When people refer to "heat maps," they can be referring to spectrum-colored maps of values (e.g., maps that show red, yellow, green, etc., based on a range of values), or actual maps of the weather (such as on the back page of USA Today :).  In this post, we'll be examining both: heat maps of heat, so to speak.

In our last few articles, we've looked at some free desktop-based applications that nonprofit staff can use to create choropleths and cartograms, two specific types of maps.  These desktop tools, while they can be free and powerful, aren't always user-friendly and many nonprofits don't have the necessary underlying datafiles to use them properly.

In last week's article about cartograms, we discussed one type of thematic map (a map that displays a theme, such as poverty statistics or disease infection rates).  This week, we'll look at some free programs to make choropleths - thematic maps that preserve distance and shape, but use colors or patterns to differentiate between different regions with different values.

Most of the maps we use look familiar to us: for instance, most Americans can recognize the shape of the continental United States whether it's appearing on a weather map or in a newspaper article about the economy.  Sometimes, however, ordinary maps depicting geographic area aren't the most useful method of visualizing our information.

Neogeographer and community technologist Ben Sheldon posted a summary of his mapping presentation from the Technology 2.0 community event - it's a good overview of the topic and there are numerous great examples.

WaterGoodness is a new site allowing users to share and find water quality reports across the United States.  The site combines water quality and pollution data from a variety of government sources with visitor reports of their local water quality.