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My boss gave me a small 3D-point-shapefile and asked me to make an DEM from it, which I should put together with an older DEM of the same site, which was made with Surfer.

I created a raster-layer from the shapefile and used the contour-tool to create level-contours. Unfortunately they seem to be lines instead of polygons. When I use the tool line-to-polygon some of the levels disappear. I am sure there is a way to create a DEM from point-data, but I have no idea what I am doing wrong.

I use QGIS Valmiera.

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    You need to use one of the many interpolation tools available to interpolate your vector points onto a raster DEM grid. Examples of interpolators include inverse distance weighted (IDW), spline based interpolators, triangulation coupled with subsequent rasterization, and kriging. This is a very broad field and will require some investigation to figure out which method is most appropriate for your data and application. The GRASS and SAGA plugins for QGIS each contain numerous tools for interpolating. Good luck. – WhiteboxDev Oct 14 '14 at 15:59
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A DEM (digital elevation model) is a raster and not contour lines or polylines. If the point data is a systematic grid of points (equal spacing) then it is as simple as converting straight to raster using a rasterize tool. In QGIS you can use "Raster > Conversion > Rasterize (Vector to raster)".

However, if the points are irregular then the becomes a interpolation problem. There are many options available. The stock QGIS "Raster > Interpolation > Interpolation" tool only has TIN and IDW available. I would not really recommend either of these methods but they may be adequate for your needs. Through the QGIS toolbox SAGA GIS (need to install) provides Kriging and Spline interpolation. There are also several very robust options in GRASS GIS (also avaliable in the QGIS toolbox).

GRASS interpolation options:

"r.surf.nnbathy" - Natural_neighbor

"v.surf.bspline" - B-splines

"v.surf.rst" and "v.vol.rst" - regularized splines with tension

"v.krige" - Kriging (GRASS 7+)

Often one try's a few methods and then selects a model based on RMSE (Root Mean Squared Error) or residual error to evaluate model fit.

  • Thanks a lot for the fast answers. I will try some of the GRASS and SAGA-Tools tomorow. The problem is, that my boss wants something that looks like a Surfer-Plot, so different colours for heights divided by contour-lines. I allready interpolatet a raster from the point-data. I did this "Raster>Conversion>Rasterize", but it had a very poor resolution. I hope this will get better with one of the other algorithms. Maybe I can then feign the Surfer-optic by using a second layer with contour-lines. Many Thanks. Lukas – akra-kachi Oct 14 '14 at 19:09

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