[Gdal-dev] Re: [GRASSLIST:6160] Re: r.proj and gdawarp give differernt results

[Maciek Sieczka]

Frank

From: "Frank Warmerdam" <fwarmerdam at gmail.com>

> Maciek,
>
> 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[164]% 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)

Maciek