How do surveyors measure accurately?

[Machine_Elf]

@Chronos

The surveying job that really impresses me is Lewis and Clark Caverns, MT. The caverns have one natural entrance, where people go in. At some point, they decided they wanted a separate exit, so tourists wouldn’t have to turn around and come back out the way they came in. To do this, they had to dig a tunnel through a few hundred feet of solid rock. And they had to dig from the outside in, because if they dug from the inside out they’d have no way to dispose of the rubble. So they had to accurately survey the three-dimensional location of their target point, through a long network of twisty, turny passages, then aim the tunnel at that point. I don’t know how many lines of sight it takes to get from the natural entrance to the exit room, but it has to be hundreds at least, and in some places the traversable cavern is only a meter or so across.

If you dig in the wrong spot, interesting things can happen.