Wednesday, December 7, 2011

Choosing Colors for Your Maps

There are a number of factors we need to consider when choosing colors for the maps we create using QGIS or any other software.  This is especially true if we are creating maps on behalf of a local unit of government.  The information on the maps must be accessible to everybody, especially to the estimated 10% of the male population that have conditions that we collectively call "color blindness."

Check out the website http://www.vischeck.com/.  Among other things, this lets you upload an image (of your map, for example) to simulate what a color-blind person sees.  The results will surprise you!

Other color-related factors we need to consider include:

  • the number of colors to use (too many may overwhelm the senses)
  • using distinct colors
  • choosing colors so they look good when printed
  • choosing colors so information is not lost when photocopied




All these are classical conundrums (conundra?) that plague modern cartographers.  (See Designing Better Maps, A Guide for GIS Users by Cynthia Brewer, an awesome Penn State cartography professor.)




One tool you definitely need to consult while cranking out your GIS map is Color Brewer (http://colorbrewer2.org/) by Cynthia Brewer and her crew at Penn State.  It'll help you select color schemes that take into account as many of the above factors as you'd like.  (Note that as you take more of these factors into account, your choice of color schemes starts getting severely curtailed...)


Once you've chosen a color scheme from Color Brewer, you'd normally have to manually input the color scheme as a "color ramp" into QGIS to assign the colors to the various multi-featured polygons in your map.  But fear not:  Anita Graser, an Austrian QGIS contributor who blogs as "Underdark" in Free and Open Source GIS Ramblings, has published a very simple methodology to import into QGIS, wholesale, the color schemes that are recommended in Color Brewer.

In her blog entry, Graser presents an XML file (as a listing which you can copy into a text editor like Notepad and then save it as "colors.xml" or something like that) which you can import into QGIS to form many color ramps.  Just follow the instructions in the blog after you've saved the xml file.  She has done all the work of transcribing the colors in the various Color Brewer color schemes into hex codes that QGIS can understand, and has made them into continuous color ramps.  This means you can use a color scheme on a polygon that has numerous features without having to worry about among exactly how many features you have to split the colors.  Happy Map Coloring!

A final thought: Check out the QGIS blogs listed in the Resources > QGIS page of our Open Source Township GIS Page!  Many of these are chock full of hints, tips and tricks like those that Anita Graser has given us!

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