LiDAR guide · an IPR project
Using it in QGIS
With the DTM loaded, four products answer most questions about a piece of land. All with free tools that already ship with QGIS.
0. Preparation
- Merge the tiles into a virtual raster: Raster › Miscellaneous › Build Virtual Raster (VRT). It takes no space and QGIS treats it as a single file.
- Check the project reference system: ETRS89 / PT-TM06 (EPSG:3763). Working in the data's own system avoids reprojection and errors of several metres.
- Clip to your area with a margin of a few hundred metres (Raster › Extraction › Clip Raster by Mask Layer). Water comes from outside the boundary: the margin matters.
1. Contour lines
Raster › Extraction › Contour. A 1 m interval for design, 5 m for a general reading. At 50 cm the lines come out very jagged: for printing, smooth the DTM first (for example by resampling to 1 or 2 m) or smooth the lines afterwards.
2. Hillshade
Raster › Analysis › Hillshade. In multidirectional mode, terraces, old tracks and ditches appear. Place it under the other layers with some transparency.
3. Slope
Raster › Analysis › Slope, in percent or degrees. Useful classes to start with: up to 5% (flat), 5 to 15% (gentle), 15 to 30% (moderate), over 30% (steep). Slope decides where a machine can go, where erosion happens and where no track should be opened.
4. Watercourses and catchments
- Fill depressions: in the Processing Toolbox, SAGA "Fill sinks (Wang & Liu)" or the WhiteboxTools equivalent. At 50 cm there are many false depressions (bridges, culverts, walls) that cut the flow.
- Compute flow accumulation and direction with GRASS
r.watershed. - Choose a drained-area threshold to draw the network (try several) and extract catchments from the points that interest you.
For catchments of several square kilometres, use the 2 m DTM: the result is almost the same and the computation far quicker.
In the field
QField (free, Android, iOS and Windows) opens the same project on your phone, offline, with GPS on top of the contours and slopes.