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Recently I encountered the following WKT CRS definition from an ArcMap project:

PROJCS["SI-D-48-Gauss_Kruger",GEOGCS["GCS_Bessel_1841",DATUM["D_Bessel_1841",SPHEROID["Bessel_1841",6377397.155,299.1528128]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Gauss_Kruger"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",-5000000.0],PARAMETER["Central_Meridian",15.0],PARAMETER["Scale_Factor",0.9999],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]

While this is identified in ArcMap as EPSG:3912, the definition varies from the standard definition of EPSG:3912. I can't find any reference to the "SI-D-48-Gauss_Kruger" projection, or the "D_Bessel_1841" datum online.

I'm seeking any background anyone may have on this particular projection (and datum), and why they have been used in the WKT definition of this CRS.

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    @mkennedy possibly you may be able to assist? – ndawson Jan 7 at 23:26
  • The "@mkennedy" here won't ping them - see meta.stackexchange.com/questions/43019/… - but I suspect they monitor the coordinate-system tag anyway. – PolyGeo Jan 7 at 23:57
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    I'm surprised that ArcMap recognized it as 3912 because that's not an Esri (nor EPSG) name! D48 is the geographic CRS name. SI is the 2-digit country code for Slovenia. CRS-EU uses SI_D48 / SI_TM so it's not that one either. – mkennedy Jan 8 at 22:45
  • @mkennedy so it's likely a result of some corruption? (Maybe a 3rd party utility created this invalid definition...) – ndawson Jan 9 at 0:26
  • Probably. WKT1 didn't have any requirements for the names so there's a lot of varieties out there. – mkennedy Jan 9 at 18:15
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As pointed by mkennedy, the Description of CRS - SI_D48 / SI_TM, from the Information and Service System for European Coordinate Reference Systems, expresses that SI is the 2-digit country code for Slovenia and D48 is the datum.
The description of the projected coordinate system defines also the parameters of the Transverse Mercator Projection conversion used (Gauß Krüger Projection as an alias).
The Helmert Transformation details from SI_D48 to ETRS89 geodetic datums are described in the Description of Transformation - SI_D48 to ETRS89.

As pointed by TomazicM, an informative description of D48 datum and D48/GK projected system can be found on this page of the Slovenian Prostor portal (translated here).

Established in 1948, it bears the designation D48, with the year referring to the calculation of the positions of astrogeodetic grid points.
[...]
Today, however, the D48 / GK coordinate system is still in transit, and will be replaced by the new national D96 / TM coordinate system.


In the PROJ (Rel. 6.2.1) database, "D_Bessel_1841" is registered as an ESRI alias name for EPSG:6004 - Not specified (based on Bessel 1841 ellipsoid) geodetic datum.


The same projected system can be found as ESRI:102060. In an OSGeo4W Shell:

C:\>projinfo ESRI:102060
PROJ.4 string:
+proj=tmerc +lat_0=0 +lon_0=15 +k=0.9999 +x_0=500000 +y_0=-5000000 +ellps=bessel +towgs84=426.62,142.62,460.09,4.98,4.49,-12.42,-17.1 +units=m +no_defs +type=crs

WKT2_2018 string:
PROJCRS["D48_Slovenia_TM",
    BASEGEOGCRS["GCS_D48",
        DATUM["D_D48",
            ELLIPSOID["Bessel 1841",6377397.155,299.1528128,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["ESRI",104131]],
    CONVERSION["Slovene National Grid",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",0,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",15,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.9999,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",500000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",-5000000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]],
    USAGE[
        SCOPE["unknown"],
        AREA["Slovenia"],
        BBOX[45.42,13.38,46.88,16.61]],
    ID["ESRI",102060]]

The same projected system, with the D48/GK alias name, can be found in the EPSG registry as EPSG:3912 - MGI 1901 / Slovene National Grid.

The transformation between D48 (as an alias) and ETRS89 datums, can be found in the EPSG registry as EPSG:3914 - MGI 1901 to ETRS89.

But in PROJ it seems necessary to choose the datum transformation, because the +towgs84 parameters of the PROJ.4 string for EPSG:3912 are not the same:

C:\>projinfo EPSG:3912
PROJ.4 string:
+proj=tmerc +lat_0=0 +lon_0=15 +k=0.9999 +x_0=500000 +y_0=-5000000 +ellps=bessel +towgs84=476.08,125.947,417.81,4.610862,2.388137,-11.942335,9.896638 +units=m +no_defs +type=crs

WKT2_2018 string:
PROJCRS["MGI 1901 / Slovene National Grid",
    BASEGEOGCRS["MGI 1901",
        DATUM["MGI 1901",
            ELLIPSOID["Bessel 1841",6377397.155,299.1528128,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",3906]],
    CONVERSION["Slovene National Grid",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",0,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",15,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.9999,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",500000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",-5000000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["easting (Y)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["northing (X)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]],
    USAGE[
        SCOPE["unknown"],
        AREA["Slovenia"],
        BBOX[45.42,13.38,46.88,16.61]],
    ID["EPSG",3912]]

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