A friend of mine gave me DEM resolutions of cells 5x5m, but there is something wrong with interpolation. When I try to make a cross-section of the terrain I get a pretty inaccurate cross-section, with weird peaks and valleys. What should I do to get a smoother DEM? These are the extensions:
The DEM seems to be the result of a Triangulation based on points and lines (probably ArcGis TIN algorithm). One thing that you can do is correct the value of the wrong cells using the Plugin Serval that allow you to change "manually" the value of any pixel. In the case of your DEM I think that the amount of the wrong pixels is small and you can try this approach. I suggest visualising your DEM with Pseudocolour Singleband that exalts the presence of pixel with different values from the surrounding area and, with the Serval tools, just proceed using the Probing mode tool to extract the value of a near pixel and copy in the wrong pixel with the Drawing mode tool.