[gdal-dev] RE: reprojection to WGS 1984 Web Mercator

[Sjur Kolberg]

John,

This looks like the kind of north/south error you get when the transform gets the ellipsoids wrong, with most serious displacement if a one of the ellipsoids is a sphere.

I am guessing that your transform ignores the ellipsoid change, try see what you get if you specify GRS 1980 also in your target CS. You can use cs2cs on single points to check the transform.

Also, verify that 6378137 is the proper Earth radius both for your sphere and for the GRS80 spheroid, this is usually not the case.

Sorry I couldnt pinpoint he exact command to solve, it's too long since I dealt with these issues :-)

Sjur



________________________________
From: gdal-dev-bounces at lists.osgeo.org [mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of John Hayes
Sent: 5 august 2009 02:55
To: gdal-dev at lists.osgeo.org
Subject: [gdal-dev] reprojection to WGS 1984 Web Mercator

Hi,
  I'm having problems warping an image to WGS 1984 Web Mercator.  The lat/long center point moved south by 10 arc-minutes.  Does anyone have some suggestion on how to fix this?

 I'm pretty sure the USA_Contiguous_Equidistant_Conic transform worked fine because I looked at the lag/long in Google Earth and saw the same buildings.

I've also done the transform in two steps Source SRS to WGS84 and WGS84 to WGS 1984 Web Mercator but get the same answer.

Bellow is the command line I used to warp the image and info dumps of the before and after images.

$ gdalwarp -t_srs ESRI::wm.prj Athens_RGB_CEC1FT_200303.tif Athens_RGB_CEC1FT_200303-wm.tif
Creating output file that is 7455P x 6882L.
Processing input file Athens_RGB_CEC1FT_200303.tif.
0...10...20...30...40...50...60...70...80...90...100 - done.

Files: Athens_RGB_CEC1FT_200303.tif
Size is 6683, 6024
Coordinate System is:
PROJCS["USA_Contiguous_Equidistant_Conic",
    GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.2572221010002,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4269"]],
    PROJECTION["Equidistant_Conic"],
    PARAMETER["standard_parallel_1",33],
    PARAMETER["standard_parallel_2",45],
    PARAMETER["latitude_of_center",39],
    PARAMETER["longitude_of_center",-96],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
Origin = (1157874.742488616100000,-478125.149393160190000)
Pixel Size = (0.169893858067214,-0.169893858067214)
Corner Coordinates:
Upper Left  ( 1157874.742, -478125.149) ( 83d24'42.40"W, 33d58'12.44"N)
Lower Left  ( 1157874.742, -479148.590) ( 83d24'47.89"W, 33d57'39.54"N)
Upper Right ( 1159010.143, -478125.149) ( 83d23'58.53"W, 33d58'7.37"N)
Lower Right ( 1159010.143, -479148.590) ( 83d24'4.02"W, 33d57'34.47"N)
Center      ( 1158442.443, -478636.870) ( 83d24'23.21"W, 33d57'53.46"N)

Files: Athens_RGB_CEC1FT_200303-wm.tif
Size is 7455, 6882
Coordinate System is:
PROJCS["WGS_1984_Web_Mercator",
    GEOGCS["GCS_WGS_1984_Major_Auxiliary_Sphere",
        DATUM["WGS_1984_Major_Auxiliary_Sphere",
            SPHEROID["WGS_1984_Major_Auxiliary_Sphere",6378137,0]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
Origin = (-9285526.535261498800000,4000908.274406374400000)
Pixel Size = (0.204760916615493,-0.204760916615493)
Corner Coordinates:
Upper Left  (-9285526.535, 4000908.274) ( 83d24'47.89"W, 33d47'31.24"N)
Lower Left  (-9285526.535, 3999499.110) ( 83d24'47.89"W, 33d46'53.36"N)
Upper Right (-9284000.043, 4000908.274) ( 83d23'58.53"W, 33d47'31.24"N)
Lower Right (-9284000.043, 3999499.110) ( 83d23'58.53"W, 33d46'53.36"N)
Center      (-9284763.289, 4000203.692) ( 83d24'23.21"W, 33d47'12.30"N)






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