[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 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?

 you can interactively play with this with i.vpoints's analyse tool if you
 like.

 ISTR in photogrametry the standard fiducial marks needed are like:

 {{{
 +          +

     +  +

 +          +
 }}}

 which compensates for rotation, scale, skew, and lens fisheye.
 (if that translates to this task geometrically..?)


 > and i.rectify fails.

 what is the error i.rectify gives?

 > If I use 7 GCP's, I get real values from g.transform and i.rectify
 works.

 ... but what if 0 rms error is real?

 > So at present, in terms of how this functions, the i.rectify man page is
 > incorrect. I don't know if there is an error in the code or an error in
 the
 > description.

 Or consider the case that the error is not with i.rectify at all.
 (sorry, I haven't tried the test data yet)


 Hamish

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Ticket URL: <http://trac.osgeo.org/grass/ticket/163#comment:5>
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