[postgis-users] Raster Processing and ST_Transform

[Paul Ramsey]

On Wed, Feb 29, 2012 at 4:41 PM, Pierre Racine
<Pierre.Racine at sbf.ulaval.ca> wrote:
>> First problem, after reprojection, all the chunks have different alignements ("The
>> two rasters provided do not have the same
>> alignment") which is unfortunate. It would be nice to be able to ensure that
>> when reprojecting all the objects get resampled into a same alignment (am I
>> right that an "alignment" is a combination of a origin, pixel size and skewness?)
>> so that they can be processed together in succeeding steps.
>
> Alignment is the combination of any arbitrary pixel corner on a raster grid, scale (what some call pixel size) and skewness. We are discussing about that with Bborie. He might come up with a slight redesign of ST_Transform. Right now you can achieve what you want with ST_Resample if I understand well.
>
>> I was hoping in my next steps to calculate slope and go from there, but I'm
>> stopped at the transform step. Thoughts? While a general utility to re-sample
>> any collection of rasters into the same basis would be good, multiple re-
>> samplings are not great, so inherently re-sampling operations like st_transform
>> should probably have a way to force the output alignment.
>
> I think we all agree on that. BTW You will have to ST_Union everything before passing it to ST_Slope as all our neighboring functions do not transparently work on a tiled coverage. For now, calling ST_Slope on a tiles coverage should result in tile border effects. The way to get all the neighboring functions to work on tiled coverage is still an open question.
>

Well, I cleverly unioned each tile with its neighbors before sloping,
then clipped them back using the original extent, at least that's the
theory, but the process never finished, which seemed unacceptable
given the data size. Still learning!

P.

> Pierre
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