Per default elevation is disabled. But you can easily enable it:
graph.elevation.provider=cgiar
or
graph.elevation.provider=srtm
then GraphHopper will automatically download the necessary data for the area and include elevation for all vehicles - making also the distances a bit more precise.
The default cache directory (/tmp/srtm) will be used. For large areas it is highly recommended to
use a SSD disc, thus you need to specify the cache directory:
graph.elevation.cachedir=/myssd/ele_cache/
All should work automatically. Another setting is to specify the location where the files are
downloaded - e.g. if the servers are not reachable, then you set:
graph.elevation.baseurl
E.g. for CGIAR there are two URLs you can use: http://droppr.org/srtm/v4.1/6_5x5_TIFs and
http://srtm.csi.cgiar.org/SRT-ZIP/SRTM_V41/SRTM_Data_GeoTiff/
where the last one is only accessibly if you specify the
full zip file
If the area is small and you need a faster import you can change the default MMAP setting to:
graph.elevation.dataaccess=RAM_STORE
The CGIAR data is preferred because of the quality but is in general not public domain. But we got a license for our and our users' usage: https://graphhopper.com/public/license/CGIAR.txt
Using SRTM instead CGIAR has the minor advantage of a faster download, especially for smaller areas.
Integrating your own elevation data is easy and just requires you to implement the ElevationProvider interface and then specify it via GraphHopper.setElevationProvider. Have a look in the existing implementations for a simple overview of caching and DataAccess usage.