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

[Frank Warmerdam]

John Johnson wrote:
> 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.

John,

Any datum completely missing a TOWGS84 clause, and not the specially
handled NAD27 datums using grid shift files would be considered lacking
datum shift information and could be flagged accordingly.   However,
I would be very hesitant to have gdalwarp just fail for these cases.
It might be better to have it issue a warning in these cases.

Internally what I imagine I would do is:
  1) Are the input and output the same datum?  (based on datum EPSG code,
     or lacking that the datum name).  If so, assume ok.

  2) Translate both coordinate systems to PROJ.4  Do both have datum
     information (ie. +towgs84 or +nadgrids= clauses)  If so, assume
     ok.

  3) A change of datum seems to be happening, but there is no apparent
     datum shift info, issue a warning.

Potentially I could accomplish this in the OGRCreateCoordinateTransformation()
function.  Possibly adding a flag indicating if missing datum shifts should
be:
  - handled as currently (quietly convert just based on ellipsoid)
  - issue a warning.
  - terminate with a failure.

Ideally I would also like to come up with a way of identifying
transformations where multiple datum shift strategies are avalable
and selecting one.  But that would require more substantial efforts.

If you would like to file a bug report on this issue, I can try and queue
up a work item.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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