5.2 Elevation Data

The Consortium for Spatial Information (CGIAR CSI) make Digital Elevation Model data covering about 80% of the globe, available for download. It has been sourced and enhanced from data gathered by the NASA Shuttle Radar Topographic Mission (SRTM).

From the main page of the CGIAR CSI website, follow the link to SRTM Data to download zip files that contain tiff files with 5m x 5m elevation data.

Extract the tiff files to a folder, e.g. /var/local/elevation-data and configure an elevation section in config.yaml, e.g.

elevation:
  tileCacheMs: 60000
  datasetDir: /var/local/elevation-data/

When the Trip Server application is started, it reads all the tiff files in the folder specified by the elevation.datasetDir parameter and creates an in memory index containing the area covered by each tile. When elevation data is required for a specific location, the relevant tile is loaded, the response provided, and the tile retained in memory for the number of milliseconds specified by the elevation.tileCacheMs parameter.

The tiff files take up a lot of space. Where space is at a premium, consider storing them in a compressed file system, e.g. on Linux use Squashfs.

e.g.

1. Download files to ~/downloads/srtm

   $ mkdir -p ~/downloads/srtm
   $ cd ~/downloads/srtm
   $ wget http://srtm.csi.cgiar.org/wp-content/uploads/files/srtm_5x5/tiff/srtm_72_22.zip
   

2. Extract the tiff files to ~/tmp/tiff

   $ mkdir -p ~/tmp/tiff
   $ cd ~/tmp/tiff
   $ find ~/downlods/srtm -name '*.zip' -exec unzip -n '{}' '*.tif' \;
   

3. Create a Squashfs compressed file containing the tiff images

   $ mksquashfs ~/tmp/tiff /var/local/elevation-data.squashfs -no-recovery
   

   The -no-recovery option is to stop Squashfs leaving a recovery file behind in the destination folder. However, it does mean that should the operation fail, there is no recovery information to unwind the command. This is probably more of a potential problem when appending to an existing Squashfs file.

4. Optionally, delete or archive the downloaded zip files to free up space.
5. Download more files, extract them and squash them using the above steps. Repeating the mksquashfs command as above will append to an existing Squashfs file.
6. You can list the contents of the Squashfs file with:

   $ unsquashfs -i -ll /var/local/elevation-data.squashfs
   

7. Test mounting the Squashfs file

   $ mkdir -p /var/local/elevation-data/
   $ sudo mount -t squashfs /var/local/elevation-data.squashfs /var/local/elevation-data/
   $ ls /var/local/elevation-data/
   $ sudo umount /var/local/elevation-data
   

8. Add an entry to /etc/fstab to mount the Squashfs file on boot:

   $ echo '/var/local/elevation-data.squashfs \
   /var/local/elevation-data squashfs ro,defaults 0 0' \
   | sudo tee -a /etc/fstab
   

9. Mount using the /etc/fstab entry:

   $ sudo mount /var/local/elevation-data
   $ ls /var/local/elevation-data
   $ sudo umount /var/local/elevation-data
   

10. If need be in the future, you can extract the files from the Squashfs file with:

    $ unsquashfs -i /var/local/elevation-data.squashfs
    

    Which will extract all the files to a sub-folder of the current working folder named squashfs-root.

    Use the -f parameter if the squashfs-root folder already exists.

11. To extract select files, create another file containing the names of the files to be extracted, prefixed by a forward-slash. e.g. /srtm_11_03.tiff.
12. $ unsquashfs -i -e list-of-files.txt /var/local/elevation-data.squashfs
13. See SquashFS HOWTO for more information