Sunday, June 15, 2008

Stop Mapping the World

The Thematic Mapping Blog has been on a 3D chloropleth jag lately, exploring all the ways to display global variables on a map of the world. The technical achievements are all very nice, but the actual display of the useful numbers, is as usual totally screwed by the intractable fact that countries are radically different sizes!

The trouble with chloropleth maps, is that you're trying to display numerical data visually, but one of the most visually arresting features of your display is a variable that you do not care about, the size of the regions.

What do I get out of the global map display of world data, that I don't get out of a simple rank-ordered table? A and B are high, and A and B are on the same continent. That could be done by coloring the rank order table by continent. A and B are high, and A and B are adjacent. That's harder.

I've really become fond of the cartogram approach, as a partial solution to this problem, but it has it's own problems.

We're in for a whole slew of this kind of stuff, with the US election offing, since the Republicans uniformly win the large less dense counties, every chloropleth map visually overstates the results in one direction, no matter what the cartographers do. How do you show San Francisco County on a map of the continental USA? Does it get 1/2 a pixel even?


Rob said...

Hi Paul,

Reading your post an old college text sprung to mind, 'How to Lie with Maps' by Mark Monmonier. You identify with the 3D chloropleth that most visually arresting features of your display is a variable that you do not care about, but is this really true?

For example, the approach taken by Bjorn would surely be of interest and benefit to mapping of numerous health care stats by Uncle Sam, conveniently disguising their success when compared to neighbors such as Cuba? Is it not the ability to lie with a map the real benefit to such maps over using a table?


Robert said...

as my pappy used to tell me... "figures never lie, but liars sure can figure"

Paul Ramsey said...


The chloropleth is only effective for lying when the lying lines up with the geography (if I want to prove Texas has more of something than Rhode Island, for example). In many respects the 3D chloropleth is superior to the 2D chloropleth because a tall Rhode Island spike is harder to ignore than 2 pixels of deeply colored Rhode Island. However, I think chloropleths (or "map shaped charts", as I like to call them) are pretty flawed for any non-uniform geographic basis.


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