[Tagging] tagging of "ele" / elevation data e.g. in the context of towers

[Tobias Knerr]

John F. Eldredge:
> If a structure is located on sloping ground, do you record the elevation of the highest point in contact with the structure, the lowest point, halfway between the highest and lowest points, or what?

This is related to the question: Where do you measure the structure's 
height from? To allow the "top = ele + height" calculation that has been 
mentioned in this thread, it would be desirable if the answer to both 
was the same.

At the last 3D rendering workshop I attended, the general idea appeared 
to be to measure height from the lowest point in contact with the 
ground. But it's indeed not properly documented anywhere afaict. And 
while it seems intuitive enough for height, it might not appear that way 
for ele.

Tobias