Re: Shape of resulting merged image is not what I expect


Sean Gillies
 

Hi,

I'm happy report that you won't have this problem with version 1.2.5, which is coming soon to PyPI.

As a part of the work to fix the bug noted at https://github.com/mapbox/rasterio/blob/maint-1.2/CHANGES.txt#L10, we replaced a call to math.ceil() to a call of round() in a key place and now the output shape of merge is what you expect.


On Thu, Jun 10, 2021 at 9:02 AM <juanjo.gomeznavarro@...> wrote:

[Edited Message Follows]
[Reason: typos]

I'm not sure if this is a bug or a subtle unexpected behaviour of the function rasterio.merge.merge.

I'm creating a mosaic with several images for the whole globe. The resolution is such that the image should be 16000x8001 (width x height). The calculation of the resolution is done through these variables:
pixels = 16000
res = 360 / pixels
xmin = -180 - res / 2
xmax = 180 - res / 2
ymin = -90 - res / 2
ymax = 90 + res / 2

Which result in xmin, xmax, ymin, ymax = (-180.01125, 179.98875, -90.01125, 90.01125). OK, the problem is that when I use merge as:  
mosaic, out_trans = merge(files, bounds=(xmin, ymin, xmax, ymax), res=res,  method='max')
The shape os mosaic is NOT (4, 16000, 8001), but (4, 16000, 8002) (note the extra row).

I think the problem comes from a rounding issue. In my python terminal (ymax-ymin)/res = 8001.000000000001 (whereas (xmax-xmin)/res = 16000.0). This rounding error of 1E-12 seems to be forcing the number of rows to be 8002 instead of 8001. This might seem like a tiny approximation error, but the problem is that controlling the shape of the image is very important for further calculations downstream of the algorithm.

Is this what it is supposed to happen? Is there a way to "force" merge to produce an image of the expected shape 16000x8001?

Thanks.
  
--
Sean Gillies

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