 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. Introduction to GIS numerator denominator