Depth below datum

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Re: Depth below datum

Postby trogman » Jan 26, 2014 5:13 pm

ian mckenzie wrote: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?


On my maps I report the "vertical extent." That is part of the cave statistics, and the dome height would be part of the vertical extent.
In AL the cave survey rules state that surveyed passage must be 'humanly traversable." That would exclude measuring cracks in the floor or ceiling or wherever and adding them to the surveyed length.
I hope that answered your question.

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Re: Depth below datum

Postby Extremeophile » Jan 26, 2014 10:21 pm

ian mckenzie wrote: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.

I'm not sure I'm completely following your argument. How do you know what the highest or lowest points are unless they're surveyed. Yes, there is judgement involved in whether or not survey shots get excluded or not, and where to survey, but the surveyed length is the summation of all of the included survey shots, and that is a precise value. I can come up with a dozen or more scenarios where the difference between the high and low point are just as open to interpretation, but again, the surveyed depth is the difference between the high and low points in the measured data and this is an objective measure. It seems to me that a 100' dome is significant, should be included in the depth, and therefore should be surveyed so that it will be included.
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Re: Depth below datum

Postby trogman » Jan 27, 2014 7:28 am

Extremeophile wrote:I'm not sure I'm completely following your argument. How do you know what the highest or lowest points are unless they're surveyed.



That's just it-they are surveyed. If you establish a station and measure an LRUD at that station, is that not part of your survey? I would qualify that by saying that it should actually be measured, either with a laser or a tape. In Groundquest's original question, that was the scenario, and it stands to reason that if he measured it, and included it in his survey, then it should be part of the reported vertical extent (or depth, whichever you prefer). Of course, there are a lot of situations where you'd have to make a judgement as to whether or not to include it, such as a floor or ceiling crack that you can measure with a laser but it is obvious a person would never fit into.

Cave surveying is not an exact science, and "standard" practices vary from one person to the next. I suppose the NSS Survey Section should consider writing up a set of standard rules for us to follow. I'll bet that would generate some heated discussions! :argue:

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Re: Depth below datum

Postby Extremeophile » Jan 27, 2014 12:27 pm

It sounds like you're saying you need to take the elevation of every station and add or subtract up and down measurements to determine vertical extent. The survey software I use doesn't do this so it would need to be done manually. Most people I know often estimate LRUDs, and often they report up and down directly from the station rather than the maximum passage dimensions, so those elevations almost never have the same accuracy as a survey station. On top of all of that, the station elevation plus or minus the up or down measurement is still only the surveyed depth, not the true vertical extent (i.e. the true high and low point might be some place other than where the survey station was set).

It's obvious to me that there's no one right way to report cave depth, just as there's no one right way to report cave length. I'll continue to use the values reported by the cave survey software (i.e. the elevation difference between the high and low station) and be content to report this on my maps as "surveyed depth".
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Re: Depth below datum

Postby trogman » Jan 27, 2014 1:39 pm

Extremeophile wrote:It sounds like you're saying you need to take the elevation of every station and add or subtract up and down measurements to determine vertical extent. The survey software I use doesn't do this so it would need to be done manually. Most people I know often estimate LRUDs, and often they report up and down directly from the station rather than the maximum passage dimensions, so those elevations almost never have the same accuracy as a survey station. On top of all of that, the station elevation plus or minus the up or down measurement is still only the surveyed depth, not the true vertical extent (i.e. the true high and low point might be some place other than where the survey station was set).




No need to do it with every station, just the highest and lowest ones.

As far as estimating LRUDs, I guess I just assumed that most surveyors nowadays use a laser. I do agree that the station does not always end up directly where the maximum passage dimensions are. As cave surveyors we can never achieve 100% perfect representation of the cave with our maps, but we try to get as close as possible. However, the original question was concerning an actual measured value, in his case the "Down" dimension. Since it was measured I see no reason it shouldn't be reported as part of the vertical extent.

Extremeophile wrote:It's obvious to me that there's no one right way to report cave depth, just as there's no one right way to report cave length.


I will certainly agree with that statement, as I've been in numerous and sometimes heated discussions over such questions in recent years.

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Re: Depth below datum

Postby ian mckenzie » Jan 27, 2014 2:21 pm

Din't mean my comment to be so complex. What I meant was: if you accept that a cave's depth is the difference between its highest point and its lowest point, then depth is an exact number not open to interpretation. This implies agreement on the definition of high point and low point, but I think most cavers will find this easy to agree on. If a cave survey includes both the high and low point, then the 'surveyed depth' will equal the cave's actual depth. The cave survey example at the beginning of this thread failed to do so by not placing a station at the lowest point i.e. the floor, but this could be corrected quite simply.

Conversely, there can never be agreement on a cave's absolute length because there are so many styles, opinions and variables in measuring. Surveyed length therefore is only a general indication of a cave's actual length, no matter how accurate the grade of survey.

The discussion about lasering a dome or aven is interesting, because it raises the question as to whether a dome is an increase in ceiling height or is a passage that legitimately adds positive depth to the cave. The requirement for the place to have been traversed by a human seems to be the answer that most go with.
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Re: Depth below datum

Postby GroundquestMSA » Jan 28, 2014 2:10 pm

trogman wrote:That's just it-they are surveyed. If you establish a station and measure an LRUD at that station, is that not part of your survey?


I think so. Survey stations are reference points, not the beginning and end of the survey.

Extremeophile wrote:Most people I know often estimate LRUDs, and often they report up and down directly from the station rather than the maximum passage dimensions, so those elevations almost never have the same accuracy as a survey station.


I was wondering about this when surveying in Maxwelton. If you record dimensions strictly from station, the sketcher then needs to take multiple measurements at the station and sketch the cross section to scale in to have an accurately sized section, and accurate ceiling heights. If you record the maximum dimensions at the station though, you can draw the section not to scale, and the cartographer can still create an accurate cross section. What to do?

In order to deal with this on many of my own surveys, I record the maximum dimension at each station. In short stream caves and others with little or no gradient, I record no inclination data, and put no stated depth on the map. This allows me to record only three passage dimensions: left, right, and total height.

Short stream caves actually help to illustrate the vagueries of cave depth. Suppose I put every station on the floor. The surveyed depth, if based on the stations, could be 1' or 2' or 0'. But what if the cave has a big entrance with a ledge on one side? If I climb up on the ledge and put the first station near the ceiling, all of a sudden the cave is 24' deep. I intended, with my original question, to find out what the accepted method was. The answer seems to be "whatever you want". That's ok with me.
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Re: Depth below datum

Postby Extremeophile » Jan 28, 2014 6:14 pm

GroundquestMSA wrote:I intended, with my original question, to find out what the accepted method was. The answer seems to be "whatever you want".

It seems so. I certainly was under the impression that ~99% of cartographers were measuring cave depth more or less the same way, and then a thread like this reveals that there is no standard. It's probably no different than the variations out there for measuring pit depth. I was also surprised (and deeply disappointed) when I found out there are still a significant number of people surveying without backsights, but that's another topic :argue:
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Re: Depth below datum

Postby ian mckenzie » Jan 28, 2014 6:53 pm

Well, there is a standard definition of depth, but it is a matter of whether the surveyors follow it or not. For example, a cave survey might not touch the floor at the deepest point, because the deepest point might not have be known at the time of survey. Certainly no surveyors set out to misrepresent depth.

I suppose there may be some dispute about where to set entrance datum, although I think the lowest rim point (or the highest closed contour) fits most circumstances. Entrance dolines and irregular entrances might invite debate.
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Re: Depth below datum

Postby GroundquestMSA » Jan 28, 2014 11:28 pm

Extremeophile wrote:(and deeply disappointed)...

Ah, but how deeply, as measured from where?

(At least I hope that was a pun. Is bad surveying really worth an emotional response?)
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Re: Depth below datum

Postby Extremeophile » Jan 29, 2014 12:14 am

GroundquestMSA wrote: Is bad surveying really worth an emotional response?)

Yes. Bad surveying is worse than not surveying.
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Re: Depth below datum

Postby Chads93GT » Jan 29, 2014 1:10 am

bad surveying gives you a false sense of what is going on. Just like bad science. LRUD's are open to your interpretation. You can do true LRUDs or you can shoot past that ledge next to the station to the wall. Either way you are going to shoot to the walls anyway to draw your cross section, assuming you use a disto, and you will be drawing in the lrud points as well.
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