DEM preparation

How to use digital elevation models (DEM)

Tangible Landscape typically runs with defined DEM (in scanning tab) which is used to georeference the scanned data, so that we can overlay other data (e.g., orthophotos) on the model.

DEM

Depending on the format of the available DEM, we import it into GRASS. The DEM is expected to be in raster format commonly used in GIS applications (for example GeoTIFF) and its spatial reference system (SRS) should be defined. To find DEMs see elevation data sources.

Using Location Wizard create a new Location based on the georeferenced raster (or XY in case the SRS is missing). You can import the raster automatically during creating the Location, or later using r.import module (File -> Import raster data).

To make things easy, the DEM should be rectangular. If it's not, trim the DEM:

1. Set the computational region to match the DEM (called dem for example). Run in GUI command console:

   g.region raster=dem

2. Zoom to see the entire raster. Then in Map Display toolbar -> Various zoom options -> Set computational region interactively -> draw the rectangle. Repeat if necessary. Use Zoom to computational region extent tool in that toolbar to inspect the selected extent. Then run:

   r.mapcalc "dem_trim = dem"

Enter the reference DEM name into the Reference DEM field in Tangible Landscape plugin (Scanning tab).

No DEM available

This is not recommended, since the system was not developed for this, but it's possible. In case you don't want to use any specific DEM, these are the steps:

1. Create new GRASS Location and in Location wizard select XY location. Start GRASS in PERMANENT or create new mapset. (Any other Location will work too, but XY Location is appropriate for this.)

2. Decide what model size you will have, and set computational region appropriately. For example, if your model is 35 cm x 30 cm, set the region to have the same aspect ratio and save it to a named region (run this in GUI command console):

   g.region s=0 w=0 e=350 n=300 t=0 b=0 save=ref_region

3. Set this named region into Reference region (see Tangible Landscape plugin: scanning parameters).

4. Then the scan raster will have horizontal coordinates between 0 - 350, however the values of the raster depend on the scale.

5. Don't forget that for computing contours, for example, first check the vertical range (r.info map=scan) and set the contour interval appropriately.