[gdal-dev] Interesting issue with ogr2ogr and -tps

[Even Rouault]

Martin,

Patch applied. Thanks !

Even

> I had a similar problem when not using armadillo, but a simple
> transformation does the trick. The problem is that usually the distance of
> different gcps compared their absolute value is very small.
> 
> This leads to a bad conditioned linear equation system (see
> https://en.wikipedia.org/wiki/Condition_number): The first column consists
> of 1s and the second and third of the x, y values of the gcps respectively.
> Now if the difference of the gcps is relatively small the second and third
> row are nearly colinear to the first one, hence the matrix is nearly
> singular.
> 
> The solution is to shift the coordinate system by the mean value of all gcps
> and solving the modified linear equations. Since the basis function of the
> TPS only depends on the difference of coordinates the resulting
> transformation is the same.
> 
> For the implementation details see the attached patch file.
> 
> Martin


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