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I am attempting to take a simple opentopo heightmap and carry out the following in QGIS 3.4.1

- Reproject to UTM 31N
- Set cell size resolution to 5mtr
- Clip to a shapefile polygon feature exactly 40960 x 40960mtr

So why when using following command in Python console:

import os
os.system(r'''gdalwarp -t_srs EPSG:32631 -tr 5.0 5.0 -of GTiff -cutline E:\Users\rosso\Arma\QGIS\UTM31N\Test23-11-18\shape.shp -crop_to_cutline "E:/Users/rosso/Arma/Terrain Heightmaps/Heightmaps Opentopo/output_srtm.asc" E:/Users/rosso/Arma/QGIS/UTM31N/24-11-18/converted2.tif''')

Do I get width and height not matching exactly to my shapefile feature I'm clipping to - (40960 x 40960) - which at 5mtr resolution size should be '8192' for both width and Height

enter image description here

If I break up the process into 3 stages:

Change CRM:

import os
os.system(r'''gdalwarp -t_srs EPSG:32631 -of GTiff "E:/Users/rosso/Arma/Terrain Heightmaps/Heightmaps Opentopo/output_srtm.asc" E:/Users/rosso/Arma/QGIS/UTM31N/26-11-18/crm.tif''')

Change Resolution:

import os
os.system(r'''gdalwarp -tr 5.0 5.0 -of GTiff "E:/Users/rosso/Arma/QGIS/UTM31N/25-11-18/crm.tif" E:/Users/rosso/Arma/QGIS/UTM31N/26-11-18/res.tif''')

Clip to Shapefile feature mask layer:

import os
os.system(r'''gdalwarp -of GTiff -cutline E:\Users\rosso\Arma\QGIS\UTM31N\Test23-11-18\shape.shp -crop_to_cutline "E:/Users/rosso/Arma/QGIS/UTM31N/26-11-18/res.tif" E:/Users/rosso/Arma/QGIS/UTM31N/26-11-18/cut.tif''')

The width and height adhere to what I expect:

enter image description here

I would prefer the process to be completed in one command, rather than splitting them out as I want to reduce the amount of processing on the original raster.

  • So I progressed a little in combining the first 2 commands for CRM and Resolution - gdalwarp -t_srs EPSG:32631 -wo SOURCE_EXTRA=1000 -tr 5.0 5.0 -r cubic -of GTiff "E:/Users/rosso/Arma/Terrain Heightmaps/Heightmaps Opentopo/output_srtm.asc" E:/Users/rosso/Arma/QGIS/UTM31N/26-11-18/crm.tif But I still can't seem to combine with the Clip to Shapefile command without it messing with width and height. – Rossoe Dec 1 '18 at 12:12
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I discovered that due to an update to GDAL (2.4.0) using 'Clip Raster by Mask Layer' no longer works as I expected - see thread I posted here - https://github.com/OSGeo/gdal/issues/1176

So I switched to following command and fixed my issue:

import os
os.system(r'''gdalwarp -t_srs EPSG:32630 -wo SOURCE_EXTRA=1000 -tr 5.0 5.0 -srcnodata "-9999" -r cubic -of GTiff -te 597700.541507 5593677.822312 638660.541507 5634637.822312 "E:/Arma/Heightmaps/Opentopo/output_srtm.asc" E:/Arma/project/converted.tif''')

Also followed a few tips from this chap: https://medium.com/planet-stories/a-gentle-introduction-to-gdal-part-2-map-projections-gdalwarp-e05173bd710a

particularly re some additional warp parameters that improved raster quality!

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