[Therion] LOX: Measure distance from station to surface

[Tarquin Wilton-Jones]

> However, my aven does not enable me to avtivate it in the view menu, its
> disabled (greyed out), i assume because i do not have surface data in
> the file?

Sounds like it. It works based on legs that have:

flags surface
1 2 9.8 123 0
flags not surface

Or if you have used TerrainTool to export it as a grid, it will have
added that for you.

This has been very easy for us in our projects because we either knew
exactly which line to follow on the surface beforehand, or we surveyed
the cave first then surveyed over the surface afterwards, staying above
the passage so we could have a useful measure of the surface above the cave.

In more complex caves, I rely on TerrainTool to cover the surface.
Looking forward to being able to use the new more accurate NASADEM so
that the measurements are actually accurate.