A map of the west half of North America focusing on the United States of America. It is overlaid with a heat map where major cities and metro areas are marked with red and fade out to blue and green with further distance from the cities. There are also a number of green and bluish purple spots across the map. There is no key indicating what the shades mean.
Description 
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Category
Source 1 of 8
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Date
2014
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Date
2014
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Date
2014
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Date
2014
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Date
2014
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Date
2014
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Date
2014
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Date
2014
Collection
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