[GRASS-dev] Re: [GRASS GIS] #163: g.transform no longer calculating error for 2nd order transformation

[GRASS GIS]

#163: g.transform no longer calculating error for 2nd order transformation
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  Reporter:  cmbarton  |       Owner:  grass-dev at lists.osgeo.org
      Type:  defect    |      Status:  reopened                 
  Priority:  major     |   Milestone:  6.4.0                    
 Component:  default   |     Version:  svn-develbranch6         
Resolution:            |    Keywords:  g.transform georectify   
  Platform:  All       |         Cpu:  Unspecified              
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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

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Ticket URL: <http://trac.osgeo.org/grass/ticket/163#comment:6>
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