[gdal-dev] GeoTransform confusion

[Simon Lyngby Kokkendorff]

Basically it seems to be an issue with interpreting "center" directives
when doing the gdal_translate. Consider:

ncols 2
nrows 2
xllcenter 0.0
yllcenter 0.0
cellsize 1.0
nodata_value -999
2 2
2 2

gdalinfo on this file, will give the expected GeoTransform:

In [2]: ds = gdal.Open("center.asc")

In [3]: ds.GetGeoTransform()
Out[3]: (-0.5, 1.0, 0.0, 1.5, 0.0, -1.0)

Seems OK to me.

If I then do a gdal_translate: gdal_translate center.asc center.tif, I get
another interpretation:

In [4]: ds = gdal.Open("center.tif")

In [5]: ds.GetGeoTransform()
Out[5]: (0.0, 1.0, 0.0, 1.0, 0.0, -1.0)

And, if I do a roundtrip: gdal_translate -of AAIGRID center.tif
center_back.asc, I get:
ncols        2
nrows        2
xllcorner    0.000000000000
yllcorner    -1.000000000000
cellsize     1.000000000000
NODATA_value -999
 2 2
 2 2

Which is definitely not the same GeoTransform as my input center.asc, but
shifted half a cellsize from what I would expect...

In [6]: ds = gdal.Open("center_back.asc")

In [7]: ds.GetGeoTransform()
Out[7]: (0.0, 1.0, 0.0, 1.0, 0.0, -1.0)



GDAL version:
GDAL 1.11.2, released 2015/02/10

Cheers,
Simon


On Wed, Nov 18, 2015 at 9:38 PM, Simon Lyngby Kokkendorff <silyko at gmail.com>
wrote:

> 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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