[GRASS-dev] import/export projection error

[Martin Wegmann]

well, as far as I understand AAIGrid now, it stores one value for raster file 
resolution only. If the values (nsres/ewres) differs slightly the single 
resolution value in the AAIGrid export file holds a wrong resolution for ns 
or ew which is only visible if number of columns/rows is high (e.g. high 
resolution imagery or continental scale raster file). 
It can be solved by setting g.region to the same nsres/ewsres and resample the 
respective raster files before exporting the raster. 

If my error detection result is correct then I would suggest that r.out.gdal  
prompts a warning for AAIgrid if g.region nsres/ewres resolution differs. 

Martin



On Monday 08 May 2006 14:31, Martin Wegmann wrote:
> the shift only happens for r.out.gdal export of AAIGrid not for geotiff or
> hfa.
> So far it seems that the 3 ( and upward) decimal place of the pixel size
> (AAIGrid exports only) differs from the original (and GRASS imported) file
> what causes the error on a very large scale.
>
> regards, Martin
>
> On Friday 05 May 2006 18:43, Martin Wegmann wrote:
> > Hello,
> >
> > I just encountered a very peculiar error.
> >
> > import of latlong/WGS84 geotif
> > import of latlong/WGS84 shp
> >
> > border fits well
> >
> > export of latlong/WGS84 asc/img etc. via r.out.gdal
> >
> > load GRASS export asc into QGIS plus shp layer:
> >
> > upper left: border fits good
> > middle left: border is shifted slightly
> > lower right: border does not fit at all
> >
> > load original geotif into QGIS, overlay with shp layer: border fits
> >
> > it is a continental raster/shp - perhaps only this extent makes the
> > problem visible.
> >
> > I attached the original raster + shp and the GRASS exported raster + shp
> > in QGIS
> >
> > if somebody wants to have the original files for testing, please feel
> > free to contact me.
> >
> > cheers, Martin

-- 
Martin Wegmann

DLR - German Aerospace Center
German Remote Sensing Data Center
@
Dept.of Geography
Remote Sensing and Biodiversity Unit
&&
Dept. of Animal Ecology and Tropical Biology
University of Wuerzburg
Am Hubland
97074 Würzburg

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