How to make a heat map overlay (like Crime in Trulia) for Google Maps
This is an experimental service from Google, with which you can upload your own data to spreadsheets, CSV or KML format in Google, and then visualize the data using the Sheets API on a map.
There's a blog post on how they did it: http://www.glenrobertson.co.nz/projects/2013/02/11/trulia-crime-map.html
It looks like they created their custom map overlays using US Census data.
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