[gdal-dev] gdalwarp -crop_to_cutline produces non-square pixels for EPSG:3857 (GDAL 1.10.1, released 2013/08/26)

[Even Rouault]

Le jeudi 05 février 2015 17:23:06, Jesse McGraw a écrit :
> I'm not sure whether this is a real issue or not but I thought I'd bring it
> up, at the very least I'll learn something
> 
> When using "gdalwarp -cutline <shapefile> -crop_to_cutline" on an input
> raster that is in EPSG:3857 with square-pixels the output raster, while
> still EPSG:3857, now has non-square pixels.
> 
> They're aren't terribly non-square but aren't they supposed to be
> completely square for EPSG:3857?

Perhaps... Actually the effet of -crop_to_cutline is exactly the same as 
manually passing the target extents with -te with the bounding box of the 
cutline.
It is not possible to both preserve the extent extents and pixel size at the 
same time, due to width and height being integer values.
So, in order to preserve pixel square, the extent should be modified a little 
bit. What is more appropriate is a matter of point of view I think.
You can always re-run "gdalwarp tmp.tif out.tif" where tmp.tif is the output 
of first gdalwarp with -crop_to_cutline. And you should get square pixels I 
believe.

> 
> (FWIW, I see that there are tickets opened that reference similar issues
> but they reference the output being shifted or the origin changing, not the
> pixel shape changing)
> 
> For example:
> #Warp our original .tif to EPSG:3857
> $gdalwarp \
>         -t_srs EPSG:3857 \
>         -dstalpha \
>         -co TILED=YES \
>         "ENR_L33.tif" \
>         "./2.tif"
> 
> #See that the output pixels are square
> $gdalinfo 2.tif
> Origin = (-8577554.996301921084523,5421778.172851986251771)
> Pixel Size = (43.677179501975118,-43.677179501975118)
> 
> 
> #Now crop the image to a cutline
> $gdalwarp \
>         -crop_to_cutline \
>         -dstalpha \
>         -cutline "./ENR_L33.shp" \
>         -cblend 10 \
>         -co TILED=YES \
>         "./2.tif" \
>         "./3.tif"
> 
> #See that output pixels are not square
> $ gdalinfo 3.tif
> Origin = (-8480047.445924906060100,5366376.137789577245712)
> Pixel Size = (43.678439570399853,-43.675457269061347)
> 
> 
> I ran some more tests and without the -crop_to_cutline option the output
> pixels remain square
> 
> Thanks,
>   Jesse

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