Improving QGIS raster calculation performance?

Question

I'm performing raster analysis on a QGIS 2.14 project CRS is EPSG: 32632. I used the official 10m DEM extracting the area of interest of 446 raster cells. I've made a slope map in 5 classes, and this task was accomplished without problems.

Then I used Raster Calculator to reclass the slope map assigning weight to each class as follows:

0-10%  -> 2; 11-20% -> 1; 21-35% -> 0; 36-50% -> -1; >50%   -> -2

So I inserted the following expression:

( "Slope_clip_UTM@1" < 11 )  * 2 +  (  ( "Slope_clip_UTM@1" >= 11 )  AND  ( "Slope_clip_UTM@1" < 20 )  )  * 1 +  (  ( "Slope_clip_UTM@1" >= 20 )  AND  ( "Slope_clip_UTM@1" < 36 )  )  * 0 +  (  ( "Slope_clip_UTM@1" >= 36 )  AND  ( "Slope_clip_UTM@1" < 50 )  )  * -1 +  ( "Slope_clip_UTM@1" >= 50 )*-2*

First I've performed raster calculation on a slope raster the CRS of which was EPSG 3003 by default and I obtained an empty raster (no cells and no values), but when I carghed Min/Max values they were both equal 2. Latter attempt was made right after saving the slope raster in EPSG 32632, the same of the Project: this time as a result I obtained an uniform raster in wich the values of each cell is equal to 0; but when I charged Min/Max this values are both equal to 1.79769e+308.

How do I fix the problem?

Answer 1

Usually I use Reclassify values under SAGA to reclassify data into specific values.

To reclassify a slope data into slope ranges, the following steps can lead us to the desired result:

• Go to Processing toolbox → SAGA → Raster tools → Reclassify values

• Select Slope percent file
• Method: Simple table

• Lookup Table click on the … a new window will open

• In the example, the slope data were classified into six classes, so you need to change the values based desired range and output

• Reclassified grid: write a name for the new classified slope data.

Please make sure that you have selected the correct projection when you created the slope percent to give you the desired output from reclassify tool.