On this occasion we will see how to generate a Digital Elevation Model (DEM) that reflects the shapes of the terrain using various vector entities.
In QGIS 3 it is possible to generate surfaces of a variable by interpolation, generally using coordinate points with elevation information.
However, when you want to model a portion of the earth’s surface the problem is different, the relief has known shapes and details that must be reproduced.
This article is part of a sequence, after the creation of the DEM (download practice exercises), will incorporate a dike, will estimate the water mirror and the calculation of the volume dammed.
Evaluation of a digital terrain model:
Like other interpolated surfaces, digital terrain models are evaluated for their horizontal and vertical accuracy.
Also, it is of interest the adequate representation of the forms of the terrain, that allows, the hydrological, geomorphological, climatic modeling, among others.
Digital terrain model in QGIS 3
In order to generate a digital terrain model according to QGIS 3 terrain shapes, it uses the TIN interpolation method (networks of irregular triangles).
The TIN method accepts different input vector entities, each influencing the triangulation differently:
- Elevation points.
- Structure lines: continuous lines that add information to the surface.
- Rupture lines: produce marked changes on the surface, for example to represent rivers, roads, coastlines.
Case study:
Generation of a DEM of a small surface of steep relief within an agricultural farm.
Procedure
Load the vector files into the map view.
You may notice that we have entities of points, lines (hydrography, contour lines, peaks and slopes) and polygons (study area).
To access the interpolation tools go to Process Menu > Process Toolbox, in the Panel select the Interpolation tab, then TIN Interpolation.
Digital model with elevation points
To compare the effect of introducing break lines, interpolation is first generated using elevation points extracted from the contour lines.
In the Vector Layer tab select the point layers, in Point Attribute select the Elevation field. Then click on the button with the green plus sign to add the variable.
QGIS 3 offers two methods of generating TIN, Linear and Clough-Toucher, the second produces a smoother surface.
On the Extension tab click on the corner button > Use Canvas Layer Extension.
In the Select Extent dialog box select the Area layer.
You can set the pixel size in the options Number of Rows and Number of Columns, a smaller pixel size generates a better defined surface.
For more details on how to adjust pixel dimensions refer to the Spatial Interpolation article in QGIS 3.
Finally you can define the output files.
In Interpolated the name and location of the surface
Then in Triangulation the vector with the network of triangles.
Digital model with breaking points and lines
Again, deploy the TIN interpolation tool and configure the layers.
Enter the Points layer in the same way as above.
Then select in the Vector Layer tab > contour lines and in Attribute the Elevation field, add it to the list, in Type > Break Lines.
Then, for the Shelves layer select the option “Use z coordinates for interpolation“, in Type > Structure Line.
For the Hydro layer select the option “Use z coordinates for interpolation”, in Type > Break Line.
Configure the other options in the same way as above.
Finally, compare both surfaces.
