[Gdal-dev] Re: [GRASSLIST:6160] Re: r.proj and gdawarp give
werchowyna at epf.pl
Sat Mar 19 15:06:38 EST 2005
From: "Frank Warmerdam" <fwarmerdam at gmail.com>
> I see the different in the warp results that you are mentioning. I have
> picked a particular pixel that differs in the two results and I have
> "manually" processed it.
> The summary is that with my understanding of the coordinate system
> conversion taking place, gdalwarp appears to have done the right
> thing. Either there is an issue in the rproj algorithm, or more likely
> there is a subtle difference in the actual reprojection operation taking
> place in rproj due to differences in coordinate system detail handling
> between gdal and GRASS.
> Particular points to watch for is whether there might be some
> reason that r.proj is actually applying a datum shift.
Sorry if I missunderstood. Do you mean that r.proj is applying a datum shift
here? That would be strange because for GRS80 Grass assumes towgs84=0,0,0.
Paul - ?.
> It is also
> possible that I kept insufficient precision in my calculations, though
> it seems to me that I kept enough.
> The point in question (feel free to check my results!) is:
> Output Location
> Value: 98 (75 in rproj)
> Pixel/Line = 20.5, 11.5
> Georef = 337276.125, 279378.375
> long/lat = 16d42'41.641"E 50d21'32.477"N (proj -I +init=epsg:2180)
> GCS Conversion
> The Geographic Coordinate System for EPSG 2180 is EPSG 4258 which is
> GRS80 with no datum shift information (in GDAL). The GCS for the source
> file is WGS84 (EPSG 4326) which is based on a WGS84 ellipsoid and is for
> the precision of PROJ.4 identical to EPSG 4258.
> SPHEROID["WGS 84", 6378137,298.257223563]
> SPHEROID["GRS 1980",6378137,298.257222101]
> warmerda at gdal2200% cs2cs +init=epsg:4258 +to +init=epsg:4326
> 16d42'41.641"E 50d21'32.477"N (input)
> 16d42'41.641"E 50d21'32.477"N -0.000 (output)
> Input Location
> long/lat = 16d42'41.641"E 50d21'32.477"N
> Georef = 621743.762, 5579950.040 (proj -f '%.3f' +init=epsg:32633)
> Pixel/Line = 10.14, 10.56
> Value: 98 (pixel 9.5,10.5 is value 75)
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