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I'm a member of a student team that is currently researching how to query raster information using PostGIS v2.0. Our raster's coordinate system is EPSG 2236 (NAD 83 / Florida East) and as we will be using the Google Maps API extensively, we believe that we should use gdalwarp to reproject the raster's coordinate system to EPSG 3857.

The WKT of our raster is:

PROJCS["NAD83 / Florida East",
    GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.2572221010042,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4269"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",24.33333333333333],
    PARAMETER["central_meridian",-81],
    PARAMETER["scale_factor",0.999941177],
    PARAMETER["false_easting",656166.6666666666],
    PARAMETER["false_northing",0],
    UNIT["US survey foot",0.3048006096012192,
        AUTHORITY["EPSG","9003"]]]

When we execute gdalwarp -t_srs 'epsg:3857' input.tif output.tif, the raster's size expands from 25,500 x 30,000 to 25,532 x 30,164. Is this normal? I know I can enforce a size on the output using gdalwarp's -ts switch - is that common practice?

Lastly, the WKT obtained using gdalinfo of the output GeoTIFF has too many unnamed and unknown parameters as shown below:

PROJCS["unnamed",
    GEOGCS["unnamed ellipse",
        DATUM["unknown",
            SPHEROID["unretrievable - using WGS84",6378137,298.257223563]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","3857"]]

And the coordinates are way off:

Origin = (-9003811.830927321687341,2993729.091127494350076)

Can someone enlighten me in what we are doing wrong?

Thanks in advance!

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1 Answer

up vote 4 down vote accepted

The increase in size is normal when you're reprojecting, the image will be slighty stretched and rotated to account for the difference in projections. You do need to make sure that a nodata value is set on the output otherwise you tend to get black areas around the edge.

As for the lack of WKT projection data, it sounds like you're using an old version of GDAL which may not have had EPSG:3857 in its database. Ideally you should be using 1.8.0 which should have this in. If not, then it sounds like a problem with your paths, you need to set GDAL_DATA to point to the data directory of GDAL.

You could also try using EPSG:900913, which was the code used by Google until the new one was assigned.

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Thank you! I hadn't realized I was using an older version of GDAL. –  alfyboza Apr 7 '11 at 23:43
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