Possible to collect good survey data without backsights?

[Extremeophile]

Now, for a serious discussion, do you guys who use backsights with two sets of instruments account for your instrument errors when you force a two degree agreement between fs/bs? So, imagine that you have one clino at -2 and another at +1, that's three degrees difference when the instrument readers read the correct shot. If you're not accounting for this in your agreement, then there's a potential that your frustrated instrument readers are going to massage the numbers make it work. Seem far-fetched? I've seen it happen, on several trips.

At Mammoth Cave they run a compass course before every survey and only use one set of instruments. Every set of Suunto instruments I've ever used there was within the instrument error on front and back sights (i.e. <0.5 degrees).

At Jewel and Wind Caves (SD) the Cave Resources staff run the instruments through calibration courses on an audit basis and return or service instruments that don't agree.

My own instruments agree within instrument error also.

The only time I had the problem you describe was when I was doing back sights in Jewel Cave with a Suunto Tandem and front sights were with a Disto-X. We were in good agreement on azimuth, but consistently 2-2.5 degrees off on clino. It was frustrating because every other reading gave a 2.5 degree agreement error and we kept re-taking readings. After half a dozen of these it was obvious the problem was due to offset. We came to learn that the Disto-X hadn't been calibrated for 4 months. If we had run a compass course before beginning the survey we would have caught this and could have adjusted.

I wouldn't dismiss the differences between doing back sights and not doing them as different approaches with the same end result. Everyone is welcome to survey the way they like on their own projects, but there are differences in quality.

[Footleg]

I'd like to hear alternative solutions to catching blunders without doing back sights, or convince me you've figured out a way to survey without making them.

These days we are surveying with a DistoX and PDA. We calibrate the DistoX at the start of the survey, in the cave and check this against a Suunto compass and clino to verify the calibration is good. For each leg we take 3 readings (the PDA software uses this to distinguish between passage dimension or splay shots and actual legs). So we get a check that the DistoX laser spot is on target. All data is transferred electronically to the PDA over bluetooth so there are no transcribing errors. I think this method produces very low blunder rates. What is really interesting however is how close in agreement overall the data we are collecting today compares with the data collected 30 years ago. Despite that being much higher in blunders, the overall effect of these produces around 30ft discrepancy in the misclosure between the end points where two caves meet several miles from either entrance using GPS positioning of the two entrances in different valleys. I wonder are we being obsessed with accuracy for accuracies sake?

[Chads93GT]

Well.............when you survey downstream and end up with an uphill gradient because you didn't do backsites........ would you call that a quality survey? lol

Chad, thanks for the rude comment.

I'm not sure that you understood the scenario, but I'd like to hear you explain in greater detail how backsights would have helped me. Backsights may have helped with any singular instrument reading errors, but the error we discovered was solved by calibrating the instrument (yeah, should've done this when I got it, I know). This recurring error would have been present had we been using backsights or not. Fortunately, this particular error is easily solved after the survey is done. Were there other errors? I don't know, but I'd be happy to show you to the cave if you want to show me up.

Now, for a serious discussion, do you guys who use backsights with two sets of instruments account for your instrument errors when you force a two degree agreement between fs/bs? So, imagine that you have one clino at -2 and another at +1, that's three degrees difference when the instrument readers read the correct shot. If you're not accounting for this in your agreement, then there's a potential that your frustrated instrument readers are going to massage the numbers make it work. Seem far-fetched? I've seen it happen, on several trips.

All-in-all the difference between the ways we survey is minimal and both ways have their merits. Tools in the toolbox, as it were.

More importantly, though, I think next time I have a quick question about survey stuff I'll just go buy a copy of Dasher's cave survey book that I lost years ago. Or, just email George personally.

-Jon

Rude, maybe, but you are the one calling us dogmatic for using backsites to check our surveys for accuracy, yet at the same time you are inquiring about why your downstream survey came out uphill...... I call it how I see it. A spade is a spade, if that makes me an ass, then Im an ass. Doesn't bother me one bit. You make it sound as if we are silly for doing backsites on our surveys.

When doing a survey with multiple sets of instruments, we check them all before hand as to how accurate they are towards each other. We keep it in the notes and make corrections accordingly. WE also initial the notes as to who is making which shots, and of course we even write down our compass serial numbers so there is no confusion down the road.

Checking the clino for accuracy before you started would obviously have helped, but if its THAT far off, I think I would be sending it back to the manufacturer to have it recalibrated, rather than having an inaccurate piece of equipment.

Sorry you don't like some of the answers that you have recieved on here, but when you get standoffish about doing backksites , then give off the impression that we are silly for doing backsites as it slows the survey down and we obviously sketch slow, dont be suprised if some of us dont warm up to you.

we all use different methods, some work better than others. When I see discussions about survey I pay attention. I like to find ways to make my job easier, but I will not sacrifice effeciency for speed. I was taught to do a thorough survey and not a half ass survey by guys who are cartographers on the largest cave project in the world. Ive seen enough maps done from "back in the day" in my area. God forbid that the maps I have a part in ever look like some of those maps.

here is a tidbit on why I am anal retentive when it comes to accuracy. One of the largest caves in the state of missouri had a radio location project done a couple years ago. One of the areas they were trying to locate was around 500 feet off as to where it was supposed to be on the surface. Accuracy.........its a good thing.

do I always do back sites? Nope, if a passage is horrible crawl and dead end, generally no backsites, i simply maike the guy who is setting stations by backing up all the way in the narrow crawlway as its too small to turn around in, we tend to simply do "backsites" no front sites and he double checks the back sites. It just depends on how grim the passage is.

As I said, sorry if I come off as an ass, but calling us dogmatic for doing backsites sort of hit a nerve. My apologies.

[wyandottecaver]

well as long as we are beating a dead horse.....

Backsights do take time, how much depends.
The value of backsights is DIRECTLY related to how good your instrument people are and how good the instruments are.
The need for backsights will be related to how accurate a survey you require....BTW without backsights you will never know how accurate it is in the first place without a radio location or loop closure or some other reference.

Finally, I will make the observation that people who cut corners here, will cut corners there. In other words, if your worried about the hassle of a backsight you probably will be worried about the hassle of an awkward station, or a precise foresight while your freezing, etc.

[Extremeophile]

I'd like to hear alternative solutions to catching blunders without doing back sights, or convince me you've figured out a way to survey without making them.

These days we are surveying with a DistoX and PDA. We calibrate the DistoX at the start of the survey, in the cave and check this against a Suunto compass and clino to verify the calibration is good. For each leg we take 3 readings (the PDA software uses this to distinguish between passage dimension or splay shots and actual legs). So we get a check that the DistoX laser spot is on target. All data is transferred electronically to the PDA over bluetooth so there are no transcribing errors. I think this method produces very low blunder rates. What is really interesting however is how close in agreement overall the data we are collecting today compares with the data collected 30 years ago. Despite that being much higher in blunders, the overall effect of these produces around 30ft discrepancy in the misclosure between the end points where two caves meet several miles from either entrance using GPS positioning of the two entrances in different valleys. I wonder are we being obsessed with accuracy for accuracies sake?

This sounds very reasonable. 3 front sight readings is validation of your data, which is effectively what back sights are doing. I really like this all digital measurement and recording system because, as you mention, it eliminates the potential for several types of blunders.

I agree that, with respect to precision, there are diminishing returns. I remember reading an article where they surveyed a large loop using various instruments with different levels of precision. The loop closure errors were all comparable. The reasoning is that random errors in the individual shots have a tendency to cancel each other out over longer distances.

I generally don't feel there's a great need for greater precision in cave survey. Going to +/-0.1 degrees instead of +/-0.5 degrees isn't going to change much about the maps that are created. I do think there is still a need to improve accuracy through elimination of blunders. Back sights are an important tool in addressing blunders using traditional survey tools. Most blunders are human caused, so an all electronic survey system such as yours can be expected to eliminate even more blunders.

[Extremeophile]

There was a discussion earlier about survey grade and how it involves accuracy and precision. Most people think of the BCRA grades, which don't specifically address back sights. They do include tolerances, and multiple readings are needed to determine whether a tolerance is met. They talk about the different grades being different levels of accuracy, but I think they mean precision (must be an English to American translation glitch).

http://bcra.org.uk/surveying/index.html

Here are the survey grades used by CRF. They expect all modern surveys to be Grade A.

[lleblanc]

I'd like to hear alternative solutions to catching blunders without doing back sights, or convince me you've figured out a way to survey without making them.

These days we are surveying with a DistoX and PDA. We calibrate the DistoX at the start of the survey, in the cave and check this against a Suunto compass and clino to verify the calibration is good. For each leg we take 3 readings (the PDA software uses this to distinguish between passage dimension or splay shots and actual legs). So we get a check that the DistoX laser spot is on target. All data is transferred electronically to the PDA over bluetooth so there are no transcribing errors. I think this method produces very low blunder rates. What is really interesting however is how close in agreement overall the data we are collecting today compares with the data collected 30 years ago. Despite that being much higher in blunders, the overall effect of these produces around 30ft discrepancy in the misclosure between the end points where two caves meet several miles from either entrance using GPS positioning of the two entrances in different valleys. I wonder are we being obsessed with accuracy for accuracies sake?

Last October, I was resurveying a gypsum cave in Southern Spain with 2 DistoX and a PDA running Auriga. Thanks to backsights, we were able to determine one single station showed a local magnetic anomaly, something you'd rather expect in a lava tube. Repeating foresights would not have shown anything special, all measurements staying consistent with each other. Instead we canceled shots from that station, only keeping shots to it. That and some care allowed us to achieve a 0.14% closure rate over a 2-km loop. Backsights are indeed worth the extra time.

[HKalnitz]

I will agree with the Backsight team.
After picking up a complex island karst project theat entailed several surveyloops on a single page of survey, I found that one set of survey did not agree with the other surveys on the same sheet. Turns out the incorrect survey had no backsights that could have caught and corrected an obvious error in the bearing. So to prevent the closure calculations from distorting the cave we chose to go back to the cave on a subsequent trip to correct it. A true waste of considerable time,money and effort that could have been used in new survey for the few minutes saved on the first survey.

I keep book and usually survey with two sets of equipment on the team, but even on my quickest days, the compass/clino team can far outpace me - they are usually waiting for me to complete the sketch (plus the profile, plus the cross section, plus inventory....). This may be better with PDA but as far as I can tell the unit of time for survey is measured as 1 Sketchtime/shot. Reducing the time for compass/clino from .5 sketchtime/shot to .35 sketchtime/shot doesn't increase my survey if I still need to spend 1 sketchtime/shot on the book itself! I place just as much emphasis on the accuracy and correctness of the book as I do the precision of survey

Howard
nss20678

[Spike]

I haven't botherd reading reading all the posts. Not too much to say on the subject that hasn't been said in the last 50 years. All I will say is that n=2 is >> than n=1. n=3 would actually start to be meaningful and n>>3 would be even better. For now I see n=2 as the basis of a reasonable survey considering our physical constraints.

[Anonymous_Coward]

In extremely delicate passages, backsights are a conservation tool. If you survey a passage correctly the first time, there is often no need to go back and impact it a second time. Backsights help ensure that it is surveyed correctly the first time. This is especially helpful in caves where you can sometimes feel bad just by visiting a passage once.

Good survey CAN be accomplished without backsights. The problem is that we are all human. Everyone makes mistakes. So when you make one (you will), would you like to discover it within minutes, or within days. When it is discovered days later out of the cave, someone has to go back and clean up your mess. Having spent a good portion of my caving career cleaning up these messes, I would say that it is very important to take backsights. I guarantee you that most of the blunders I have fixed were committed by "experts" who "knew what they were doing" or "had time constraints" or "ran their instruments through a compass course" or "it was too hard to take backsights with only two people."

Whatever your excuse for not doing them, that's all it is, an excuse. If you want to get your feelings hurt over it, then fine. I'm tired of cleaning up your mess.

[hunter]

Got to pitch in with the pro backsight crowd as far as more accurate survey data goes. Seen and personally made to many errors, including a distoX disagreeing with itself on the backsight, to say otherwise. I think a really reliable distoX type instrument and bluetooth will help a great deal but at the same time backsights will be that much easier. I've also run across the magnetic anomaly issue and found out both times via backsights. Once was a buried piece of metal that had washed in and we never would have known our foresight was way off.

Oh, and as someone said on some other thread. Cavechat is about expressing opinions and I'm just expressing mine.

James

[Stan Allison]

While I think it is possible to collect good survey data without backsights, I think that it is much easier to collect good survey data by using backsights. While surveying in the cave here are the advantages of backsights:
Backsights reduce the chances of making a blunder when reading the instruments.
Backsights reduce the chance of making a blunder when conveying the instrument readings to the sketcher.
Backsights average out the errors inherent in a single set of instruments when entered in the survey data.

After exiting the cave and while entering the survey data at the computer the backsights also serve to reduce data entry blunders. Both Walls and Compass have the ability to compare backsight and foresight entries to make sure that they are within 2 degrees. I have found that this is a good way to catch data entry errors. Taking backsights in the cave is only half of the process, anyone who is taking backsights in the cave, but not entering the data is missing out on an easy data improvement.

With good instrument readers, I find that taking backsights doesn't slow the survey process down significantly even when doing two person surveys. With an inexperienced instrument reader or an instrument person who doesn't read instruments well, it does slow the process down significantly, but that is exactly the situation where it should be slowed down to get good data.

I still shudder a bit to think of when I and others first learned to read survey instruments and were cut loose on surveys without backsights. Backsights are an excellent way for both experienced and inexperienced cavers to ensure that they are reading the instruments properly.

For full disclosure, there is one alpine caving project I am working on in Montana that doesn't do backsights in mid 30F cold, wet caves. I also don't take backsights on dead-end shots or splay shots.

Stan Allison

[Squirrel Girl]

I would just do what that guy Stan Allison says to do!

[Spike]

Luc,

By chance on the station where there was a Magnetic anomaly, did turning the angle at that station produce a vector that was 180 from the backsight of the 2nd shot? I had heard of lava tubers doing this and I have used this method when surveying around pesky handrails in comercial sections of Mammoth.

[JR-Orion]

In the case of 2 man surveys we still do front and back sites, even in crawl ways, as like I said, we all have instruments to take shots. I generally do book 99% of the time, but I still have my tandem with me to make sure the reading given to me (backsite as the lead guy is doing backsites, tape and setting stations) while I pull back tape, do book and do instrument readings in crawlways. In easy walking passage, if back sites arent done, is simply lazy ;)

Crawlways suck to survey no matter what and sometimes like said above, you simply cant do a backsite from where a station is. It happens. But if someone is neglecting to do backsites in a borehole because they simply dont want to walk back to the front fo the shot to double check, then thats just being lazy IMO.

We use small LED lights (fishing bobber lights) for survey stations, so no manpower is being used to keep a light on station. we simply stick it to the station with a dab of mud and do our thing, that way its easy for us to do front and back sites.

I've used those lights many times and really like them. You get a steady, perfect little dot of red light. Are they fairly popular with surveyors? I'm a fan, that's for sure.