i wanted to know what % of my city was road/building/etc, so i made an app for it
predownloaded geojson datasets are in geo/. use it by putting the name in a browser hash, like p27.ca/mapsort#sanantonio. datasets can be downloaded from overpass-turbo.eu.
once a dataset is downloaded, pan the map so the whole thing is visible, and click dump. a bunch of images will be created at the bottom of the page.
at this point, the pixels in these images would be counted for a % of total image reading.