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For a suitability analysis I need to define the suitability of slope from DEM values (in %) to build new trails.

Knowledges says that flat and rather steep areas are not suitable due to water runoff. Unfortunately, it is not possible to define threshold like

<5% nogo, 5%<x>60% go, >60% nogo.

So I was thinking about a fuzzy set. I use QGIS and do not want to install GRASS GIS (due to security settings on my macOS). SAGA GIS fuzzify allows to define gradients between 0 (fully flat) and 1, see

http://www.saga-gis.org/saga_tool_doc/2.2.3/grid_calculus_11.html

andenter image description here

Basically, this works for me. But: For further calculations my output should be in the range from 1 to 5, whereas

0 - 1 = NoGo
1 - 2 = not good
2 - 3 = take care
3 - 4 = good
4 - 5 = must go

So I wanted to recalculate the 0 to 1 gradient from SAGA GIS into my gradient.

0   =! 1
...
0.5 =! 3
...
1   =! 4

How can I do this? Is there a better fuzzy logic algorithm available than this one from SAGA GIS (apart from GRASS GIS)?

If I didn't find any solution, I would probably create a discrete assignment between the slope and my categories.

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    Whats the problem defining a region that satisfies <5% nogo, 5%<x>60% go, >60% nogo? Its a standard raster algebra calculation. Assuming you mean 5% < x < 60%... – Spacedman Jan 24 '18 at 20:18
  • it is the problem one often faces when categories are not clear to separate. think of this: 0-3 is absolute no go, 4 is medium, 5 is go. Okay, I could do more categories. But doing this again: 3.5 is better than 3 and worse than 4. I could – again – create another category. Finally, that would create an indefinite number of categories. And therefore I'd prefer a fuzzy set. – Kevkev770 Jan 25 '18 at 8:38
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Basically you are asking how to rescale a raster whose values range from 0-1 into a new range of 1-5, correct? The answer to this question highlights the formula how to do this https://stackoverflow.com/questions/1456000/rescaling-ranges.

Using the formula from that question

x := (c - a) * (z - y) / (b - a) + y

Where the current range is defined as; a = 0, b = 1. The new range is y = 1, z = 5 and c is the pixel value being rescaled. This makes a formula like:

x := (c - 0) * (5 - 1)/ (1 - 0) + y

Which simplifies to

x := 4c + 1

So in the raster calculator in QGIS you need to use the formula like this on the grid.

(raster@band * 4) + 1
  • Thank you @Liam! Basically, this is what I needed...I was just looking at the wrong place, though. I will do (raster@band * 3) + 1as I do not want a "Must Go" in this parameter (I just need the 5 classes for other indicators in my work as well). I will accept the answer as soon as I will have tested it! – Kevkev770 Jan 25 '18 at 8:41

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