Hi List, I have a geoid in AAIGRID-format, with the following header:
ncols 1001 nrows 731 xllcenter 400000.0 yllcenter 6045000.0 cellsize 500.0 NODATA_value 9999 GDAL will read this just fine, and report: Driver: AAIGrid/Arc/Info ASCII Grid Files: dkgeoid13b.utm32 Size is 1001, 731 Coordinate System is `' Origin = (399750.000000000000000,6410250.000000000000000) Pixel Size = (500.000000000000000,-500.000000000000000) Metadata: AREA_OR_POINT=Point Corner Coordinates: Upper Left ( 399750.000, 6410250.000) Lower Left ( 399750.000, 6044750.000) Upper Right ( 900250.000, 6410250.000) Lower Right ( 900250.000, 6044750.000) Center ( 650000.000, 6227500.000) Band 1 Block=1001x1 Type=Float32, ColorInterp=Undefined NoData Value=9999 As expected, the upper left corner is shifted half a pixel. It is my understanding, that GDAL (GetGeoTransform) always uses the xulcorner, yulcorner (pixel) interpretation. However, I was suprised to see that after a gdal_translate dkgeoid13b.utm32 hello.tif, I get: >gdalinfo hello.tif Driver: GTiff/GeoTIFF Files: hej.tif Size is 1001, 731 Coordinate System is `' Origin = (400000.000000000000000,6410000.000000000000000) Pixel Size = (500.000000000000000,-500.000000000000000) Image Structure Metadata: INTERLEAVE=BAND Corner Coordinates: Upper Left ( 400000.000, 6410000.000) Lower Left ( 400000.000, 6044500.000) Upper Right ( 900500.000, 6410000.000) Lower Right ( 900500.000, 6044500.000) Center ( 650250.000, 6227250.000) Band 1 Block=1001x2 Type=Float32, ColorInterp=Gray NoData Value=99 The GeoTransform is not shifted half a pixel, and is not the same as the AAIGRID i ran gdal_translate on?? So, I would have to interpret now the GeoTransform as xulcenter and yulcenter. This came as a suprise, and led to some bugs in our processing software. Is this to be expected?? Cheers, Simon Kokkendorff
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