In a previous question (convert-xyz-points-to-topography-and-calculate-mean-slope), I asked how to interpolate point elevation data to a raster in order to measure slope. Thus I use the Raster > Interpolation tool, and create the raster with TIN method, grid 300x300.

This works, however when I calculate slope (raster->Analysis -> DEM) it gets awful.

Elevation raster Slope raster

I have few points, but I know some softwares such as Surfer provide tools to obtain a quite smooth surface by resampling.

Is that possible in QGIS? Do I need to resample the created raster? Or is there another way to get more "continuous" slope values ?

  • Is it possible to obtain contours, or even create contours from your point data? then try interpolating from the contour lines.
    – Goldring
    Apr 26, 2016 at 16:17

2 Answers 2


I'm not sure of the best answer to this at the moment, but I can explain why you're getting that result. I think this is probably more suited to Kriging. As pierma says, this is available using SAGA (via processing)

The Slope tool is doing what it's supposed to. The reason for this output is that you're using a TIN.

A TIN is a mesh of triangular faces, joined at their edges. Each face is made of 3 coplanar points, which is a fancy way of saying the face's surface is 'flat' - although it can be tilted. The angle of tilt is the slope. Any 3 points in space are coplanar, but you can't say the same about 4 or more.

By definition, slope measures the local slope at each pixel. That's why each pixel which forms a triangular face has the same value - they all lie on a plane with the same slope.


This aspect come from TIN interpolation. You can try others interpolation methods. You'll find more algorithms in the processing toolbox, especially the SAGA tools.

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