[Therion] LOX: Measure distance from station to surface

[Tarquin Wilton-Jones]

> enable surface in aven?
> If i try to enable the surface legs, aven prompts me to open some file...

I assume you are trying to "file - open terrain" rather than "view -
surface survey legs".

Terrain is a different thing, which I have never used, and I don't think
you can select parts of it for use with measuring. I think though that
Survex's terrain is similar to the type of terrain used by Therion though.