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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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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.

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