[Gdal-dev] NAD27 to WGS84

[Frank Warmerdam]

Dadi, Upendra wrote:
> My question is how does GDAL handle coordinate system transformation
> from NAD27 to WGS84? You can see that there is no towgs84 parameter in
> the projection above. I am guessing it handles NAD27 as a special case
> and uses a more complex algorithm for transformation. What happens if I
> add towgs84 parameter in the string? Will it overide the internal
> algorithm used? Hope I am clear. 
> Any insight on this is appreciated. 

Upendra,

You are correct, NAD27 is handled as a special case, and datum
shift grid files are applied.  I suspect supplying a towgs84
parameter would not be enough to stop this from happening, you would
need to completely obscure the fact that it is nad27 (but you should
still declare it as Clark 1866 ellipsoid)

Changing the GEOGCS portion to something like the following should
do the trick.

     GEOGCS["custom",
         DATUM["custom",
             SPHEROID["Clarke 1866",6378206.4,294.9786982139006,
                 AUTHORITY["EPSG","7008"]]],
         PRIMEM["Greenwich",0],
         UNIT["degree",0.0174532925199433]],
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