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I've been trying to get a slope raster from a DEM, so I can reclassify it and aggregate it with land use data.

The problem is that my slope raster seems to have a lot of distortion (diagonal lines). I've tried changing some settings but I really have no idea what can be causing this. I'm using a projected CRS, EPSG 31983. I've got the DEM from whole state where I live in Brazil, then I had to clip it to my AOI, and that is all the processing I made with the DEM before trying to get the slope.

Some information about the DEM file from the metadata:

Driver: GDAL provider
GTiff GeoTIFF

Dataset Description
/Users/Guilherme/Desktop/FINAL/DEM/DEM_SIRGAS_CLIPPED.tif

AREA_OR_POINT=Area
DataType=Generic

HighCellSize=0.039888
LowCellSize=0.000277
MaxCellSize=1.994400

Band 1 SourceBandIndex=0

Dimensions - X: 692 Y: 346 Bands: 1

Origin: 227546,7.41239e+06

Pixel Size: 28.8724,-28.8724

No Data Value: 0

Data Type: Float32 - Thirty two bit floating point

Pyramid overviews

Layer Spatial Reference System: +proj=utm +zone=23 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

Layer Extent (layer original source projection) - 227546.4710596329532564,7402400.2644091211259365 : 247526.1508292471116874,7412390.1042939284816384

Band No: 1
Min Val: 531.9765014648
Max Val: 960.2605590820
Range: 428.2840576172
Mean: 608.2492439474
Sum of squares: 1395234469.1412367821
Standard Deviation: 76.3367047628
Sum of all cells: 145634332.9768066406
Cell Count: 239432

DEM file:

DEM file

Slope generated from DEM (a lot of odd diagonal lines):

Slope generated from DEM

EDIT:

I went to the source again, and got the raw DEM to process/clip it again. This time around everything seemed to work just fine. I believe what happened was some kind of mismanage when warping the raster.

I didn't get the chance to try Freighty's solution because SAGA and Orfeo are not working properly for me, but I believe if you have access to them, it's a feasible solution.

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I had a play with your data and there are stripping errors in your DEM. Where did you obtain it from?

You can reduce the errors by smoothing your raster either using the 'Simple Filter' SAGA tool in the processing toolbox; or the "Smoothing' Orfeo Toolbox, again in the processing toolbox in QGIS if you have both providers installed. Then you can calculate your slope raster from this smoothed dataset. However you will lose information so this may not be desirable.

You can look at other methods such as using a fast fourier transformation to remove the stripping as outlined here http://www.ctmap.com/assets/pdfprojects/destripe.pdf

  • Well, I think this might solve my problem. I don't care about losing some information. However, I can't access both SAGA tools and Orfeo from the toolbox... maybe something related to my mac as I've seen here. Are there any other tools that can do that? The link you gave me is not working here. – Guilherme Salles Nov 21 '17 at 1:02

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