Le samedi 20 septembre 2014 18:56:11, Jesse McGraw a écrit : > Is it possible to force gdal_translate to only rotate/scale an image > when given a list of more than two GCPs? > > I'm manually georeferencing maps which are all North-up and I'd like > to supply as many GCPs as possible to average out my errors but if I > provide more than two it will potentially skew the image.
Jesse, GDALGCPsToGeoTransform() would indeed return an affine transform matrix, computated with least-squares, but with potential shearing. If you want only rotation and scaling (I assume uniform scaling), that would mean to solve Xi = X0 + S * cos(theta) * xi - S * sin(theta) * yi Yi = Y0 + S * sin(theta) * xi + S * cos(theta) * yi where the unknown variables are X0, Y0, S and theta. Unfortunately, I think this would be difficult to analytically solve it because of the non-linear terms in cos(theta), sin(theta). Perhaps a pragmatic way would be to use GDALGCPsToGeoTransform() that would return (X0, A, B, Y0, C, D) such that Xi = X0 + A * xi + B * yi Yi = Y0 + C * xi + D * yi And then you can try the following approximation : S ~= sqrt(A* D - B * C) theta ~= 0.5 (atan2(-B,A) + atan2(C,D)) (that would be exact if the transform is only scaling and rotation) You can then recompute X0 and Y0 as the average of Xi - ( S * cos(theta) * xi - S * sin(theta) * yi ) and Yi - ( S * sin(theta) * xi + S * cos(theta) * yi ) This will be not the mathematical optimal solution, but hopefully it would not be very far from the optimal provided that the amount of shearing is low. If you want something more exact, you can use this solution as the initial solution for an interative method as described in http://en.wikipedia.org/wiki/Non-linear_least_squares Note: I didn't try, so there might be errors in the formulas. And they are based with indexing pixels from the bottom left corner of the image, so some sign tweaking will be necessary. Even > > Thanks! > -Jesse > _______________________________________________ > gdal-dev mailing list > gdal-dev@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/gdal-dev -- Spatialys - Geospatial professional services http://www.spatialys.com _______________________________________________ gdal-dev mailing list gdal-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/gdal-dev
