Web Mapping Illustrated

Appendix A. A Brief Introduction to Map Projections

Map projections are a critical component of any mapping application, whether for a hardcopy printout or an interactive web map. If you are new to mapmaking, you may see projections as a confusing and intimidating topic. If so, keep in mind that even some of the more advanced mapmakers have just enough map projection knowledge to do what they need. The theory runs as deep as math itself. With a minimal amount of orientation, you can get started with map projections.

This guide will prepare you for using map projections with MapServer. For more information about MapServer, see the other chapters in this book. In this section, general concepts are discussed, and a few popular projections are introduced. The syntax for setting projections in a MapServer map file is also covered.

The Third Spheroid from the Sun

The Earth is round, or so they say. Video games, globes, and graphic art may depict the Earth as a perfect ball shape or sphere, but in reality the Earth is a bit squished. Therefore, we call the Earth a spheroid, rather than a sphere. It is sphere-like, but somewhat elliptical.

To take it one level further, we all know that the surface of the Earth isn't perfectly uniform. There are mountains and valleys, bumps and dips. Geoid is the term used for a more detailed model of the Earth's shape. At any point on the globe, the geoid may be higher or lower than the spheroid. Figure A-1 shows an example of the relationships among the sphere, ...