[gdal-dev] Cassini_Soldner warping using WGS84 GCPs is distorted

[Martin Geng]

Hi Frank,

thank you for your answer. I have tried
gdalwarp -t_srs EPSG:4326 -tr 0.0000031156 0.0000019021 pic_gcp.tif result.tif
but neither with QGis nor with SharpMap the map is rendered undistorted.
And that although the sizes now have proper values...

Any idea?

Best regards,
Martin

..\FWTools2.4.2\bin>gdalinfo result.tif
Driver: GTiff/GeoTIFF
Files: result.tif
Size is 15141, 11373
Coordinate System is:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0],
    UNIT["degree",0.0174532925199433],
    AUTHORITY["EPSG","4326"]]
Origin = (13.437037980137905,52.481881537880760)
Pixel Size = (0.000003115600000,-0.000001902100000)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  (  13.4370380,  52.4818815) ( 13d26'13.34"E, 52d28'54.77"N)
Lower Left  (  13.4370380,  52.4602490) ( 13d26'13.34"E, 52d27'36.90"N)
Upper Right (  13.4842113,  52.4818815) ( 13d29'3.16"E, 52d28'54.77"N)
Lower Right (  13.4842113,  52.4602490) ( 13d29'3.16"E, 52d27'36.90"N)
Center      (  13.4606246,  52.4710652) ( 13d27'38.25"E, 52d28'15.83"N)
Band 1 Block=15141x1 Type=Byte, ColorInterp=Palette
  Color Table (RGB with 256 entries)
    0: 0,0,0,255
    1: 1,1,1,255
    2: 2,2,2,255

-------- Original-Nachricht --------
> Datum: Fri, 18 Sep 2009 10:39:09 -0400
> Von: Frank Warmerdam <warmerdam at pobox.com>
> An: Martin Geng <martin.geng at gmx.de>
> CC: gdal-dev at lists.osgeo.org
> Betreff: Re: [gdal-dev] Cassini_Soldner warping using WGS84 GCPs is distorted

> Martin,
> 
> Examining the GCPs in WGS84 space I see:
> 
>  >>> (13.4841338099227  - 13.4370322252183) / 15118
> 3.1155962894827404e-06
>  >>> "%.10f" % ((13.4841338099227  - 13.4370322252183) / 15118)
> '0.0000031156'
>  >>> "%.10f" % ((52.4818154665202 - 52.4602476117909)/ 11339)
> '0.0000019021'
> 
> 
> Basically, in WGS84 space your source pixels are distinctly
> non-square and gdalwarp distorts your image to produce square
> pixels in the output projection.  If you want to preserve
> the original aspect ratio try:
> 
>    -tr 0.0000031 0.0000019
> 
> on the gdalwarp commandline.
> 
> But basically WGS84 and Cassini-Soldner have quite different
> characteristics in the area of your image, and WGS84 may not
> be a good working coordinate system for the image.
> 
> Best regards,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent

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