The Pros & Cons of Choropleth maps (Blot maps / Patch maps)

The choropleth map is one of the most frequently used maps in geography. It uses a coloring scheme (different colors or a graduated color scale) inside defined areas on a map in order to show value levels and indicate the average values of some property or quantity in those areas.

Pros:
– Can effectively be used to report area values at virtually any scale, from global to local – and the data can be thought about in many different ways at many different levels of analysis, from general overall patterns to the detection of details.
– Helpful for finding intriguing hot spots, detecting relationships between the encoded variable and geographic location (and the many variables entangled with location), or letting people know how their area compares with others.

Cons:
– Since the choropleth map uses an average number to represent defined areas, the viewer can not gain detailed information or perspective on any area’s internal conditions. This can be solved by making the map interactive (like “The Geography of Government Benefits” from The New York Times)
– The areas are not uniform: equating the visual importance of each county with its geographic area rather than with the number of people living in there, giving sparsely populated areas great visual emphasis. This can be solved by using the method of mesh/grid-square mapping (dividing the map into equal sized units/squares and then color each one according to the data being encoded). An example of a mesh map (the grid squares are 1km on a side) :

Remember to be careful and find out if the choropleth map is telling you the truth. You will need a lot better data than county-by-county aggregate numbers to learn what is really going on.

A nice way to do a choropleth map: “Measuring the U.S. Melting Pot” by David Yanofsky from Bloomberg News:
Being able to select two heritages and see how they compare with each other, instead of just selecting one heritage, gives you a relative scale and the map becomes a whole lot more useful (of course I had to compare the Norwegains with the Swedes):

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