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sitedesign:site_terrainmod

Terrain modelling for site design

As well as for hydrological modelling, GIS provide extended capacity for modelling landscape. Again we will use the Digital Elevation Models to extract key features for keyline and permaculture site design. Specifically we are interested in the following feature:

  • Terrain slope and locations of extremes steep slopes
  • Terrain slope orientation
  • Contour lines
  • The main ridge
  • The primary valley
  • The primary ridge

For this tutorial we have also used informations from the article on Keyline design -Soil, Water & Carbon for Every Farm - Building Soils, Harvesting Rainwater, Storing Carbon. Abe Collins & Darren J. Doherty.

As a preliminary step of data processing we will overwrite the projection of our DEM map to the EPSG: 26915 code and we will use this projection reference system. Thi can be done in Qgis using the Raster tab > Projections > Warp tool.
Or using the terminal command line using the following code:

gdalwarp -t_srs EPSG:26915 \\
ost4sem/siteplan/data/raster/farm_dem.tif \\
ost4sem/siteplan/data/raster/farm_dem26915.tif

Terrain slope

The stepensess of the landscape is a crucial element for the water flow leading to soil erosion or flooding events to the downstream slopes. A key feature of the Keyline design is the definition of Keypoint (i.e. the point at which the primary valley gets suddenly steeper).
We use GIS for extracting slope angles from DEM and then highlight areas of extremes slope steepens.

  • In Qgis we use the Processing tab > toolbox
  • GDAL/OGR
  • Analisys
  • Slope and select the below options:

To produce the following map with low to high steep surfaces in white to black colours.

Terrain orientation

Terrain orientation (in GIS aspect or slope exposure) is computed in Qgis using the Processing > toolbox > e GDAL/OGR > Analysis > Aspect function. We can use the parameters below: And produce the Orientation map using colours in dark-light yellow for south exposures slopes and blue-grey colours for n-e n-w slopes.

Contour lines

In cartography, a contour line (or isoline) traces points of equal elevation (height) above a given level, such as mean sea level. In Qgis

  • Processing
  • Toolbox
  • GDAL/OGR
  • Extraction
  • Contour
  • Select farm_dem26915 as Input layer
  • Select 1m as interval between contour lines
  • In contour click on Save file /home/user/ost4sem/siteplan/data/vector/contour/contour-1m.shp
  • Run

The output vector map overlayed with DEM:

Landforms - Geomorphons

In a stabilized landscape, there are three land shapes we consider in relation to Keyline development: the main ridge, primary valley and primary ridge.

The main ridge

The main ridge is the summit line of two valley, it begins at the convergence of two water drainage lines. The crest of a ridge is synonymous with a water divide line. The crest of a ridge is usually less steep than the sides of the ridge.

The primary valley

The primary valley is run from the ridge to the downslope water creek or main stream. A primary valley has a primary ridge on either side. Primary valley are located at the extremes convex points of contour lines descending from a primary ridge to the bottom valley.

The primary ridge

Primary Valleys are divided by Primary Ridges. Primary ridges are located at the extremes concave points of contour lines descending from a primary ridge to the bottom valley.

In GIS we might use different terminology for landforms. In GRASS GIS there is an excellent algorithm r.geomorphon able to produce the above landforms and classify the landscape into: 1 flat, 2 peak, 3 ridge, 4 shoulder, 5 spur, 6 slope, 7 hollow, 8 footslope, 9 valley, and 10 pit.
We can use the University of Cincinnaty's web processing service to process data online using GRASS and download the geomorphon features. Upload the DEM and using a search radius of 25m we produce the map below. In geomorphons terminology, the Main ridge correspond to class 3- Ridge; Primary valley correspond to class 5 - Spur and Primary ridge correspond to class 7- Hollow.

sitedesign/site_terrainmod.txt · Last modified: 2021/01/20 20:36 (external edit)