[Gdal-dev] Help handling image with gcp.

[Frank Warmerdam]

Ken Sewell wrote:
> I just started receiving some NITF files that only have GCP's.  I need the
> ability to query the pixel's lat/lon.  Can I do this with GCP's or is
> there a utility in GDAL that will georegister the image from the GCP?
> This what gdalinfo shows:
> 
> Driver: NITF/National Imagery Transmission Format
> Size is 640, 473
> Coordinate System is `'
> GCP Projection = GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS
> 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","
> 6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9108"]],AXIS["Lat",NORTH],AXIS["Long",EAST],AUTHORI
> TY["EPSG","4326"]]
> GCP[  0]: Id=UpperLeft, Info=
>           (0,0) -> (-116.633,35.2542,0)
> GCP[  1]: Id=UpperRight, Info=
>           (640,0) -> (-116.66,35.2758,0)
> GCP[  2]: Id=LowerLeft, Info=
>           (0,473) -> (-116.608,35.2736,0)
> GCP[  3]: Id=LowerRight, Info=
>           (640,473) -> (-116.624,35.2889,0)

Ken,

You could run the image through "gdalwarp" which would rectify it giving
you an image with a nice geotransform.  It would build a 1st order transformation
to accomplish the image warp.

Alternatively you could build an approximate geotransform (the same as it
would use for a warp) from the GCPs.  This can be accomplished with the
function GDALGCPsToGeoTransform().  Note that you will need to pass bApproxOK
as TRUE.

Best regards,
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