The heatmap shows the places people like, based on the number of panoramio photos at each place in the world. The dark areas have few photos, the red areas have more and the yellow areas have a large number of photos geotagged.
The hottest places have markers linking photos, streetview, wikipedia, wikivoyage, foursquare and google plus articles about the site. The place names are selected by the wikipedia readership numbers and foursquare checkins. Area populations are based on the geonames database.
Street level heatmaps are available for top 15000 places in the world, sorted by the number of photos in an area of a size of a few square kilometers around the place center. The popularity ranking of places in high-res area maps is computed by combining place hotness with popularity rankings from wikipedia, foursquare and real-time google places selection.