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.
Best Answer
Short Answer:
QGIS handles SRS much the same as ESRI. Dive into QGIS and you'll find all your favorite SRS waiting for you.
Long Answer: EPSG is the European Petroleum Survey Group. It provides 'official' spatial reference systems and spheroids and its codes are ubiquitous. ESRI takes most of its SRS from the EPSG dataset. So you are already using the EPSG SRS, except ESRI just doesn't tell you the catalog number in an obvious way. ESRI has a few 'interesting' SRS that are not available in the EPSG catalog (e.g. the Dymaxion and a cube projection) but they are not in common use.
QGIS uses the EPSG naming convention but provides common names too so you should not get lost if you are familiar with ESRI. Coming from ESRI, the EPSG codes can seem a bit alien at first, but you quickly get used to it as you realise that you've been using it all along.
Lastly we have the user-defined SRS group. These have no official status. It is possible to define your own projection system (as you can in ArcGIS) but the need to do so is rare as there are so many tried and tested SRS out there and correctly defining an SRS is (IMO) a non-trivial task if you want to have something reliable and then have to convince all your co-workers that it is both credible and necessary. Personally, I would be a bit suspicious of any of the User-defined SRS as I do not know their provenance or accuracy and would have to spend time researching their composition. I'd rather use an EPSG SRS that has good documentation and is well supported.
While we're on the subject, ESRI also borrows parts of GDAL so you are also using some of QGIS' raster handling capabilities (particularly for converting between ratser data types)... you may not be aware of that either, but it should give you a bit of additional comfort moving to QGIS.