Transformations of cartographic and geographic coordinates in
Portugal using NTv2 datum shift grids
(Portuguese)

 

Portugal has adopted a new map projection, based on the ETRS89 datum, as a national coordinate system for topographic mapping purposes. This new map projection is designated as PT-TM06 (EPSG:3763). The Portuguese Geographic Institute (IGP) has observed with GPS more than 1000 points of the national geodetic network and provides their coordinates.

 

Older reference systems are still in use, in particular Datum 73 (epsg:4207) and Datum Lisboa (epsg:4274), both based on the Hayford ellipsoid. Usual projections of these datums are identified with epsg codes epsg:27493 (D73) and epsg:20790 (DLisboa). Coordinates of more than 8000 points (1st, 2nd and 3rd order points of the geodetic network are provided by IGP) as well as Bursa-Wolf parameters to convert from these to ETRS89. Coordinates in these projections can be transformed to ETRS89 using PROJ.4 command cs2cs :

 

cs2cs +init=epsg:27493 +towgs84=-230.994,102.591,25.199,0.633,-0.239,0.900,1.950

      +to

      +proj=longlat +ellps=GRS80 +towgs84=0,0,0

 

cs2cs +init=epsg:20790 +towgs84=-283.088,-70.693,117.445,-1.157,0.059,-0.652,-4.058

      +to

      +proj=longlat +ellps=GRS80 +towgs84=0,0,0

 

The RMSE of coordinates transformed using these parameters are of approximately 0.40 m (for D73) and 1.50 m (for DLisboa).

 

Using around 900 of the points provided by IGP, datum shift grids were calculated, using standard kriging interpolation, for the differences: Datum 73 to ETRS89 (pt73_e89.gsb) and Datum Lisboa to ETRS89 (ptLX_e89.gsb). These files are in format NTv2 and can be placed in the the directory pointed to by environment variable PROJ_LIB (for example, "c:\Program Files\proj\proj_lib"). Using cs2cs command with the grids:

 

cs2cs +init=epsg:27493 +nadgrids=pt73_e89.gsb

      +to

      +proj=longlat +ellps=GRS80 +towgs84=0,0,0

 

cs2cs +init=epsg:20790 +nadgrids=ptLX_e89.gsb

      +to

      +proj=longlat +ellps=GRS80 +towgs84=0,0,0

 

Tests made with a set of around 140 points not involved in the generation of the grids provided RMS errors of 5 cm in the case of Datum 73 and 8 cm for Datum Lisboa.

The grid files can be also easily configured in ArcGIS (as a Custom Geographic Transformation) and other commercial and open-source GIS programs.

Publication (portuguese only)

Gonçalves, J., 2009. Conversões de Sistemas de Coordenadas Nacionais para ETRS89 Utilizando Grelhas. Paper presented at the 6th National Conference of Geodesy and Cartography. (paper)

 

Program to create NTv2 files (GSB extension)

 

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