[gdal-dev] gdalwarp cutting off projected image

Frank Warmerdam warmerdam at pobox.com
Tue Dec 9 21:41:25 EST 2008

Scott Lewis wrote:
> Frank,
> The image I've been trying to convert is at this URL.  It's about 6MB, 
> hopefully that's not too big.
> ftp://sidads.colorado.edu/pub/incoming/glewis/polar.gtiff.tif
> Basically, I'm trying to convert it to a plain-old EPGS:4326 (latlong 
> projection).
> I appreciate you taking a look.


I have established the problem is the logic that computes the desired
extent of the output file.  It walks the edges of the input file,
transforming to WGS84 and ends up with incomplete extents because there
is no recognition that there is a topological discontinuity in the
transformation (the pole issue).

Normally my advice would be to use the SAMPLE_GRID=YES warp option to
force internal sampling of the region when computing the bounds, but
I have discovered that only applies to deciding what source region
to read during warping - not to the computation of the original warp

So, in the meantime you can just force selection of output extents.
Something like:

gdalwarp -t_srs EPSG:4326 polar.gtiff.tif out.tif -te -180 28 180 90

When I have the ticket in hand, I'll plan to modify the
GDALSuggestedWarpOutput() function to allow forcing of internal gridding.
It would also be nice if I could automatically recognise this sort of
case to trigger it automatically, but I'm not sure how to do that
without quite a bit of extra work.

> PS - I'm still new to the GDAL list; when you say file a Trac ticket, 
> where would I do this?



There should be brief instructions on getting an OSGeo Userid which you
can use to login to the site, and then hit "New Ticket" to file a bug
report.  If you find this path isn't deducable from the main GDAL web
page, please email me suggestions on how to improve things.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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