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Having issues with slope analysis. I click raster, analysis and then slope.

I enter my input and output and run the tool. The output I get doesn't appear to have been calculated correctly. Instead of seeing a continuos gradual change in slope through various colors, I just see a scattering of small squares.

It looks like only the max and min values are being displayed an nothing else (not sure).

Do I need to use the terrain analysis plugin for the tool to work? Or is there something in my workflow that I need to change/add? I am using version 3.0.3

  • You can get these small squares (i.e. 3x3 boxes of very high, nearly 90-degree, slope) surrounding outliers in the DEM. Outliers are single grid cells that are either significantly above or below the surface. This is very common in LiDAR data, if that happens to be what you are using. They should be removed from the DEM prior to calculating slope. – WhiteboxDev Jul 22 '18 at 15:29
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Just guessing here. If your horizontal coordinate system is in degrees (such as wgs84) and your heights are in meters you calculation will not be accurate. can you reproject to a metrics crs and calculate slopes again?

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Turns out the study area was too large. I cropped it to a small area, and the slope ran fine.

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    It doesn't make sense that the spatial extent would cause these errors. It is more likely that by cropping your image, you removed your outliers. That is, that you ended up with a smaller extent that did not contain any elevation outliers. – WhiteboxDev Jul 25 '18 at 13:27
  • This is probably right. Ran another slope recently in another area and did not encounter the issue i had previously/no need to create a subset of the data. – Kevin Kronzo Mathew Aug 2 '18 at 7:00

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