I'm trying to understand EPSG codes. I've been to the spatialreference.org site, and in its "about" link, it states:
"There are currently three distinct catalogs of spatial reference systems on this website -- EPSG, ESRI, and User-defined (called sr-org)."
What is the difference between these catalogs? Maybe a better question is why are there separate catalogs?
I'm moving from the ESRI desktop world to QGIS, and I want to make sure that I have a firm understanding of how QGIS handles coordinate systems.