#163: g.transform no longer calculating error for 2nd order transformation -----------------------+---------------------------------------------------- Reporter: cmbarton | Owner: [email protected] Type: defect | Status: reopened Priority: major | Milestone: 6.4.0 Component: default | Version: svn-develbranch6 Resolution: | Keywords: g.transform georectify Platform: All | Cpu: Unspecified -----------------------+---------------------------------------------------- Comment (by msieczka):
Replying to [comment:5 hamish]: > Replying to [comment:4 cmbarton]: > > Well, when I use exactly 6 GCP's the RMS values returned by g.transform are 0, > > IIUC that is RMS *error* value is 0, not results. By specifying the minumum number of points have you just fit a perfect surface to the points, and so no error? or is the actual fit bad? On GDAL API reference pages [1] it reads that "Note that 2nd order requires at least 6 GCPs, and 3rd order requires at least 10 gcps". GDAL uses the same algorithm as GRASS and this info is coherent with GRASS manual. AFAIK, for any transformation supported by i.rectify, adding an extra point above the minimum lets the algorithm calculate residuals. If only the very required number of points is provided, the transformation result is still valid, but you don't know the RMS. Given that, g.transform should return 0 with 6 values, but i.rectify should work. If it doesn't it's strange. [1]http://www.gdal.org/gdal__alg_8h.html -- Ticket URL: <http://trac.osgeo.org/grass/ticket/163#comment:6> GRASS GIS <http://grass.osgeo.org>
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