Depth below datum

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Depth below datum

Postby GroundquestMSA » Jan 22, 2014 7:30 pm

I know I've read a bit about this before, probably on this forum, but now I can't find it.
When indicating the depth below or above 0 datum, is it acceptable to measure to the floor, or must the station be used as the reference? For example I have this shot in my book from last weeks project: 56.5' distance, 85 az., -7.5 inc., 15/19/13/5.5 LRUD (at to station).

The vertical difference between the stations is 6.6', but the floor is 5.5' below the to station. Assuming the from station sat on the floor and is my 0 datum, should my depth symbol read -6.6' or -12.1' ?
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Re: Depth below datum

Postby ian mckenzie » Jan 23, 2014 7:32 pm

You can easily resolve this by adding another station to your notes, with a distance of -5.5ft, clino -90 degrees, compass 000 degrees. If you're in the middle of a survey transit, just consider this vertical shot as a branch, e.g. station 45 to station 45a.
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Re: Depth below datum

Postby GroundquestMSA » Jan 23, 2014 9:05 pm

That would certainly do the trick, and satisfy anyone who insists that the depth of a cave is the distance between highest and lowest stations. I process my data with a calculator and plot everything by hand, so this step isn't really neccesary since I can get the same result by subtracting the distance down from the below or above datum figure. Of course, if I did add a station at every point I wanted to display depth on the map, I could increase the surveyed length of the cave...

I realize that it doesn't much matter, but I thought I would ask. The cave I just surveyed had it's first station on the ground, and the next 14 or so at head level. It was a very flat cave, so the depth could be 18' or 12', depending on your methods of measuring depth. I tend to think that the cave is the same depth, no matter who surveys it or how. I generally measure depth from the highest accessible floor to the lowest accesible floor.
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Re: Depth below datum

Postby Chads93GT » Jan 23, 2014 11:58 pm

by hand? wow old school. You need to learn to use walls :)

Keep up the good work.
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Re: Depth below datum

Postby Extremeophile » Jan 24, 2014 8:07 am

I think you're overthinking this. Nearly all cartographers just use the elevation of the station. If the cross-section dimensions are greater than the overall cave dimensions then you need to find some bigger caves to survey :big grin:
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Re: Depth below datum

Postby driggs » Jan 24, 2014 9:42 am

It is more correct to list depth/elevation consistently at floor level than at arbitrary station locations, provided you do it consistently.

Subtracting the "down" measurement from your station elevation doesn't add footage to your cave any more than doing splay shots at every station would double the amount of passage in your cave. It's simply a splay shot down.

For dealing with absolute cave depth (or absolute passage length), simply shoot your final survey shot to the floor or passage terminus. Do double-forward shots if necessary. In tiny caves, the couple extra feet you gain are significant!

PS: Seriously, start using Compass or Walls or Survex or Therion or anything rather than a pencil and trigonometry. The benefits are overwhelmingly numerous. Spend your time drafting, not doing 9th grade math and then re-doing all your drafting after you discover you botched a shot.
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Re: Depth below datum

Postby GroundquestMSA » Jan 24, 2014 10:38 am

Extremeophile wrote:I think you're overthinking this. Nearly all cartographers just use the elevation of the station. If the cross-section dimensions are greater than the overall cave dimensions then you need to find some bigger caves to survey :big grin:


I do have a problem with overthinking. However, I don't understand what you are saying in the third sentence. Sure, I'm surveying very trivial caves, but I don't know what that has to do with my question. The cave I'm working on now is 1089' long, and depending on where I measure my elevations, either 30.9' or 25.4' deep. Again, it really doesn't matter, but I think the 30.9' figure is more accurate.

driggs wrote:It is more correct to list depth/elevation consistently at floor level than at arbitrary station locations, provided you do it consistently.

That's how I feel, and what I've been doing.

driggs wrote:Subtracting the "down" measurement from your station elevation doesn't add footage to your cave any more than doing splay shots at every station would double the amount of passage in your cave. It's simply a splay shot down.

I see. I was assuming that entering these shots into Compass (or similar) would result in them being counted as cave length. Obviously, I don't know much about that.
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Re: Depth below datum

Postby driggs » Jan 24, 2014 10:50 am

GroundquestMSA wrote:I was assuming that entering these shots into Compass (or similar) would result in them being counted as cave length. Obviously, I don't know much about that.


Compass lets you assign "flags" to any shot, which instruct the software to exclude the shot from cave length, from plotting, from loop closure, etc.

It will also display elevations on your lineplot... but will do so at the survey station rather than the floor level. Back to the ol' calculator! :wink:
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Re: Depth below datum

Postby GroundquestMSA » Jan 24, 2014 10:57 am

driggs wrote:PS: Seriously, start using Compass or Walls or Survex or Therion or anything rather than a pencil and trigonometry. The benefits are overwhelmingly numerous. Spend your time drafting, not doing 9th grade math and then re-doing all your drafting after you discover you botched a shot.


I would like to. I have Compass and Inkscape on my computer but I don't know how to use them. That would certainly make my maps look better. The online tutorials are hard for me to understand. I didn't own a computer until after I got married, and I'm slow to learn the language of technology. Still, I enjoy plotting the survey with a pencil and basic math. And while I have screwed up a shot and had to re-draw an entire line plot, it doesn't much matter when the cave is 112' long. As soon as I get my first mile-long cave, I'll really try to learn Compass.
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Re: Depth below datum

Postby Extremeophile » Jan 24, 2014 1:01 pm

GroundquestMSA wrote:I don't understand what you are saying in the third sentence. Sure, I'm surveying very trivial caves, but I don't know what that has to do with my question. The cave I'm working on now is 1089' long, and depending on where I measure my elevations, either 30.9' or 25.4' deep. Again, it really doesn't matter, but I think the 30.9' figure is more accurate.

You'll have to forgive me for making light of a situation where the station reference point significantly impacts your overall cave depth. You are right that 30.9 vs 25.4 doesn't matter. I don't think you have measurement precision of plus or minus 0.1 feet to begin with, and what possible difference does 5 feet of depth make to anyone :shrug: Maybe one number is more accurate than the other, but it's only accurate relative to some arbitrary definition you determine that isn't necessarily the same definition used elsewhere. My advice is to just use the station elevation for depth and spend your time worrying about other more significant accuracy concerns.
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Re: Depth below datum

Postby GroundquestMSA » Jan 24, 2014 6:27 pm

Hey, I don't really care. I simply suspected that there may be a standard way of thinking about this. No, 5' makes no difference to anyone. I could also guess every other azimuth, estimate passage dimensions, and draw 367' of imaginary passage in the back of the cave and no one would ever know the difference. As is the case with most (all?) small cave surveys, neither precision nor accuracy are life and death concerns.

Extremeophile wrote:I don't think you have measurement precision of plus or minus 0.1 feet to begin with

Hey now, are you questioning my inclinomoter reading skills, my laser button pushing skills or my math skills? :big grin:
You're right, I'm sure some rounding is in order.
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Re: Depth below datum

Postby Extremeophile » Jan 24, 2014 6:41 pm

Almost certainly beating a dead horse here, but one final thought ... I always label my maps as "surveyed length" and "surveyed depth". The actual length and depth are wide open to an endless debate, and all surveys contain errors that impact accuracy (random, systematic, and blunders).
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Re: Depth below datum

Postby ian mckenzie » Jan 26, 2014 2:32 am

Ah, but... surveyed length is always just a relative indicator of a cave's length since you can never really agree on how to measure all of a cave's length (there is much debate on which survey shots qualify), while surveyed depth is an exact number as you certainly can measure all of a cave's depth, provided you adhere to certain commonly accepted rules.

In other words: Length can vary depending on how you treat large rooms vs tight passages, alcoves vs side tunnels etc. - but the only variation to a cave's measured depth is a change in accuracy of your measurements as the highest and lowest points don't change.
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Re: Depth below datum

Postby trogman » Jan 26, 2014 10:21 am

I always interpret vertical extent as the true vertical extent of the cave, and not of my arbitrary choice of where to place survey stations. Lets say I am surveying a mostly horizontal cave, and then come into a dome that rises 100' up. My survey station may be on or near the floor, but the cave itself extends 100' higher. Therefore that is part of the vertical extent of the cave. I'm not sure why you would report it any other way. I wouldn't count the 100' high dome as part of the surveyed passage, unless I or someone else had climbed it. But with laser measurement devices, one can accurately measure such features, and they should be included in survey statistics.

Trogman :helmet:
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Re: Depth below datum

Postby ian mckenzie » Jan 26, 2014 11:41 am

Agree, and the commonly-accepted rules of surveying for depth agree too. But I have a bit of a question about your language: you wouldn't include the dome in the "surveyed passage", but would include it in the "cave statistics" - how would you do that?
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