Chapter Ten: Thematic Mapping in ARC/INFO
Because of the great flexibility of ARC/INFO, a wide variety of maps can be generated within the system. This guide's focus has been on the design of environmental maps. These maps include maps of point data (Toxic Release Inventory sites, for example), linear data (STORET information), and area data (demographic/Census data). With the GRID and TIN modules, ARC/INFO has the ability to show area data with grid cell, dot density, isoline and fishnets, as well as the choropleth and graduated circles methods that are part of ARCPLOT.
Summary of Presented Methods
The methods presented in this guide have been based on the theoretical construct, `visual variables.' Use of this construct allows the specification of rules for matching data to symbolization types. Each of these rules is indicated in the subsection headings in Chapters Three through Nine. For point and line data, select the number of variables that need to be represented with one symbol, then match the appropriate data levels to the symbolization method by following the headings (Ratio Data--Size, etc.). For area data, the character of the data must be considered. Data that are continuous and changes abruptly at boundaries should be displayed with choropleth maps (for example, sales tax rates). Data that are continuous and smoothly changing should be displayed with isoline of fishnet maps (for example, elevation). Data that are discrete and smoothly changing should be displayed with dot density maps (for example, population density). Data that are discrete and abruptly changing should be displayed with graduated symbols (for example, TRI point source emissions).
Areas of Possible Continued Research
As a simple continuation of these AMLs, there are many possibilities of combinations of data types (particularly for area data) and visual variables that are not presented here. This expanded set of AMLs, could be developed on an as needed basis.
A potentially valuable addition to the ARC/INFO repertoire is the development of animation. Time can be used to show temporal changes in data, for touring a data set, or for display of some other variable. These abilities can be used to both impress (as in visual communication) and highlight otherwise obscure relations (as in visual thinking).
I would like to thank my colleagues at the National GIS Program: Tommy Dewald, Jeff Booth, Dave Wolf, and Ed Partington for seeing the need for an ARC/INFO cartographic guideline and reviewing the text. Dave Rejeski originally foresaw the need for a guideline, from which this has developed.
At the Pennsylvania State University, Deryck Holdsworth has provided valuable guidance, for the version of this guideline that is my master's thesis. Anthony Williams also provided helpful comments.