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SDR in 3.4.0rc2 - cell alignment shift compared to input DEM
Hi, I've just used the pre-release version 3.4.0rc2 to run the SDR in East Kalimantan, Indonesia.
Thanks for all of your work on this release.
Do you know why the outputs have the same projection and cell size as the input DEM, but are shifted (by less than 1 cell) relative to the DEM's cell alignment?
The size of this shift seems to be larger when larger watersheds are used. The shift can be in different directions if different watershed shapefiles are used (i.e. alternative, adjacent watersheds with identically aligned boundaries based on the same input DEM)
The DEM cell size is 30.84220252 m.
Output TIFs all have the same cell size, but are shifted by up to 30% of one cell length. (e.g. shifted to the east by approx. 13 m and south by 3 m.
The shift is largest (c. 13 m) when I run the model using watersheds covering all of East Kalimantan, and is less (c. 5 m) when run for a single large watershed (e.g. Berau 14,000 km2.)
This is true for the preliminary outputs, flow direction, stream TIF etc., so it may arise during one of the early reprojection steps?
This isn't a massive spatial shift, but I don't know why it happens, and regret that:
1. separate runs for adjacent watersheds do not align (their shifts are slightly different, when the only difference in the model setup is the use of an adjacent watershed with an identically aligned shapefile boundary)
2. the outputs have less accurate stream locations, compared with delineation from the original DEM.
Perhaps this is not large enough to be considered a problem, but I thought I should ask in case there's anything going wrong with the initial alignment of all datasets to the DEM.
This also happens to some extent (each slightly different) in previous versions including 3.3.3 and 3.3.3.post147+nc374360e6f37.
In case it's helpful to see the data, all of the inputs and example 'watersheds' files (SubWS) are on Dropbox here:
(Parameters file: sdr_kal_example.json)
Some background on the other inputs: (I don't think anything suggests a reason for the cell alignment shift)
The watersheds align exactly with the DEM (from which they were delineated).
All layers are in the same UTM projection (WGS 1984 UTM N50)
Cell size is similar or slightly larger for other inputs (e.g. LULC 33 m), but I thought resampling was performed for all layers to align with the DEM, so this should have no impact on the alignment of the outputs.
Thanks for any light you can shed on this, or even just reassurance that this spatial shift doesn't sound like a major concern relative to other uncertainties.