What's the difference between Geographic coordinate system 4019 and 4121?

Question

I have a DEM with cell size 100,100 with geographic coordinate system GRS_1980_Transverse_Mercator, Datum: D_GRS_1980. I created some extra shapefiles with projected coordinate system: Greek_grid, projection: Transverse_Mercator and geographic coordinate system: GCS_GGRS_1987 and datum:D_GGRS_1987. Adding them in ArcMap I have the message that my layers use different spatial reference which is reasonable because the shapefile datum was different from the DEM I am using. I checked online and found in http://edndoc.esri.com/arcims/9.2/elements/gcs.htm the geographic coordinate systems I am using. They are the 4121 and 4019. They look to have the same characteristics. Does anyone know why they exist with different names? Or am I missing something?

4121    GCS_GGRS_1987
GEOGCS["GCS_GGRS_1987",DATUM["D_GGRS_1987",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]

4019    GCS_GRS_1980
GEOGCS["GCS_GRS_1980",DATUM["D_GRS_1980",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]

Thanks in advance, chch

Answer 1

http://spatialreference.org/ref/epsg/4019/html/

http://spatialreference.org/ref/epsg/4121/html/

If you look at these well known text definitions, you'll see that the spheroid, prime meridian, and unit (degree) are identical. The only difference is that 4019 seems less sure about its datum.

http://spatialreference.org/ref/epsg/4019/proj4/

http://spatialreference.org/ref/epsg/4121/proj4/

In the proj4 definitions, which probably won't be available to you in ArcMap, 4121 includes some values that will help when translating to WGS84. I'd say that 4121 is ultimately more correct. However, you could probably use either without any real problems.