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?
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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):
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:
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.