a waypoint-block per line in the surface statement should be enougth to create manually something and still control it free. I've attached an idea.
Your sketched idea looks nice. Now for someone who can write the code! I expect it is not a small task to write that and integrate it into Therion. :) Of course there is a work around you could use, with current Therion capabilities. http://therion.speleo.sk/wiki/doku.php/contrib:terrainmodelling#surface_prof iles ie using GPS Visualizer to create a profile or create your own gps trail by walking the route, then creating a surface survey as described on the wiki link above. Now you can draw scraps on the surface survey to create pdf representations and label it as you wish. Maybe a new line type is needed, u:ground that would look similar to wall:sand or wall:clay, to distinguish between 'above ground' and 'below ground'. That would go a small way to working around the problem you mentioned of the background colour filling in the sky. Bruce