Allowable Error vs. Shot Length

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Re: Allowable Error vs. Shot Length

Postby reeffish1073 » Apr 16, 2010 5:51 am

OK?

What about shots over 50ft. with a disto? Correct me if i am wrong, but are disto's held to the same accuracy as a suuanto? Are they not calibrated to a standard of say +or- 2deg.? As for passage sketching, i think it is up to the sketcher his/her self as to what they are comfortable! I have seen sketches at 1:20ft that are verry rough with little detail, (small shot length) and i have seen sketches 1:40ft ( Long shot length) that are just awsome! Loaded with detail! I think alot of it is the sketchers ability to interpet the data, and make a quality sketch of the passage! I am verry fond of disto's myself as they make getting the distance shots and LRUDS more quickly! and the ones that have the compass feature, increase the surveyed footage of a trip more quickly.

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Re: Allowable Error vs. Shot Length

Postby Dan Henry » Apr 16, 2010 8:46 am

If you're talking about a DistoX, they report their angles to a tenth of a degree, so you shouldn't really go over 50' with them either, depending on what sort of real accuracy you're trying to get. Again, the accuracy of the distance will be far greater than the accuracy of the angle, which gets worse and worse the longer you make the shot length.

All instruments have their limitations. Even a 5 second theodolite is only really accurate to about 500' , the distance where 5 seconds of arc-angle equals 0.01', or the same accuracy as the electronic distance meter in that instrument.

Suuntos are like shotguns. They work great close up and you're going to get a lot of lead on the target. On shots over 50', you're trying to use a shotgun like a sniper rifle. you might get lucky and get a pellet or two on the target, but the spread is much greater and less effective if you want an accurate result.

Also, take sketching out of the equation. Sure, most sketchers are not comfortable with super long shots, but what if you find a sketcher who feels fine with a 200' shot. Will you let them do it just because they're comfortable with it, even though you know it doesn't fit best practices for surveying with Suuntos? Ultimately, we're talking about the quality of the data here. Just like some projects require backsights within a certain tolerance as one of their adopted standards, a lot of projects do not want you to take shots over 50' for the same reasons we've been hashing out. Data quality and sketch quality are different things.
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Re: Allowable Error vs. Shot Length

Postby George Dasher » Apr 16, 2010 10:23 am

Dawn: VERY good point. That's my strategy also.

John: Using a Disto for shots over 50 feet kills the sketcher. It just shouldn't be done, unless you're shooting to the top of a dome or someething like that.

Dave: I'll provide the reference once you provide the review and I can publish. :bananawhip:
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Re: Allowable Error vs. Shot Length

Postby Bob Thrun » Apr 16, 2010 3:21 pm

Heinz Schwinge (The Accuracy of Cave Survey, NSS Bull., Vol. 24, No. 1, Jan. 1962) showed that, given the accuracies of length and angle measurements, there is an optimum shot length. This occurs when the error due to angle measurement is the same as the error due to length measurement. He considered only horizontal measurements and did not consider station position error. You can look at the situation in several ways. Given some combination of angle error, length error, or shot length, what should you do for the others?

Given the compass and clinometer errors that cannot be improved upon, there is no need for overly precise length measurements. One meter or one foot is not good enough. I use the next smaller tape markings even though they are better than needed. I do not record any finer measurements. It is possible to go the other way with angle measurements that are too accurate.
I have notes from a surface survey with turning angles measured to 0.1 arcseconds and distances measured to 1 meter.

It is possible, under ideal conditions, to get reproducible measurements to 0.1 or 0.2 degrees. I managed this on the DC Grotto compass course. The ultimate limiting factor, even with a perfect compass, is the variation of magnetic north during the day.

From the DC Grotto compass course data I found that the standard deviation between what Suunto compasses consider to be magnetic north is 0.55 degrees. In my paper at the 2000 NSS Convention I compared cave surveys to BCRA Grade 5. No cave survey met the Grade 5 standard. The closest was Hamilton Cave, an easy horizontal maze. I suspect the reason it did not meet the Grade 5 standard is that there was no compass calibration to see their different magnetic norths.

I was notetaker on a surface survey where one person read the same set of instruments for both foresights and backsights. The readings were consistently within a +/-1 degree limit
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I am not qualified to comment on this, but perhaps someone else is... I believe that the BCRA survey grades actually define standard deviations for instrument slop rather than hard values. Can someone (Bob Thrun?) expand further on this point? Edit: Despite what I had inferred from the Survex data format, it appears that this is not the case. See: http://bcra.org.uk/surveying/


The 1976 BCRA Grades said "Angles accurate to +/-1 degree". This is the wording of the machine shop, not that of statistics. I interpreted the limits as hard limits. Anything out the limits is unacceptable while anything within the limits is equally acceptable. I used a uniform distribution for my paper at the 2000 NSS Convention. Others interpreted the wording to mean 1 or 3 standard deviations Almost nobody tried to establish that their readings were accurate enough. In practice, anyone who used instruments marked in 1-degree increments claimed Grade 5. The grades were revised in 2002 to match actual practice: "Angles measured to +/-1 degree". I wrote an article that touches our present topic in Compass Points 31 http://www.chaos.org.uk/survex/cp/CP31/CP31.pdf The same issue has a letter of mine commenting on differences between compasses and compass readers.

The BCRA Grades and the CRG Grades before them specify station position error. It is difficult to put the target and instruments at exactly the same place. The position error will affect the readings on short shots more than long shots.
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Re: Allowable Error vs. Shot Length

Postby Chris Chenier » Apr 19, 2010 12:06 pm

Dan Henry wrote:It's possible to have a correction factor for each set of instruments and enter the correction on a per shot basis in WALLS, but is more of a pain than it's worth. The exception to "single set of instruments, single reader" is extremely tight passage, where sometimes the point and instrument persons pass the single set of instruments back and forth to keep the gymnastics to a minimum, but this is still preferable to using 2 sets.


While I agree that it's no fun to be entering 2 azimuth values for each shot (in Walls or other), it is still much less trouble than to have to crawl back-and-forth, either to measure or to simple hand someone else an instrument. Entering a 2nd azimuth might take 1-2 seconds: not a very high cost per shot. My approach is "if you've measured it, record it!". Same goes for any rounding one might do in cave (eg. rounding a Disto length reading to the nearest inch/cm). Enter whatever you read and let the computer do the rounding later.

If you're entering data electronically in the cave, then the software can immediately take into consideration the calibration of both instruments to determine the difference between fore and backsight, which of course would be error prone if done in your head.
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Re: Allowable Error vs. Shot Length

Postby Roppelcaver » Apr 19, 2010 2:42 pm

In Roppel, we shoot for +/- one degree (after correction). For shots less than 10 feet, it is 2 degrees. We often do use two sets of compasses (I like just using one, and encourage it), but this is due to small parties (two people), or egregious conditions (where turning around is difficult/impossible). In these cases, both sets are shot on the course and appropriate corrects are made in the data.

We do have to keep track of whose compasses shot which direction, which is usually unchanged. But, it does introduce possible error (botching the corrections) and adds more steps to recording and processing data.

I will note that I still get recorded blunders (10 or 100 degrees off) despite all this. When we don't, or I can correct them, the resulting accuracy in Roppel is quite good. Rarely are our loops more than a few feet off, which is quite pleasing. When there is a "large" error, I go into blunder tracking mode and usually can find the offending shot with a high degree of probability. We later correct it, or increast the traverse's variance (if confident of its location).\

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Re: Allowable Error vs. Shot Length

Postby George Dasher » Apr 23, 2010 9:55 am

Peter Bosted gave a talk on loop error, etc. at the International last year.

He said he compared Roppel's survey data to Lech's, and that he expected the quality of the Lech data to blow Roppel out of the water, because Roppel is such a difficult cave.

Just the opposite happened.

The factor that made the difference was that fact that the Roppel Project has, from the word go, paid very close attention to survey quality. So the difference was in the decisions made regarding the data collection, which directly affected the quality of the surveys.


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Re: Allowable Error vs. Shot Length

Postby Dan Henry » Apr 26, 2010 9:13 am

The prosecution rests, your honor.
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Re: Allowable Error vs. Shot Length

Postby JP McLendon » Nov 29, 2010 10:20 pm

What about loop closure error expressed as a percentage, i.e., a loop consisting of 1000 feet of passage misses closing by 10 feet = 1% loop closure error? What is considered acceptable? I've only started surveying/mapping recently and our loops usually have a closure error of less than 1%. A recent trip yielded a 1600' loop with an error of over 2% (a displacement error of around 35'). This seems high to me and I suspect a blunder in the data but some may think this is fine for general mapping of small to moderate sized caves.
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Re: Allowable Error vs. Shot Length

Postby George Dasher » Dec 1, 2010 2:38 pm

Don't worry about the percent error--only the linear distance the closure is off. And then make a decision whether you can live with that error. If not, then you have to resurvey or mathematically close the loop.

But ALWAYS look at the unclosed data. You can often times figure out where the survey went bad, particularly if it is a blunder. That's why it is important to work the data up as soon as you can after the caving trip.
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Re: Allowable Error vs. Shot Length

Postby Chris Chenier » Dec 1, 2010 3:20 pm

George Dasher wrote:Don't worry about the percent error--only the linear distance the closure is off.


I think that percentage gives you a good idea of the quality. If you have a 1000 ft loop that is off by 10 feet (1%), this is much better than a 100 foot loop also off by 10 feet (10%). The first is probably good; in the second, there is a blunder and it needs to be fixed. It's easy to close a small loop within a foot or two, but you need to be rather lucky to close a mile-long one by that distance.

Arguably the best way of measuring closure is to use standard deviations with expected instrument accuracy as input, but let's face it, that's too much math for most of us! :hairpull:

You always have to ask yourself about the purpose of the survey, etc. If you need a very high accuracy (for landowner issues or drilling, for instance), you'll need better quality. If you're off in some remote area in a cave where nobody will ever return to with almost no chances of connecting to anything, most people will have a more relaxed attitude.

In most of my surveys, I start asking questions if the loop closes by more than about 2.5 %, but there are times I will re-shoot if I get 2 %, just like there are times I will ignore a larger error (say 5 % in a miserable loop off to the side and not connecting to anything else).
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Re: Allowable Error vs. Shot Length

Postby Crockett » Dec 1, 2010 5:14 pm

Why have the same "allowable error" for both the compass and the inclinometer? Do they not measure two different things? Do gravity readings, by their nature and if done properly, yield better results and allow for a tighter survey?
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