Hi, I have georeferenced an image with gdal_translate and then reprojected it from OSGB36 to googleMaps with gdalwarp. However the warped TIFF latitude/longitude coordinates are wrong. The re-projection appears to stretch the map correctly, but I discovered there was a problem when I ran gdal2tiles and the map was not aligned correctly.
For the georeferenced file, buckland.tiff, here..... https://docs.google.com/leaf?id=0B93lksnTC7_cNTg3OGU0MTEtNjNiMC00NWM5LTg5ZjQtMmQyNDIzNmM5NGE5 According to gdalinfo the coordinates of the lower-left corner are: easting=241538.5, northing=119812 This corresponds to a latitude of 50.955887 I then run the warping as follows... gdalwarp.exe -s_srs EPSG:27700 -t_srs EPSG:900913 -r cubic -of GTiff buckland.tiff buckland_warped.tiff ..and according to gdalinfo the coordinates of the lower-left corner are: 4d15'30.51"W, 51d 8'38.06"N I.e. a latitude of 51.143906 which is miles off (infact no latitude on the map is near this value). It should be 50.955887. Warped file, buckland_warped.tiff, is here.... https://docs.google.com/leaf?id=0B93lksnTC7_cNzIzZWQ0MDYtMTY5Yy00NjY0LThiMjktZjc2YzA3MmU5NjI4 When I run gdal2tiles directly on the unwarped map it does it correctly but it is not very good quality (it doesnt seem to use such a good pixel interpolation as gdalwarp). Does this look like a bug in gdalwarp or am I doing something stupid. I'm using gdal16 from osgeo4w on Windows 7 64-bit. Output of gdalinfo on the georeferenced and warped images is below. Thanks BUCKLAND.TIFF... C:\OSGeo4W\apps\gdal-16\bin>gdalinfo e:buckland.tiff Driver: GTiff/GeoTIFF Files: e:buckland.tiff Size is 600, 400 Coordinate System is `' GCP Projection = PROJCS["OSGB 1936 / British National Grid",GEOGCS["OSGB 1936",DATUM["OSGB_1936",SPHEROID["Airy 1830",63 77563.396,299.3249646000044,AUTHORITY["EPSG","7001"]],AUTHORITY["EPSG","6277"]],PRIMEM["Greenwich",0],UNIT["degree",0.01 74532925199433],AUTHORITY["EPSG","4277"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER ["central_meridian",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",400000],PARAMETER["false_northi ng",-100000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","27700"]] GCP[ 0]: Id=1, Info= (0,400) -> (241538.5,119812,0) GCP[ 1]: Id=2, Info= (600,0) -> (244713.5,121928.66666667,0) GCP[ 2]: Id=3, Info= (0,0) -> (241538.5,121928.66666667,0) GCP[ 3]: Id=4, Info= (600,400) -> (244713.5,119812,0) Metadata: AREA_OR_POINT=Area Image Structure Metadata: INTERLEAVE=PIXEL Corner Coordinates: Upper Left ( 0.0, 0.0) Lower Left ( 0.0, 400.0) Upper Right ( 600.0, 0.0) Lower Right ( 600.0, 400.0) Center ( 300.0, 200.0) Band 1 Block=600x3 Type=Byte, ColorInterp=Red Mask Flags: PER_DATASET ALPHA Band 2 Block=600x3 Type=Byte, ColorInterp=Green Mask Flags: PER_DATASET ALPHA Band 3 Block=600x3 Type=Byte, ColorInterp=Blue Mask Flags: PER_DATASET ALPHA Band 4 Block=600x3 Type=Byte, ColorInterp=Alpha BUCKLAND_WARPED.TIFF... C:\OSGeo4W\apps\gdal-16\bin>gdalinfo e:buckland_WARPED.tiff Driver: GTiff/GeoTIFF Files: e:buckland_WARPED.tiff Size is 611, 419 Coordinate System is: PROJCS["Google Maps Global Mercator", GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84",6378137,298.2572235630016, AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich",0], UNIT["degree",0.0174532925199433], AUTHORITY["EPSG","4326"]], 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 = (-474051.292871501410000,6617009.987366309400000) Pixel Size = (8.392224865731517,-8.392224865731517) Metadata: AREA_OR_POINT=Area Image Structure Metadata: INTERLEAVE=PIXEL Corner Coordinates: Upper Left ( -474051.293, 6617009.987) ( 4d15'30.51"W, 51d 9'49.57"N) Lower Left ( -474051.293, 6613493.645) ( 4d15'30.51"W, 51d 8'38.06"N) Upper Right ( -468923.643, 6617009.987) ( 4d12'44.69"W, 51d 9'49.57"N) Lower Right ( -468923.643, 6613493.645) ( 4d12'44.69"W, 51d 8'38.06"N) Center ( -471487.468, 6615251.816) ( 4d14'7.60"W, 51d 9'13.82"N) Band 1 Block=611x3 Type=Byte, ColorInterp=Red Mask Flags: PER_DATASET ALPHA Band 2 Block=611x3 Type=Byte, ColorInterp=Green Mask Flags: PER_DATASET ALPHA Band 3 Block=611x3 Type=Byte, ColorInterp=Blue Mask Flags: PER_DATASET ALPHA Band 4 Block=611x3 Type=Byte, ColorInterp=Alpha
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