GeoRSS Weblog

For those who know what a coordinate reference system (CRS) is, move along to the next paragraph. But for the curious, here’s a quick lesson in coordinate reference systems. The one everyone knows is that used by Global Positioning System satellites, which uses latitude and longitude coordinates expressed in decimal degrees and tied to the reference system and model of the shape of the earth known as WGS84. This is the default CRS for GeoRSS. It works OK for a global standard, but the accuracy is only plus or minus a meter, which just doesn’t cut it when you’re building a bridge or surveying property lines. So engineers often use CRSes that are accurate to inches or better, using models that maximize accuracy for one small part of the world, but are worse than even WGS84 when you move to other areas.

GeoRSS GML has always supported CRSes other than WGS84 lat/lon (which is the default for both GeoRSS Simple and GML), but until now we’ve been extremely lazy and haven’t had any examples of how to specify a CRS. Guilt has overcome laziness finally and I’ve gotten one example posted here (ignore the fact that the coordinates don’t make sense for CRS urn:ogc:def:crs:EPSG:6.6:26986, which by the way is Massachusetts Mainland Stateplane meters–maybe I’ll fix that by 2008).