[Gdal-dev] gdal warp code silently skipping datum shift

[John Johnson]

Occasionally I run across a raster file whose projection uses a datum that
has more than one transform to WGS84. Britain's 'OSGB 1936' is a great
example. Because the EPSG database has multiple transforms, GDAL's
data/gcs.csv file doesn't get populated with the transform value - it is
ambiguous after all.

But here's the bad part. If I warp from from a projection using the 'OSGB
1936' datum to a projection using 'WGS84', no shift happens. But no error is
generated either. So I get an incomplete warp and the image ends up being
misalligned (normally by less that a few hundred meters). When I notice
this, I can go back and add a TOWGS84 clause to the DATUM section our my SRS
and redo the warp to get the correct alignment.

So here's my question. Is there any way that GDAL can detect that it
"should" be doing a datum shift, but is missing the necessary parameters? If
so it could fail to warp wather than silently producing a bad warp. Looking
at the data structres in data/gcs.csv I'm not sure there is currently enough
information to distinguish between a datum that doesn't need to be shifted
and one that needs to but is missing the necessary parameters.
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