Mapping Hacks

Plot Statistics Against Shapes #44

Chapter 4, Mapping (on) the Web

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HACK

(e.g., “Number of Voters”). Look carefully at choropleth maps you see in

the press; what they represent can easily be illusory! Not all data should be

uploaded to Indymapper; if you can normalize it, for example, by turning

“Polling place incidents” into “Polling place incidents per 1,000 registered

voters,” then do so. Absolute values are best represented on a propor-

tional symbol map, showing an icon shrink or grow according to the

mapped value.

The rest of this hack should give you a clearer idea of what

SVG::Shapefile

and Indymapper can do, and illustrate how to “roll your own” map with

code available from the Perl CPAN repository.

Rolling Your Own

Installing modules from CPAN is easy. As your system’s root user, type:

#> perl -MCPAN -e shell

cpan> install SVG::Shapefile

cpan> install DBD::Excel

For more in-depth instructions on installing modules from CPAN, see “Set

Up an OpenGuide for Your Hometown”

[Hack #97].

You can use

SVG::Shapefile to plot a range of colors: one representing a low

value, the other a high value, with a blend to and from white between them.

Figure 4-15. Plotting stats against shapes on the Web