Here’s a 5-chapter tutorial on the new QGis feature in version 3.26: elevation profiles.
Chapter 3: Profiles from point clouds (Lidar)
Chapter 4: Profiles layout
Chapter 5: Profiles in QField
In this third chapter, we’ll look at two topics:
- The first is an option that exists for vector layers: feature EXTRUSION.
- Secondly, we’ll look at how to create profiles from point clouds, in particular LIDAR data.
The data required for this tutorial is available here.
Vector layer extrusion
Vector layers of entities other than those used for profile construction appear in profiles as points, in the case of point or line layers, or as lines in the case of polygons. This representation is consistent with the notion of intersection between these types of layers and a profile line. If the entities are of type XYZ, or if a Z value is assigned if they are of type XY, the entity representations will appear positioned on the profile.
Let’s take an example of a building layer (BDTopo), of type polygonZ.
As this is an XYZ layer, the intersections of buildings and the profile line appear positioned according to their Z with the terrain profile derived from contour lines (in green).
If the building is not positioned on the surface, this is because, in this layer, the buildings have a Z in the geometry, which corresponds to the average height of the buildings. In the attribute table, we observe several height data:
For a more accurate representation, we’re going to change the Z used for this layer to the minimum ground elevation. To do this, in the Properties window, set “Fixed to terrain” in Elevation lock and assign the field “Minimal_ground_altitude“.
Now the base of the buildings is slightly underground, which makes sense.
To take full advantage of the information available, we’ll use the extrusion function to best represent the buildings in cross-section.
In the Properties window, check the Enable extrusion box and assign the value of the “height” attribute
We can now see the cross-sections of the buildings on the profile
Profiles from point clouds (LIDAR)
If we load a LIdar point cloud into our project, it is automatically displayed on the current profile.
You’ll then notice the adjustment of extrusions and lidar points.
Unlike building polygons, which have X and Y dimensions, Lidar points have virtually no dimensions. It is therefore highly unlikely that our line will intersect any cloud points exactly. The extension uses a parameter called TOLERANCE to define a window size to the left and right of our profile line. All points within this band will be displayed in the profile.
In this image, we see the TOLERANCE value (5 metres) and its representation on the map window: the 5-metre band on either side of the profile line.
If we change the TOLERANCE to 15 meters on each side, we see how the point cloud becomes denser, as it takes into account many more points present in the tolerance band.
Finally, here’s the profile with only the LIDAR values: