Use_SRTM_data_as_Digital_Elevation_Model_(DEM)

Oliver Eichler Martin Swift

Digital Elevation Models (DEMs) allow you to:

  • see the elevation below the cursor,
  • use the contour shading, and
  • display the map in 3D.

You can find a good collection from the SRTM at Viewfinder Panoramas' DEM page.

Once you have downloaded the appropriate files for your area you have to convert them into a format that QLandkarte GT understands. This is very easy for vector maps, as they can adjust to the projection and datum of the SRTM data. For raster maps, however, the data must also have the correct projection and datum. No panic! QLandkarte GT will tell you if something is wrong.

Preparing an SRTM data file

To prepare your own SRTM data file, you can use GDAL which (if you've gotten QLandkarte GT to work on your computer) you already have installed on your computer. Windows users: GDAL provides a shell (a text input window where you type in commands). Use it.

To convert a file from the server into a geotiff you need to run:

gdalwarp -t_srs "+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs " -r cubic <sourcefile> <target>.tif

See the gdalwarp help page for information about the various options to the command.

Again: For raster maps, you will have to use the correct projection and datum. Replace the projection string (

+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs

) with that which is used for your map. If there is a mismatch between the map and the DEM data QLandkarte GT will tell you and display the map's string, too.

To stitch several files together use:

gdalwarp -r cubic  <file1> <file2> .... tmp.tif

And finally you optimize the file size by:

gdal_translate -co tiled=yes -co blockxsize=256 -co blockysize=256 -co compress=deflate -co predictor=1 tmp.tif srtm.tif

Done. To automate this, QLandkarte GT comes with

srtmconvert.py

, a Python script found in the root of the source archive (the bundle you downloaded). The script will convert all files in a particular directory, stitch them and optimize the result.

To use the created file as digital elevation model you have to link it with the map. Simply right click with your mouse on the map entry in the left hand map list. Choose "Add DEM" from the context menu.If everything went well you should see elevation information in the statusbar when moving the mouse. If that works all the other features like 3D view and elevation profile for artificial tracks should work, too.


Related

Wiki: A_Grand_Day_Out
Wiki: QLandkarte_GT
Wiki: Quick_introduction_to_QLandkarteGT_for_Power_Network_users