2.1. Lat49 Ad Geography

This section provides a brief description of zoom levels and the geographic bounds of ads.

In the Lat49 scheme, each ad covers a certain geographic area. The world is parcelled into these areas in the same way that Google Maps version 2 and Live Search Maps (Virtual Earth) divide their maps into image tiles. At the lowest zoom level of 0, most of the Mercator projection view of the world (between +/-180 degrees of longitude and +/-85.051 degrees of latitude) is fit into a single square tile. At the next zoom level of 1, the view is subdivided into a 2x2 array of tiles. Each subsequent zoom level is further subdivided by a factor of two. Thus, at zoom level zl, the Mercator world view is subdivided into an NxN array of square tiles, where N = 2^zl (i.e. 2 to the power zl).

Currently, ads are only being sold for four zoom levels: 7 (Regional), 10 (Urban), 13 (Neighborhood) and 16 (Local). Ads for other zoom levels that are requested based on geographic coordinates are automatically selected from one of the four offered levels, if appropriate. For example, a request at zoom level 8 or 9 will automatically return an ad from zoom level 10 covering the same latitude and longitude. Requests for ads at zoom level 17 or above will get ones from level 16. No ads are returned for zoom levels 4 and below because the ad areas from the Regional level would be too small and closely packed at those scales.