• PROJECTED COORDINATE SYSTEMS •
There is lots of good information on the web, especially when the federal government is up and running.
- Map Projections is a web-based technical manual from the Michigan Department of Natural Resources. The section titled "What is a Map Projection?" is one of the clearest, most concise explanations available. Note their use of the word "curvaceous".
Why So Many Coordinate Systems?
Coordinate systems are designed by scientist-mathematicians called geodesists. Each system that is published has to meet user specifications with regard to distortion (there are four kinds - distance, direction, area and shape). But the larger the footprint that the system will be applied to, the more difficult it is to limit distortion. So geodesists choose from a variety of projection algorithms to get the best balance of distortion vs aereal extent for a given group of users. The distortion-extent tug-of-war is the main reason for the proliferation of coordinate systems.
Another factor is the continuous work being done on measuring the size and shape of the earth. When an earth model (datum) is published that is substantially more accurate than it's predecessors, coordinate systems based on the new earth model are rolled out.