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I'm trying to extrat ridges from ALOS images on qgis. I would like to have polylines so I can use the plugin AutomaTracks from Sebastien Peillet.

I did inversed the MNT using raster calculator then used Streams tool. The end results are the rasterised ridges.

I'm trying to binarize them before converting them into polylines but the method isnt working, i'm left with "nan" for the ridges and 1 for the nodata pixels.

Raster Calculator : ("Ridges@1">1)*1

Also, i tried the tool r.reclass but it isnt working using the following rule : 1 thru 700 = 1

So my question is, is it possible to extract polyline from raster using QGIS ?


Solution : I finally find a way to extract ridgelines. I inverted the DEM by multiplying it by -1 in the raster calculator. Then I extracted the streams with r.stream.extract (set an adapted threshold), which were then thinned by r.thinned and converted to vector with r.to.vect

(DEM)*-1 > r.streams.extract > r.thinned > r.to.vect

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