[GRASS-dev] Re: [GRASS GIS] #352: r.in.gdal region troubles in LL

[GRASS GIS]

#352: r.in.gdal region troubles in LL
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  Reporter:  neteler      |       Owner:  grass-dev at lists.osgeo.org
      Type:  defect       |      Status:  new                      
  Priority:  major        |   Milestone:  6.4.0                    
 Component:  default      |     Version:  unspecified              
Resolution:               |    Keywords:                           
  Platform:  Unspecified  |         Cpu:  Unspecified              
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Comment (by glynn):

 Replying to [comment:2 neteler]:

 > The user took this map

 > and passed it to R, and then redefined:

 > and then he wrote b as npp.tif using the QGIS plugin manageR.

 > Maybe there is a precision issue before, somwhere in the R or QGIS part?

 It appears so. The inaccuracy is present in the TIFF file; it isn't being
 introduced by libtiff, GDAL or GRASS.

 I'm not quite sure where it would come from; 1/4 is exactly representable
 in single-precision floating-point. My first guess would be something
 calculating 360*(1/1440) rather than 360/1440 (1/1440 isn't exactly
 representable). This can occur if code is compiled with -funsafe-math-
 optimizations or similar.

 In any case, there's still the issue that GRASS takes a lat/lon region
 which is actually 360.000000000288 degrees across and treats it as if it's
 0.000000000288 degrees across. Wrapping coordinates is one thing; wrapping
 intervals is something else entirely.

 Ultimately, you can't take algorithms which work for Euclidian geometry
 and make them work for spherical geometry with nothing more than a couple
 of quick hacks.
 On a related note, I still haven't got anywhere with the "r.resamp.stats:
 nulls along western boundary" issue reported by Hamish (this should
 probably be added as a ticket).

-- 
Ticket URL: <http://trac.osgeo.org/grass/ticket/352#comment:3>
GRASS GIS <http://grass.osgeo.org>