Yesterday I added a retrieval method to the AtlasDAO which goes from a LonLat and an atlas id to a matching tile. Wrapped it up into a test application.

Unfortunately, the coordinates are way off.

On staring at the tile extents file, it became apparent that it's not expressed in degrees after all. Bah! Tried to figure out the actual units used.

The longitude scale runs from -2003772600 to 2003772600.
Degree range is -180 to 180.
=> 1 degree of longitude = 11132070 units. Comparing coordinates from both systems in the gazetteer, this seems to be a constant factor.

Latitude is trickier. The conversion factor varies as latitude changes.

I think the map coordinates may be using the Mercator projection.

Have set up a latitude to mercator converter, now testing data.

T

- posted by Tim Bruce @ 11:13 AM