Brewer & Marlow

QGIS3 Processing Chain to analyze DTM with slope (steepness) and aspect (direction).

Processing Chain based on this article by Jon Reades, I recommend you to read it for further information : https://kingsgeocomputation.org/2016/03/16/aspect-slope-maps-in-qgis/

Data needed for this operation

• DTM (.tiff, .jpeg, .asc, etc...)
• Style file

First Step

Fetch the DTM in the input path, check their projection and then mosaic them. The script is using : - gdal tool "assign projection". - saga tool "mosaic raster layers"

Second Step

Use the Mosaic to create two rasters : - a raster for slopes - a raster for aspect

The script is using grass tool "r.slope.aspect"

Third Step

Those rasters need to be reclassified. We need to create rules for each raster. Those rules can be changed if needed.

aspect = [
    "0.0 thru 22.499 = 10 \n",
    "22.5 thru 67.499 = 20 \n",
    "67.5 thru 112.499 = 30 \n",
    "112.5 thru 157.499 = 40 \n",
    "157.5 thru 202.499 = 50 \n",
    "202.5 thru 247.499 = 60 \n",
    "247.5 thru 292.499 = 70 \n",
    "292.5 thru 337.499 = 80 \n",
    "337.5 thru 360.5 = 10 \n"
]

slope = [
    "0.0 thru 4.999 = 0 \n",
    "5.0 thru 9.999 = 2 \n",
    "10.0 thru 19.999 = 4 \n",
    "20.0 thru 44.999 = 6 \n",
    "45.0 thru 100.0 = 8 \n"
]

We can now launch the reclassification using grass tool "r.reclass".

Fourth Step

Combine the rasters to make one file. Every pixel have a value between 10 and 88. The tens digit indicates the aspect, a number represent an angle of 45°. The units digit indicates the slope :

- 0 : gentle slope
- 2 : moderate slope
- 4 : strong slope
- 6 : very strong slope
- 8 : steep slope

The script is using grass tool "r.mapcalc.simple". Once the raster is created we add the style. The colors are based on the aspect. The saturation on the steepness.

Processing Chain based on this article by Jon Reades : https://kingsgeocomputation.org/2016/03/16/aspect-slope-maps-in-qgis/