Simple elevation profile service similar to google's elevation
service [1].

Demo installation using freeDEM [2] data:

http://karme.de/elevation-profile/demo?path=48.5,9|48.5,9.1&upsample=100&format=svg

(additional) features:

- open source

- seamless stacking of multiple DEMs (it is really simple to add
  another DEM dataset if the format is supported by gdal [3])

- upsample polyline (keeping original input points and inserting
  equidistant additional points if needed)

- wgs84 spheroid distance calculations

git repository at:
http://karme.de/git/elevation-profile

If http overhead is a problem you might wish to use the
elevation-profile tool providing a simple command line interface. You
can use it either directly via pipes or in combination with good old
(x)inetd [4,5] or socat [6]. For xinetd an example configuration is
included and installed by default. The default configuration binds to
127.0.0.1 only. If you want to limit access to a specific user on
localhost you can try using ident and hosts.allow/deny (unfortunately
the lua socket library doesn't support unix domain sockets, otherwise
I would change the default config to unix domain sockets). The lua
client example in lua/ uses socat.

Using apache2 I observed problems using mpm_event and I suggest to use
mpm_prefork for now:
a2dismod mpm_event ; a2enmod mpm_prefork

[1] https://developers.google.com/maps/documentation/elevation/index
    https://developers.google.com/maps/documentation/javascript/elevation
[2] http://karme.de/freedem
[3] http://www.gdal.org/formats_list.html
[4] http://en.wikipedia.org/wiki/Inetd
[5] http://www.xinetd.org/
[6] http://www.dest-unreach.org/socat/socat-version2.html