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I'm trying to extract one single contour line at given elevation from a DEM. Taking into account that I want only one contour and not all contours with the given elevation, is there a way to do it in QGIS? What I really want it is a tool similar to the contour drawing in ArcView. Just one click on DEM or GRID into the main view e the contour is drawn. I'd like to reproduce something like that.

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    You can either create the contours then select the line with the elevation value you're after, or you can use raster calculator to select the cells of the value you're after, then convert those to contours... – DPSSpatial Oct 17 '14 at 16:04
  • do you speak python+gdal? – user1269942 Oct 17 '14 at 16:54
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You can use the SAGA Contour lines from grid tool, and there set both the minimum and maximum to your elevation contour of choice

Here's an example with an elevation of 500 meters

enter image description here

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I think your best bet is to use r.contour.level from GRASS in the Processing toolbox. This allows you to provide a list of specific contours (a list of one in your case). In this way you should be able to specify the individual contour you are interested in. The Raster->Extraction->Contour tool requires you to specify just a contour interval which may not give you the control you need and will result in a lot of unwanted contours making it a little harder to pick out the one you want.

You said "I want only one contour and not all contours with the given elevation" which I take to mean that you will not want all contours the above process creates even though they will all be at your given elevation. So you have some options. The simplest is to select the individual contour you want and then save your contour vector file using Save AS from the layer properties and tick the box "Save only selected features".

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