My main goal this block is to finish the GIS training that I’m doing during my shifts in the GIS lab. As I walked into the lab this afternoon, I was ready to crank out some of the modules. But alas, my plans were foiled by geographic and projected coordinate systems. It took me a while to even begin to understand it, so disclaimer: this post is going to be a basic one, because I’m still learning this (and rather slowly).
To put it in esri’s words (which are far more concise than mine), a geographic coordinate system “is a reference system for identifying locations and measuring features on the curved surface of the earth.” Locations are designated in degrees in terms of longitude and latitude, which are defined by angles between the earth’s center and a point on the earth’s surface (see esri’s diagram). This gets a bit more complicated when we have to account for the fact that the earth is not actually a perfect sphere, and its surface is not smooth, but rather undulating. Because of this, we have to use datums for different locations on the earth, and measure the coordinates from there.
http://training.esri.com/Courses/LearnArcGIS_10/index.cfm?c=283
The tricky part is when we start trying to project these locations onto a two-dimensional map. There’s a funny 1940s video that explains better than I could how difficult it is to convert locations on a globe to a map you can hang on the wall, but I’ve forgotten the name of it. So for now here is a funny clip from West Wing which addresses some of the implications of the map we currently use, and I’ll get back to you on the difficulties of an accurate flat map. Otherwise, you can learn more online at ArcGIS Help 10.1. If you scroll through the options on the left hand side, pages on geographic coordinate systems as well as projected coordinate systems are available.
