Altimetry (DEM)

Keywords

DEM, MNT, raster, QGIS

Refer to this section to learn about the available downloadable data sources.

Warning

  • We recommend not importing a DEM with too precise resolution for performance reasons. For example, the BD Alti DEM with a 25m resolution is perfect to cover a department.

  • If you downloaded the BD Alti or RGE Alti, you will need to convert the .asc DEM to .tif format (e.g. with QGIS) before uploading it on the server.

In QGIS, you can visualize your DEM, or merge several tiles together (in Raster > Misc > Merge).

Generate a GeoTIFF, and upload both files (.tif + .tfw) on the server. And use the Geotrek-admin command to load it into PostGIS :

sudo geotrek loaddem <PATH>/dem.tif

Note

  • This command makes use of GDAL and raster2pgsql internally. It therefore supports all GDAL raster input formats. You can list these formats with the command raster2pgsql -G.

  • The elevation data of DEM must be integer values. If the elevation data are floating numbers, you can convert them in integer values with the Raster calculator processing of SAGA in QGis (Processing > Toolbox > SAGA > Raster calculus > Raster calculator) with formula parameter set to int(a).

  • If you only have a .tif file, you can generate the .tfw file with the command gdal_translate -co "TFW=YES" in.tif out.tif. It will generate a new .tif file with its .tfw metadata file.

  • If you want to update the altimetry of the topologies you need to use the option --update-altimery

Load DEM

sudo geotrek help loaddem
usage: manage.py loaddem [-h] [--replace]
                                   [--update-altimetry]
                                   [--version]
                   [-v {0,1,2,3}] [--settings SETTINGS]
                   [--pythonpath PYTHONPATH] [--traceback] [--no-color]
                   [--force-color] [--skip-checks]
                   dem_path

    Load DEM data (projecting and clipping it if necessary). You may need to create a GDAL Virtual Raster if your DEM is composed of several files.

    positional arguments:
        dem_path

    optional arguments:
    -h, --help            show this help message and exit
    --replace             Replace existing DEM if any.
    --update-altimetry    Update altimetry of all 3D geometries, /!\ This option
                                  takes lot of time to perform
    --version             Show program's version number and exit.
    -v {0,1,2,3}, --verbosity {0,1,2,3}
                                  Verbosity level; 0=minimal output, 1=normal output,
                                  2=verbose output, 3=very verbose output
    --settings SETTINGS   The Python path to a settings module, e.g.
                                  "myproject.settings.main". If this isn't provided, the
                                  DJANGO_SETTINGS_MODULE environment variable will be
                                  used.
    --pythonpath PYTHONPATH
                                  A directory to add to the Python path, e.g.
                                  "/home/djangoprojects/myproject".
    --traceback           Raise on CommandError exceptions.
    --no-color            Don't colorize the command output.
    --force-color         Force colorization of the command output.
    --skip-checks         Skip system checks.

Import command examples :

sudo geotrek loaddem \
.var/conf/dem.tif \
--replace \
--update-altimetry

docker compose run --rm web ./manage.py loaddem \
.var/conf/dem.tif \
--replace \
--update-altimetry