Web Mapping Illustrated

Using Map Projections with MapServer

The projection settings in a MapServer map file are at two levels: the output map projection and the projection of the input layers of features to be drawn. Example A-1 shows a sample of the projection settings for an output map.

Example A-1. Example settings for the map to be rendered using an Albers Equal Area projection

PROJECTION
  "proj=aea"
  "ellps=WGS84"
  "lat_0=10"
  "lon_0=-90"
END

The settings may look confusing to anyone unfamiliar with projections, but they have a straightforward purpose. If you can get these four settings clear in your mind, you'll be able to handle most projections with ease.

PROJ.4: Projection Library and Utilities

The terms used in Example A-1, proj, ellps, lat_0, lon_0, are all keywords for the projection library used behind MapServer, called PROJ.4. There are other options (e.g., datum, units, etc.), but these are the most common. PROJ.4 isn't just a set of libraries that MapServer uses. It also comes with some command-line utilities. One of these is a program called proj, which lists the available projections, ellipsoids, etc.

Running the command:

    > proj -l

returns a list of the available map projections, including the abbreviation used in the proj= setting for MapServer.

This utility can also project coordinates interactively by keyboard or from a text file; it's perfect for bulk projection of coordinates.

For more information on PROJ.4 see http://proj.maptools.org/.

The first line in Example A-1, "proj=aea", is the most ...