Add SRTM15_PLUS
We might want SRTM15_PLUS as well (http://topex.ucsd.edu/WWW_html/srtm30_plus.html --> first link).
The topo-file will be approx. 3GB. @carsten-schirnick: Is 3GB allowed by the max. request entity? I remember that you increased this limit to something like 2GB a while ago. Is this still valid?