Pear Map Projection Classes
Pear Map Projection Classes is a collection of classes used to project GPS data points onto maps.
When plotting large areas of a sphere's surface on a two dimensional map, a linear projection from latitude/longitude to x/y is often undesirable, due to to distortions that appear around the edges. Many projections exist that deal with these distortions: Mercator's projection was designed to aid navigation, while equal-area projections are intended to preserve a constant scaling factor from the sphere to the map.
A list of projections and their properties can be found at http://mathworld.wolfram.com/topics/MapProjections.html.
That package can be used to convert GIS data to a few popular projections used in atlases and by map image providers, such as http://nationalatlas.gov or http://www.mapresources.com. If the specific parameters used to create a map are known, it is possible to use this package to plot geographical points to that map, or determine geographical position from map position.
The package also has built-in methods for performing 2D transformations on the projected data - the raw output of a projection almost always must be scaled, rotated and translated to be useful. Map_Projection accomplishes this by using the package Math_Matrix.