The most tedious part of writing your own map provider is calculating the relationship between the map projection and the tile coordinates. This calculator is designed to make the process less annoying. Each map provider (e.g. BlueMarbleMapProvider) knows its own map projection and transformation. To use this calculator you must know what projection your base map uses.
Pick three widely-spaced sample points on the map (a broad triangle covering a large portion of the map is best), for which you know the precise geographical location, and tile coordinate. Pierre Gorrisen has an excellent, simple tool available for clicking on a map and getting geographical coordinates. Enter the latitude, longitude, coordinate row, column, and zoom for each marker point below.
Clicking Calculate Transformation will generate the
ActionScript code required in your sub-class of
AbstractMapProvider.
Have a look at other map providers included with Modest Maps
for an example of how the Transformation and *Projection classes are used.