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I need to do a slope analysis, and would like to make a TIN from 2m contours lines. Or do a slope analysis directly from contours? The only DEM available is not precise enough. I do not have any elevation raster files.

And how do I then make slope classes? The TIN properties does not seem to be able to classify anything other than elevation.

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2 Answers

You can use the interpolation plug in. It is located in the raster menu.This is the dialog box

enter image description here

The output file is a raster that you can use with the raster/analysis/dem (terrain models) to produce the slope (either degrees or percent) analysis.

hope it helps

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Thanks. But I forgot to mention that I tried that, and it crashes QGIS. Both when I use DEM points and add contours, and use just contours. Works ok on points only, if zoomed in close. –  Allison Feb 6 '13 at 21:49
    
A long time ago I used a free software to convert dxf polylines to a text file (x,y,z triplets) you can look at it here: guthcad.com/freestuff.htm may be it can help you, since you can use the file to interpolate by points. –  Gerardo Jimenez Feb 7 '13 at 14:07
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The crashing during raster interpolation was a memory issue, so I simplified the geometry of the contour lines (Vector > Geometry Tools) to vastly reduce the number of nodes without seeming to lose much accuracy. The raster interpolation plugin worked fine once the number of nodes was down enough.

Then Raster > Terrain Analysis for slope classes.

(This may seem obvious to some, but I am just learning and have not found much instruction for newbies)

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