[gdal-dev] Python API ReadAsArray axis order

[Even Rouault]

Chris,

ReadAsArray() returns a NumPy array, and the first dimension in NumPy is 
the slowest varying one, that is y.

GDAL's geotransform is

Xgeoref = gt[0] + column * gt[1] + line * gt[2]

Ygeoref = gt[3] + column * gt[4] + line * gt[5]

and GDAL's Raster API have their arguments with column first, line second.

So, there's no transposition. It is just a different convention on how 
to present dimensions.

Note: if you used GDAL's multidimensional API, it would be similar to 
NumPy convention, with slowest varying dimension being the first index 
value.

Even

Le 04/12/2022 à 09:02, Chris Crook a écrit :
> Hi
>
> I am using the Python GDAL API and looking at a simple TIFF image with 
> a test program:
>
> from osgeo import gdal
> dataset = gdal.Open('test-gdal.tif')
> transform=dataset.GetGeoTransform()
> nx=dataset.RasterXSize
> ny=dataset.RasterYSize
> data=dataset.ReadAsArray()
> print(f"Raster xsize, ysize ({nx},{ny})")
> print(f"ReadAsArray dimensions {data.shape}")
> print(f"GeoTransform: {transform}")
>
> The output is:
>
> test_python_api.py
> Raster xsize, ysize (3,4)
> ReadAsArray dimensions (4, 3)
> GeoTransform: (22.15, 0.7, 0.0, -69.5, 0.0, 1.0)
>
>
> I am confused that the array appears transposed from the raster x,y size.
>
> Is this always the cased.  As far as I can tell the geotransform is 
> assuming x,y (0-2,0-3 values) to map indices to location, which is 
> what I want to do.
>
> Can I assume that the array is always transposed?  Or if not how do I 
> identify when this is the case?
>
> Many thanks in advance
>
> Chris Crook
> Land Information New Zealand
>
>
>
>
>
>
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