I have been trying to add the ESPG:7019 to my postGIS DB using INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext). I am trying to find the right definition to use inside this function. I tried espg.io and spatialreference.org and but can't find the definition. Some answers suggest that it's because GRS 1980 (EPSG:7019) is just an ellipsoid and not an real CRS. But how can we have an ESPG code for an ellipsoid? Also,select distinct ST_SRID(geom) from mytable returns 7019 in PostGIS -so it has to be one of the recognised CRS. So where can I find the right PostGIS deifnition?
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2All objects in the EPSG geodetic database have an ID: units, ellipsoids, coordinate operation methods, etc.– mkennedyCommented Feb 25, 2022 at 3:41
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1 Answer
GRS 1980 is defined by two measurements:
Semi-major axis = 6378137
Inverse flattening = 298.257222101
With this, you've defined an ellipsoid, but you do not have a coordinate system. Where is Europe on this ellipsoid? North America? That is why you need a datum. The datum aligns the actual Earth onto this mathematical construct.
Once a datum is in place with the ellipsoid, you can use degrees or other angular measurements to pinpoint locations that are along the surface (given a Prime Meridian) and that is the basis for a Geographic Coordinate Reference System. (GeoCRS or GCRS)
Bottom line is that you can't use just an ellipsoid since they are abstract concepts and (in of itself) not tied to the Earth.
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If you could also add why GRS 1980 still has
espgcode assigned to it (Refer mkennedy's comment) then this becomes a complete answer to my question for me to accept.– rm167Commented Feb 28, 2022 at 17:15