Couldn't you just do this with a total station??ÿ
The problem is that you can plumb one point down into the tunnel to set up on. But then what are you going to backsight to orient your traverse from there?
Well, not knowing the shape or depth of the hole.. I assumed one could set a control point a foot or two below the rim and backsight that.
Thanks for all the tips and ideas guys..this thread was an education in itself. I just got back from this project... I didn't have to go through the manhole...I found another way in..I set a GPS control pair then total station traversed through a window into the old WW II era building and down a set of stairs and then through a hole in a wall in to the tunnel. No idea what it was used for back in the 1940's... it was 10 ft high and 9 feet wide for a 1/4 mile with a downward slope of maybe 10 degrees and ?ÿvery dark...head lamps and iPhone flashlights to help me find back sights and foresights ... nothing in there except asbestos ... the end of the tunnel lays ?ÿmany feet under the kitchen area of a modern building that belongs to the Alaska Marine Lines.?ÿ
wow! any cool pics you can share?