I want to classify the slope of the DEM data, by dividing the slope by 4 classes: flat, ramps, steep, very steep.

How to do this with QGIS?

  • 1
    You want to reclassify the slope raster. See for example this question: gis.stackexchange.com/questions/17712/…
    – BERA
    May 2 '17 at 13:17
  • A slope raster derived from a DEM must have either the angle or percent of the slope for each individual pixel. You have to decide what range of values are you going to consider a flat, ram steep very steep and reclassify the raster accordingly. After that,in order to get the area, you can convert the reclassified slope raster to vector (polygons). May 2 '17 at 16:10
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    – underdark
    May 3 '17 at 17:35

You may apply an on-the-fly reclassification by right-clicking on the layer in the Layers Panel and then clicking on Properties: from the dialog that appears, go to Style and then set these parameters (they are self-explanatory, but you can adapt them following your specific needs):

enter image description here

Please note that in step No.5 you set Equal interval as Mode, but you only do this for the possibility of editing the number of classes (actually, you may choose a different treshold for different categories, even if the Equal interval is set).

Finally, click the Apply button for applying the changes. You will see something like this:

enter image description here

as expected.

Instead, if you want to create a reclassification on a new file, you may use several tools from the Processing Toolbox (as an example, the Reclassify values SAGA tool or the r.reclass GRASS module) or directly using the Raster Calculator from the Raster Menu. Some of these options are well explained in the link suggested by @BERA in his comment.

  • Thanks for the answer, but how to calculate the area / percentage area of the class?
    – syafrauf
    May 2 '17 at 14:02
  • @syafrauf, you can't do it on-the-fly but, for example, you may see this question for more information. If it is not enough and you don't find other useful results after having searched on GIS SE, the best would be asking a new separate question by describing what exactly you need.
    – mgri
    May 2 '17 at 14:18
  • I would like to classify the DEM data from a sub-district into 4 categories and want to calculate the percentage of each classification. How to do in QGIS ?. Thank you for the help.
    – syafrauf
    Jun 22 '17 at 20:04
  • @syafrauf this would be a new separate question (being sure it wasn't already asked before).
    – mgri
    Jun 23 '17 at 6:00

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