I'm making two maps of world population density. The two are using different projections, so each country's population density is a bit different in the two maps.

Which classification method should I use for these choropleth maps since my aim is to be able to compare them?

I would instinctively choose 'natural breaks' if I weren't aiming to compare the two, but I've read that using natural breaks makes it hard to compare multiple maps.


First you need to calculate the average difference between the population density of the same areas in the two maps.

Once you have the average difference, you can use it when you create your breaks.

To start, create natural breaks on one of the maps. Note the break values that are created. Then create the breaks for your other map manually, setting each break to the break value of the first minus the average difference (or plus depending on which way the average goes for the second map). The maps will then be visually comparable.

  • Is there a way to calculate the average difference between the population density of the same areas in the two maps through arcmap? I'm not sure how to add both fields in the same attribute table and use a field calculator to calculate a third field of the difference since they're of different projections and thus in different data frames. Thank you!! – J. Doe Nov 11 '16 at 3:40
  • You can add both layers to the same data frame. ArcGIS uses on-the-fly transformations to display data in different coordinate systems. You may be better off selecting the areas that are the same and copying the values into excel to do your calculations. I'm not sure how you would join the layers unless there is a common field between the two. IF you can join them, do that and use field calculator to calculate the average. – jbalk Nov 14 '16 at 3:53

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