[gdal-dev] How to set GDAL/cs2cs/proj to use NTv2 grid shift file when using EPSG codes?

[Thomas Campagne]

Hi,

I am using the GDAL API for Python for a script that takes only EPSG codes for inputs to specify source and target coordinate systems.
While the cs2cs command seems to handle quite well transformation between NAD27 (EPSG:4267) and NAD83 (EPSG:4326) coordinates in North America, I would like to have similar capabilities for AGD66 (EPSG:4202) to GDA94 (EPSG:4283) and AGD84 (EPSG:4203) to GDA94 (EPSG:4283) in Australia.
When using the -v option with cs2cs to look at the proj string being used i get the following:
    >cs2cs -v +init=epsg:4267
    ># ---- From Coordinate System ----
    >#Lat/long (Geodetic alias)
    >#
    ># +init=epsg:4267 +proj=longlat +datum=NAD27 +no_defs +ellps=clrk66
    ># +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat
    ># ---- To Coordinate System ----
    >#Lat/long (Geodetic alias)
    >#
    ># +proj=latlong +datum=NAD27 +ellps=clrk66
    ># +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat
The nadgrids parameters tells cs2cs to use the best fitting datum grid shift file from conus, alaska, ntv2_0.gsb, and ntv1_can.dat.
For AGD66 (EPSG:4202) cs2cs returns:
    >cs2cs -v +init=epsg:4202
    ># ---- From Coordinate System ----
    >#Lat/long (Geodetic alias)
    >#
    ># +init=epsg:4202 +proj=longlat +ellps=aust_SA
    ># +towgs84=-117.808,-51.536,137.784,0.303,0.446,0.234,-0.29 +no_defs
    ># ---- To Coordinate System ----
    >#Lat/long (Geodetic alias)
    >#
    ># +proj=latlong +ellps=aust_SA
    ># +towgs84=-117.808,-51.536,137.784,0.303,0.446,0.234,-0.29
There is no nadgrids parameter but a towgs84 instead. A grid shift file from the ICSM; "A66 National (13.09.01).gsb" contains the shift between AGD66 and GDA94.
How can I make GDAL/cs2cs/proj identify this file when using EPSG codes?
I tried to reinstall GDAL/cs2cs/proj using the osgeo4w installer with A66 National (13.09.01).gsb within the directory containing the other datum shift files prior to the reinstallation but without success.

I also posted an enhancement ticket on Trac:
http://trac.osgeo.org/gdal/ticket/5085

Cheers

Thomas

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Thomas Campagne, M.Sc. Geophysics, Consultant
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