Unit 15 - DTM reprojection

In this unit EU-DEM global dataset provided by Copernicus programme will be used. Jena region is covered by tile E40N30. Be aware that EU-DEM tiles covers large regions (4GB per tile).

Normally r.import could be used to import and automatically reproject data into target location. In this case there is no need to import into GRASS a whole big DEM tile. By using -r flag of r.in.gdal GRASS module only a subregion covering AOI could be imported. Unfortunately r.import do not offer such flag. In this unit manual reprojection will be presented. A new location for imported data (EPSG:4258) must be created and data reprojected into target jena-region (EPSG:32632) location manually.

Note

In order to save space of disk the original EU-DEM tile was clipped by larger region than Jena AOI. Modified tile is available from geodata/eu-dem directory.

Create location for data import

EU-DEM dataset is provided in ETRS-89 (EPSG:4258). Start a new GRASS session and create a new location jena-eu-dem based on input geodata/eu-dem/dem_sample.tif data (similarly as done in Unit 02). Skip data import provided by location wizard since only subregion defined by Jena city region is planned to be imported.

Fig. 87 Define name for a new location.

Location will be created based on input sample DEM file (Read projection and datum terms from a georeferenced data file).

Fig. 88 Create a new location based on a sample DEM file.

Fig. 89 Don’t import input data from Location Wizard.

Enter new location (PERMANENT mapset) and reproject Jena region vector map by v.proj (Vector ‣ Develop vector map ‣ Reproject vector map from different GRASS location) or simply from Data tab in Layer Manager just by copying and pasting the maps, see Fig. 90.

Fig. 90 Easy and quick way to reproject (copy-and-paste) raster or vector maps across locations from Data tab in Layer Manager.

v.proj location=jena-region mapset=PERMANENT input=jena_boundary

Before importing DEM set computation region extent based on reprojected Jena city region.

Fig. 91 Set computational region extent based on Jena city region.

Launch r.in.gdal (File ‣ Import raster data ‣ Import of common raster formats) to import DEM tile, don’t forget to enable -r flag.

r.in.gdal -r input=/home/user/geodata/eu-dem/dem_sample.tif output=dem

Fig. 92 Imported DEM limited to current region extent. Displayed with elevation color table.

Reproject DEM into target location

Now switch back to jena-region location (Settings ‣ GRASS working environment ‣ Change location and mapset).

Fig. 93 Switch GRASS to jena-region location and PERMANENT mapset.

Reproject DEM from jena-eu-dem location by r.proj (Raster ‣ Develop raster map ‣ Reproject raster map from different GRASS location) or directly from Data tab in Layer Manager. Set desired resolution (25m) and choose appropriate resampling method (in our case bilinear).

Note

If a mask is enabled, it’s good idea to remove it before importing.

Fig. 94 Reproject raster data in Layer Manager.

Example of reprojecting raster data using r.proj command below.

r.proj location=jena-eu-dem mapset=PERMANENT input=dem method=bilinear resolution=25

Fig. 95 DTM in 3D view.