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This question already has an answer here:

I'm new to using QGIS. I have purchased some LiDAR data which downloads as .asc format, is there a way to analyse the terrain slope angle from this data or does it need to be converted into DEM? Ideally I would like to display the steeper slope angles (>35 degrees) as a different colour to the shallower (less steep) slope angles.

I'm using QGIS 2.0 Dufour on windows vista platform.

marked as duplicate by Andre Silva, whyzar, xunilk, BERA, aldo_tapia Jan 5 '18 at 18:40

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    Since you're new, why don't you search our site? A search on LIDAR slope turns up some promising information which I believe will answer this question, such as the answer at gis.stackexchange.com/questions/41991/…. – whuber Jan 25 '14 at 19:38
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    Please note that I did not vote to close this thread as a duplicate, nor did I assert that the question I linked to was a duplicate. I merely suggested it contains useful related information. – whuber Oct 10 '14 at 20:22
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Assuming your LiDAR .asc file is a bare-earth model, use the plugin Raster based terrain analysis.

  1. Set the analysis as "Slope". From QGIS 1.8 documentation:

    Slope: Calculates slope angle for each cell in degrees (based on first order derivative estimation).

    To calculate the slope on each cell of the raster it uses a 3x3 cell grid (the 8 neighboring cells surrounding the location).

  2. Add the input layer (the .asc file).

  3. Choose the place to save the output layer (which will be the raster with slope information).
  4. Set the output layer file extension (Output format).

Here is a screenshot to help you out (I'm using qgis version 1.7.4).

enter image description here

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