Made by John Nelson of IDV Solutions.
“Of course, as is the case for any observation data, there is a strong tendency towards echoing a population map. This is certainly the case with this sighting data, as well. In order to visualize the actual sighting phenomenon, I needed to normalize by the underlying population. The first, more prominent map shows a simple ratio of the sightings by population. A per-capita approach. The second, smaller and slightly more complex map, shows a bi-variate mapping of sightings in the color dimension (dark slate for low-sightings and bright green for high-sightings) and population density in the opacity dimension (denser populations are more transparent). The result is a map that is more nuanced regarding the problem of variable populations and area sizes.”
Please check the link for the expandable version and more information.