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Mapping with ArcGIS Pro by Ryan Malhoski, Amy Rock

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Value-by-alpha

Value-by-alpha is a technique developed by cartographers Robert Roth, Andy Woodruff, and Zach Johnson, to enable the inclusion of a second variable relating to size. Traditionally, this has been done with cartograms. This technique involves distorting the size of an area, such as states or counties, to represent their share of some variable, such as population. The value-by-alpha technique is generally easier for map readers to interpret, as it preserves the expected size and shape of the area, and instead expands upon the darker equals more concept that map readers already interpret intuitively. Let's go back to that diverging scheme and make some adjustments. Instead of transitioning through white, we want that center value ...

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