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Correct way to measure pit depth

PostPosted: Dec 24, 2013 10:57 am
by BryantG
I have a fairly simple question about the appropriate way to survey a pit.

Description of pit:
A very steep funnel shaped sink with a narrow entrance at the bottom that the caver must semi-corkscrew through to pass. After this the caver drops roughly 10 feet to a muddy ledge where the next drop below the ledge is 50-60 feet. At this point the caver is standing on another muddy, steep ledge which slopes down to a fluted well which appears to be 80-100 feet deep. At the bottom of this the caver can safely get off rope and get out of the rockfall zone.
Note: Depths are conservatively estimated but I rigged a 200 ft rope to a tree directly beside the entrance sink and all that was touching the base of the pit was my knot and 2 feet of rope.

Question:
What is the best way to delineate the depth of this pit via survey?

Any answers will be greatly appreciated. Thanks.

--Bryant

Re: Correct way to measure pit depth

PostPosted: Dec 24, 2013 12:21 pm
by trogman
BryantG wrote:I have a fairly simple question about the appropriate way to survey a pit.


A "simple question?" LOL that's funny! Cavers have debated and discussed this question for years. I hope you get some good answers, but I will withhold my opinion until I see what more knowledgable persons have to say on the subject.

Trogman :helmet:

Re: Correct way to measure pit depth

PostPosted: Dec 24, 2013 3:01 pm
by Chads93GT
I start the vertical measurement where the sink breaks from slope to vertical where the rope is rigged. That doesnt mean a 90' pit I survey is a 90' rappel, it just means its vertical drop from the lip is 90' I also put notes on the map noting the total depth of the entrance. IE if you fall in, how far are you going to fall before you finally stop bouncing off the walls and ledges.

I just finished a map that had an 84' drop but a total of 90' to the true floor of the pit area, ie, wher eyou would most like come to a rest if you fell in, or threw a rock in. thats just how I do it. use notes on the map. there are no rules. The more information the better.

Re: Correct way to measure pit depth

PostPosted: Dec 24, 2013 3:54 pm
by BryantG
That is helpful to me in deciding what to consider the starting point of my pit. I like the idea of starting from the top of the sink itself.

In my instance the pit that will be surveyed has two distinct shafts with muddy slopes at the top of each of them. On either slope I would not even begin to consider getting off rope as it is all very steep, loose, muddy/wet. What is the best way to separately measure the shafts?

--Bryant