What is the difference between catalogs: EPSG, ESRI, and User-defined?

What is the difference between catalogs: EPSG, ESRI, and User-defined?

I'm trying to understand EPSG codes. I've been to the 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.

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.

Although QGIS and ESRI use the same srid numbers for popular CRS, there still is a difference:

GDAL and QGIS have bundled projection definitions with a single datum shift, whereas ESRI software offers all datum shifts that are defined in the EPSG database. This applies for older CRS based on the ellipsoids Bessel, Airy, Everest etc.

THE EPSG database includes all known projections that are used by surveyers. ESRI has defined its own projections for continentwide and worldwide maps. For example, you will not find a Mollweide or Eckert projection in the EPSG database.

Unfortunately, GDAL and QGIS do not reference those projections as ESRI, but as EPSG.

On the other hand, QGIS has incorporated CRS definitions form other sources, like IGNF, ZANGI and IAU2000. I'm not sure if ESRI (and GDAL) knows these srid's as well.

GIS databases like postgis and spatialite only care about srid numbers, regardless of the namespace. EPSG and ESRI don't overlap, but sr-org numbering might do.

Watch the video: Correctly Using Projections in ArcGIS Geographic vs. Projected Coordinate Systems