[Gdal-dev] Help handling image with gcp.

[Frank Warmerdam]

Stephane Routelous wrote:
>>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.
> 
> 
> Hi Frank,
> 
> is that different than using 
> void* transformer = GDALCreateGCPTransformer(...)
> followed by 
> GDALGCPTransform(transformer,...) for the 4 corner points and from those
> points computing the transformation with
> 	georef[0] = m_upperLeftX;
> 	georef[3] = m_upperLeftY;
> 	georef[2] = 0;
> 	georef[4] = 0;
> 	georef[1]= (m_lowerRightX - m_lowerLeftX)/((double)m_nSizeX);
> 	georef[5]= -(m_upperRightY - m_lowerRightY)/((double)m_nSizeY);
> ?
> In this case, there is no approximation ?
> Am I wrong ?

Stephane,

The "gcp transformer" just builds an approximate 1st order polynomial
internally so the approximation is just moved somewhere else.

Furthermore, the geotransform you show computing is "north up" while
my proposed solution can also account for shear and rotation.

Best regards,

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