[GRASS-dev] Re: [GRASS GIS] #929: g.transform: dump coeffs and transform sparse points

[GRASS GIS]

#929: g.transform: dump coeffs and transform sparse points
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  Reporter:  hamish       |       Owner:  grass-dev at lists.osgeo.org
      Type:  enhancement  |      Status:  closed                   
  Priority:  normal       |   Milestone:  7.0.0                    
 Component:  Imagery      |     Version:  svn-trunk                
Resolution:  fixed        |    Keywords:  g.transform              
  Platform:  All          |         Cpu:  All                      
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Comment (by hamish):

 Replying to [comment:5 glynn]:
 > I would expect that you're typically trying to find a suitable
 > approximation to a complex analytic function (i.e. a typical
 > cartographic or geometric projection).
 > [[BR]][[BR]]
 > For many projections, a quadratic polynomial will be a poor fit.
 > The main reason for using cubic functions for interpolation is
 > that you can specify both the value and first derivative at each
 > end (4 degrees of freedom).

 Sure, if I were reprojecting or suspected the scan was distorted order=3
 would be appropriate-

 I guess I should have mentioned that I always try very hard to match the
 target location's projection to the scanned-map's native projection. So
 I'm using i.rectify mainly as a georeferencing tool, not a reprojection
 tool. (so rotation+scale but no warping)

 e.g. if the quality-scan is a nautical chart in Mercator projection, with
 a known +lat_ts and grid lines in lat/lon, I'll put GCPs on all the grid
 line confluences* and enter the coords as degrees, and finally run a
 little script to reproject the target POINTS to the custom merc projection
 (see wiki addons).

 Then run r.proj to pull it from the custom merc location into whatever
 working location it is needed.


 Probably it is overkill, but trying to do the right thing..


 Hamish


 [*] totally unrelated but interesting: http://confluence.org/

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Ticket URL: <https://trac.osgeo.org/grass/ticket/929#comment:6>
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