[gdal-dev] Re: Status of RFC 4: Geolocation Arrays especially Envisat wrt GCPs

Frank Warmerdam warmerdam at pobox.com
Mon Sep 22 18:46:41 EDT 2008

Brent Fraser wrote:
> Frank,
> The short story:
>  I've got some Single Look Complex EnviSat data I need to process.  Will 
> gdalwarp do a piecewise (triangulated) warp using GCPs or only(!) use 
> them to calculate an image-wide polynomial?


There are two choices from GCPs. Either polynomial (1st to 3rd order)
or thin plate splines.  Thin Plate Splines are nice in that the
transformation is exact at all GCPs, and it will give similar, but
more continuous results to piecewise triangulated warps which I
don't support.  But TPS is likely to be slow on a large dataset with
a lot of GCPs.

> The longer story:
>     ESA's Best (http://earth.esa.int/services/best/) does an OK job 
> extracting the amplitude from the data, but because it uses tiff files 
> for intermediate storage, and their implementation is limited to 2gb 
> tiffs, it's necessary to tile an input image.  Best assigns a different 
> affine transformation to each output GeoTiff tile, which causes a 
> mis-alignment of the tiles when re-assembling them (using gdalwarp 
> etc).  Best does output an extensive metadata text file with each tile 
> including a geolocation "array" of latitude and longitude.  
> Unfortunately the "array" is not regular (eg, my test tile has values 
> for lines 1, 811, 812, 1622, 1623, 2240; every 165 pixels except the 
> last interval which is 195 pixels).  If gdalwarp would use this 
> semi-regular array to triangulate the pixel locations, that would be 
> great, otherwise I'll have to use something else...

There is no mechanism to use geolocation arrays that are irregular in
this way, and even for regular geolocation arrays the existing code is
a bit questionable.

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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