Datum Transformations

A datum transformation is the process of bringing coordinate values referenced in one defined datum into another datum system.

To perform an exact datum transformation the point's position must be recalculated using the original measurement observations that produced the latitude longitude position.

NTv2 (National Transformation Version 2)

NTv2 (National Transformation version 2) is a grid of accurate datum shifts in a specific format originally devised by the Geodetic Survey of Canada and adopted for use by Australia, France and Germany, etc.

Datum grids are stored in GSB "grid shift" files in the Cadcorp SIS 9.0\NTv2 sub-folder and are described in a JSON file (NTv2.json) that contains one or more Geodetic Datum pairs, referenced by EPSG number, for use with the grids.

SIS Desktop 9 provides GSB files for Canada (CAN folder), Australia (AUS folder, France (FRA folder) and Great Britain (GBR folder). Users can add additional grid shift files as required and modify the NTv2.json file with the relevant Geodetic Datum pairs and name and location of the .gsb file.

Since NTv2 transformations are performed for a specific region, only those Geodetic Datums, the EPSG numbers of which exactly match the Geodetic Datum names specified, will be transformed using NTv2. Other datums will be transformed using the general purpose routines within SIS Desktop 9.

When the transformation starts, Cadcorp SIS Desktop will locate the .gsb file and accompanying .json file and automatically enable the conversion from one Geodetic Datum to a second datum, using the grid file (.gsb) specified in the NTv2.json file.

The source datum codes in the supplied NTv2.json file are:

• 6267 for North American Datum 1927 (NAD27)
• 6202 for Australian Geodetic Datum 1966 (AGD66)
• 6203 for Australian Geodetic Datum 1984 (AGD84)
• 6275 for Nouvelle Triangulation Francaise Greenwich (NTF)
• 6807 for Nouvelle Triangulation Francaise Paris (NTF)
• 6277 OSGB 1936

If you open the NTv2.json file you will see these Source Datum codes, along with the target datum codes and names of the associated GSB files:

{
   "NTv2": {
       "Enabled": true,
       "NTv2Files": [
           {
               "Description": "From North American Datum 1927 (NAD27) to North American Datum 1983 (NAD83)",
               "Enabled": true,
               "Datums": {
                   "Source": 6267,
                   "Target": 6269
               },
               "FileName": "CAN\\NTv2_0.gsb"
           },
           {
               "Description": "From Australian Geodetic Datum 1966 (AGD66) 
		to Geocentric Datum of Australia 1994 (GDA 94)",
               "Enabled": true,
               "Datums": {
                   "Source": 6202,
                   "Target": 6283
               },
               "FileName": "AUS\\A66 National (13.09.01).gsb"
           },
           {
               "Description": "From Australian Geodetic Datum 1984 (AGD84) 
		to Geocentric Datum of Australia 1994 (GDA94)",
               "Enabled": true,
               "Datums": {
                   "Source": 6203,
                   "Target": 6283
               },
               "FileName": "AUS\\National 84 (02.07.01).gsb"
           },
           {
               "Description": "From Nouvelle Triangulation Francaise Greenwich (NTF) 
		to Reseau Geodesique Francais 1993 (RGF93)",
               "Enabled": true,
               "Datums": {
                   "Source": 6275,
                   "Target": 6171
               },
               "FileName": "FRA\\ntf_r93.gsb"
           },
           {
               "Description": "From Nouvelle Triangulation Francaise Paris (NTF) 
		to Reseau Geodesique Francais 1993 (RGF93)",
               "Enabled": true,
               "Datums": {
                   "Source": 6807,
                   "Target": 6171
               },
               "FileName": "FRA\\ntf_r93.gsb"
           },
           {
               "Description": "From OSGB 1936 to WGS 1984 (actually ETRS89)",
               "Enabled": true,
               "Datums": {
                   "Source": 6277,
                   "Target": 6326
               },
               "FileName": "GBR\\OSTN02_NTv2.gsb"
           },
           {
               "Description": "From OSGB 1936 to ETRS89 (see 6277 to 6326)",
               "Enabled": true,
               "Datums": {
                   "Source": 6277,
                   "Target": 6258
               },
               "FileName": "GBR\\OSTN02_NTv2.gsb"
           }
        ]    }
}

To add more grid shift files, modify the NTv2.json file with the relevant Geodetic Datum "Source" and "Target" and add the "FileName" and location of the .gsb file.

Entering the source datum codes

Select the source datum code is selected from the drop-down list from the top entry box in the Geodetic Datum tab of the Coordinate Reference System dialog.

Source datum codes cannot be entered directly and must be selected from the drop-down list:

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