Allowable Error vs. Shot Length

[driggs]

I should have clarified in my previous post that assuming no systematic error, random error will average out and should converge on the true value.

It was clear that you were talking about random error; I wasn't trying to correct you, just discussing error in general as related to your question.

I like Peter Bosted's approach of holding 50' shots to 1 degree and 5' shots to 5 degrees. I will probably use this ethic in the future.

Distance isn't the only determining factor on when it may be acceptable to relax the standard. Given two shots - a 50-footer at 0-degree inclination, and a 50-footer at 45-degree inclination - the error due to compass slop in the high-angle shot is considerably less because it is essentially a 35-foot shot in the horizontal plane. A painful 60-degree clino shot is reduced to only 25-feet on the plan view. The moral is that difficult high-angle shots may also be given a more relaxed standard in the name of survey efficiency.

Now, the real question becomes... what do you do when the cartographer or project leader throws a fit because your 4-foot long, 75-degree ear-dip shot has a 4-degree discrepancy which violates "policy"?

[jharman2]

Distance isn't the only determining factor on when it may be acceptable to relax the standard. Given two shots - a 50-footer at 0-degree inclination, and a 50-footer at 45-degree inclination - the error due to compass slop in the high-angle shot is considerably less because it is essentially a 35-foot shot in the horizontal plane. A painful 60-degree clino shot is reduced to only 25-feet on the plan view. The moral is that difficult high-angle shots may also be given a more relaxed standard in the name of survey efficiency.

This is very true and something I didn't think about. This means that a plumb shot (+90deg or -90deg) that is off by a couple degrees from true plumb makes no difference.

Now, the real question becomes... what do you do when the cartographer or project leader throws a fit because your 4-foot long, 75-degree ear-dip shot has a 4-degree discrepancy which violates "policy"?

THAT IS the real question. I'd tell him to drag his @$$ back there and reshoot it, of course that may be the last data set that I am allowed to turn in. I think from the outset organization should make provisions for these special cases in their survey ethic.

I will be off the grid for the next few days. It will be interesting to see how this convo develops!

[reeffish1073]

Ok, lets throw another question into this mix. What about shots over 50ft.? Large borehole passage shots. Do you shoot shots over 50ft. at +or - 1deg. of accuracy? Different surveyors do things differently. I myself being the cartographer on our project have found that our loop closures are accurate according to compass. Just wanted to see what others opinions are.

Thanks
John

[George Dasher]

Long shots kill the sketcher and decrease the quaility of the sketch.

I limit the shots to 50 or less, with the sketcher having total control over station placement. In complicated rooms or passages, don't even shoot to 50 feet. In other words, sketching is the control, not the instruments, etc.

And I try to use 2° as the maximum error for both compasses and clinometers. Although there are always some "higher error" shots that slip through.

It isn't Peter Bosted's computer programs that make his projects accurate, it is the fact that Peter is willing to sit down and spend massive numbers of manhours straightening out data that someone else screwed up. In other words, it is not the computer program--it is the person.

[Tlaloc]

Sure, in an ideal world there would be zero difference between instruments and readers and every shot would be in dry borehole with stations at shoulder height...

Actually what you're describing is a "floating station" which is not an actual point on the passage ceiling, floor or wall. This is a bad practice because you can't guarantee that the instrument man's eye will be in the same spot as the guy with the tape was. If they are different heights or if the instrument guy uses an offset such as his right eye, this will introduce systematic error. It is more accurate to measure the distances to real hard points. Also it can be difficult to find the survey station again if you find a side passage. If your project is concerned with accuracy then they should discourage this.

[Jeff Bartlett]

Yep, they're slow, cold and long...footage varies. The survey is dead on and we're all having a blast. Isn't that why ya do it?

If the foresight and backsight person are dead-on (within, say, 0.1 degree) for more than a couple shots in a row, I can basically guarantee that the backsight person is "cheating" by remembering the reading of the foresight person. This completely negates the value of having such a narrow tolerance.

[Scott McCrea]

If the foresight and backsight person are dead-on (within, say, 0.1 degree) for more than a couple shots in a row, I can basically guarantee that the backsight person is "cheating" by remembering the reading of the foresight person. This completely negates the value of having such a narrow tolerance.

This brings up another thought on improving accuracy—having only one instrument person. He does fore and back sights on the same instrument/s.

Oops.

[driggs]

This brings up another thought on improving accuracy—having only one instrument person. He does fore and back sights on the same instrument/s.

"One instrument, one reader" is CRF policy, and while I feel that this configuration improves the accuracy of the data (by eliminating instrument differences and reader differences), it most certainly does not solve the "cheating backsights" problem. If the same person shoots front and back, he's much more likely to be biased with the second reading because it is human nature to be "internally consistent", and he's likely to remember the values he just read 30 seconds ago. Also, in a body-sized sewer tube passage, after the instrument reader and point-man have already leapfrogged each other once so he can read the backsight, do you think he is more or less likely to "cheat" to avoid crawling back through the passage and flopping around to re-read that shot? I firmly believe that having two instrument readers helps prevent cheating, because we'd rather show that the other guy is wrong.

I like to think that I'm an "honest" backsight instrument reader because I'm spaced-out enough that I don't pay attention to the foresights when shouted out, and if I do hear them, my memory is bad enough that I forget them almost instantly!

Ooops, I did it too!

[Jeff Bartlett]

Disregarding whether there is one instrument reader or twelve: if the readings all agree perfectly, someone is lying.

[Scott McCrea]

We can add another one to the "Caving lies" thread: "The backsight matches perfectly."

[George Dasher]

Use two instrument readers.

It is more important to keep the survey moving, and Suuntos (which most people use) usually only have an accuracy of 2°.

[MUD]

Yep, they're slow, cold and long...footage varies. The survey is dead on and we're all having a blast. Isn't that why ya do it?

If the foresight and backsight person are dead-on (within, say, 0.1 degree) for more than a couple shots in a row, I can basically guarantee that the backsight person is "cheating" by remembering the reading of the foresight person. This completely negates the value of having such a narrow tolerance.

Disregarding whether there is one instrument reader or twelve: if the readings all agree perfectly, someone is lying.

Well I guess I'm a cheat and a liar. And when the instruments turn up missing? Yep, ol MUD did it!

The readings may not be *spot on* all the time. I do believe you can shoot many shots with no fs/bs error. Of course passage detail and the instrument readers themselves have alot to do with this!

[driggs]

Suuntos (which most people use) usually only have an accuracy of 2°.

Can you provide a reference to back this statement up and/or explain exactly what you mean by this George?

Edit: The Suunto website lists the compass accuracy of a Suunto Tandem as 1/3°.

[Dan Henry]

Barlett's right. If somebody gets perfect agreement on every shot, there's a huge chance they are introducing reader bias into the readings by remembering and matching the foresight. If you're in a cave, and you're wiping the slate between every shot in terms of forgetting what the other one was, you should be getting some variance. When shots agree perfectly, I often casually ask the reader "What did you get on the foresight, I didn't write it down (devious lie)?" When they instantly rattle off the number, we either switch readers or have a long talk about reader bias and how it makes your data suck.

The reason the CRF likes to use only one set of instruments is that they have a compass course upon which they read the instruments before every trip. With these values for every single trip's survey, and the known true north related azimuth of the compass course, compass correction numbers can be determined to correct the variation in each trip's survey so all the data points the same direction. If you use 2 sets of instruments with a degree or 2 of difference on the same trip, you are basically negating the accuracy you're trying to acheive by using the compass course, because part of your data is not oriented the same as the other set of instruments, so when you average the FS/BS, you get an answer that's not truly related to either reading. 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.

I know of several long cave projects where they don't bother using a compass course, but just enter each book and apply the magnetic declination to each, which doesn't at all account for the systematic error of using mutiple instruments that point different directions, which also defeats the point of taking forersights and backsights that agree within tolerance. You might as well just match 'em to the closest 5 if you're not going to go the extra bit and account for systematic error.

I don't think you should take shots over 50' with Suunto's (John Christie's comment). If you measure your distances to the 10th of a foot, you should try and use methods that allow you to get the same degree of accuracy from the angular measurements. To get a coordinate difference of a tenth of a foot over 10 feet, you need to read the instrument to the nearest half degree, which is do-able. To get a the same 0.10' difference at 50' you need to read to the tenth of a degree, which is beyond the range of the Suunto, but has become acceptable practice. At 100 feet, to get the accuracy of the angle measurement to the same closest tenth precision as the tape, you need to read the suunto to 1/20th of a degree. Impossible! In short, the greater the distance, the more accurate an instrument you need to acheive the same accuracy in the angular measurement as you can get in the distance. If you're going to take 100 foot shots and measure the angles with suuntos, you might as well take the distances to the nearest foot too, because you're not going to get results that are ultimately any more accurate than that.

[Dawn Ryan]

Ok, lets throw another question into this mix. What about shots over 50ft.? Large borehole passage shots. Do you shoot shots over 50ft. at +or - 1deg. of accuracy? Different surveyors do things differently. I myself being the cartographer on our project have found that our loop closures are accurate according to compass. Just wanted to see what others opinions are.

Thanks
John

I prefer a 50 ft shot or less to work with. That's just a personal preference. It's easier for me to sketch and take in what's around me. And it fits on the paper I use. Two of the caves we're currently surveying for the Minnesota DNR wants 10ft to the inch scale. So anything over 50ft would mean the shot not fitting on the paper or having to use "big boy paper." My other half John Lovaas, on the other hand, uses "big boy paper" and he can put out great sketches at large or small scale.

Dawn Ryan