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Does QGIS - through one of its rich algorithms library - allow for reduction of raster DSM/DEM resolution? I want to speed up the analyses and reduce effect of detailed raster on results readability.

The problem lies in the detail level of the raster. Input dsm was made based on the uav survey (output format: .tiff), so its size (area) is not large, but very, very detailed. This makes results of every analysis run on it difficult to view - i.e. QGIS calculates the aspect for every bump even 10 cm high, or streams for catchments for completely irrelevant areas.

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there are a lot of ways of achieving this in QGIS as you surmise. One is to use the raster calculator and adjust the rows and columns to suite your new desired resolution (do this via Raster->Raster Calculator and then edit the appropriate values in the dialog box).

QGIS uses GDAL under the hood so you can also achieve the same resolutions changes via either of the GDAL commands Warp or Translate (Raster->Projections->Warp or Raster->Conversion->translate). The QGIS dialogs in each case offer opportunities to change the output resolution, or you can edit the GDAL command directly (in the bottom section of the dialog) by following the GDAL documentation (linked).

Using the Processing Toolbox you also have access to SAGA and GRASS utilities that can do the same thing. For continuous data like a DSM or DEM you could use r.resamp.interp.

  • thank You for your answer. Will check how it works as soon as I'll get beck home and play with data :) – proteus Nov 5 '17 at 11:15
  • Raster Calculator works really fine. I am pleased with the results. – proteus Nov 6 '17 at 8:50
  • Just another thought came to my mind: is it possible to do reduce the resolution to a given centimeter GSD value with raster calculator? Reduction through number of lines is definetely ok, but I am curious whether we can push the functionality a bit further – proteus Nov 6 '17 at 10:02
  • It is easier to use one of the other functions I mentioned (warp/translate/GRASS/SAGA) to define an exact resolution – MappaGnosis Nov 6 '17 at 11:37
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In addition to GRASS r.resamp.interp tool suggested by @MappaGnosis, there is also SAGA Multilevel B-spline interpolation (from raster) tool in QGIS 2.18.14 . It is located under Processing Toolbox | SAGA | Raster creation tools.

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Setting:

  • Method: [1] with B-spline refinement
  • Cellsize

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This example shows upscaling cellsize from 90m (LHS) to 200m (RHS), with small ravines still visible (in some degree).

  • Thanks for your suggestion. This method however does not want to yield proper results. The only thing I get is error message with info that grid was not correctly generated. Do You know what might be a reason for that? – proteus Nov 6 '17 at 9:02
  • @proteus Thanks for your feedback. That's unfortunate. In my limited experience the tool failed to generate result when I worked on rasters in geographic (latlong) coordinates (which is characterized by small cell size 0.0000xx). However I honestly have no idea if it fits your case. – Kazuhito Nov 6 '17 at 9:06
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    SAGA GIS has also a Resampling Tool (Raster Tools > Resampling), where you can set a new resolution (cellsize). I would prefer Biliniear Interpolation for DEMs – Stefan Nov 6 '17 at 14:23
  • Thanks @Stefan I had not noticed its upscaling/downscaling methods. Quickly tested it and Bilinear did the job nicely. (and it was fast!) Could you post it as an answer? – Kazuhito Nov 6 '17 at 18:21
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You can also use the align raster tool, just below raster calculator in the raster tab. I once used it to smooth a DSM and it did the job. The tool is mainly used to align two or more rasters, but can be used to set a new resolution to your image with various resampling methods. I don't know if it exists in versions prior to 2.18 though

This link explains it better: https://docs.qgis.org/2.18/en/docs/user_manual/working_with_raster/raster_analysis.html#raster-alignment

  • Thanks for this. I am experimenting with raster calculator at the moment and this looks promising. – proteus Nov 8 '17 at 19:48
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SAGA GIS has also a Resampling Tool, where you can set a new resolution (cellsize).

For versions < 3.0.0: Raster Tools > Resampling

For versions >= 3.0.0: Grid > Tools > Resampling

I would prefer Biliniear Interpolation for DEMs.

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