Possible to collect good survey data without backsights?

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Possible to collect good survey data without backsights?

Postby Footleg » Feb 1, 2011 11:53 am

This seems like a hot topic deserving of it's own thread having read the debate sparked by Jon's post on clino errors.

Is is possible to collect useful survey data without taking back sights? Personally I think it is, and enables me to survey more cave in the time available than a more rigorous technique of taking backsights for every leg. Am I taking a big risk? Well a risk I agree, but how bad is it really? Do a few errors in the survey make it a complete waste of time? Or it that tolerable for the value that bringing back more data in a limited time gives? We get more of a picture of the potential cave development to better plan return trips with more data. e.g. Surface prospecting can go ahead based on the data collected the previous trip. If we do ever find another entrance or close a big loop then we discover how big an error there might be in the survey, but the loop closure sort of pulls everything back into line. So what's the problem?

:shock: <--Dons safety hat!
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Re: Possible to collect good survey data without backsights?

Postby Chads93GT » Feb 1, 2011 12:04 pm

I don't get why people think that doing back sites adds so much time to their survey and is such a big hindrance. If you ALWAYS start sketching right after the front site, then do it. But if your instrument reader is too damn lazy to walk/crawl to the next shot and do a back site, then you don't have a very dedicated team. You can also rid that problem by having the lead tape and instrument reader both have a set of instruments.

Damn near everyone on our team has a set of instruments for taking shots. It takes absolutely no time what so ever to do back sites if you are already sketching. If you are doing single shots per page, then you dont even need the bearing to get started on your sketch, all you need is the distance and the clino if its a high angle shot.

If people think it takes so much more time to do something so simple as a back site to check for errors, then I would love to hear the explanations. Some shots are harder to take than others, your instrument reader could be having a bad day. A head lamp could be held too close to the compass and BAM...........plot line is off, you would figure that out in a hurry however, if you were doing back sites.

I guess it just depends on the QUALITY of the survey you want and how dedicated your crew is.

Here in missouri, we aren't lazy ;)
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Re: Possible to collect good survey data without backsights?

Postby George Dasher » Feb 1, 2011 12:40 pm

The short answer is yes. And sometimes you just have to do--survey without backsights.

An example of mine (where we had to do this) was that we only had so much time to survey the cave--thanks to the
bureaucratic requirements placed on us to be out of the cave at X Time. Fortunately we had two factors in our favor: the cave was following the strike of the strata and was fairly straight, so we could keep an good eye on the azimuths, and we were able to take backsights at the beginning of our survey, and our two instruments readers had been dead on with their checks.
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Re: Possible to collect good survey data without backsights?

Postby Footleg » Feb 1, 2011 12:57 pm

I think I spot the thing that makes it much more work for us to measure back sights than you find on your trips. You are talking about lead tape, instrument readers, sketcher. There are often only 2 of us doing the survey. One is marking forward stations and sketching. The other is doing all the measuring. So person 1 locates a forward station and provides a target, person 2 takes the shot from the rear station (using a DistoX these days, but back a year ago the sketcher was also lead tape and station setter), then person 1 comes up to the forward station and person 2 shows them exactly what place was used for the station. Person 1 then takes the passage dimension measurements (LRUD). This enables person 2 to sketch before moving forward to find a new station and the process repeats. If we were to take back sights then this would require person 2 to return to the previous station to provide a target and then go back up front to find a new station, slowing things down considerably.
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Re: Possible to collect good survey data without backsights?

Postby Chads93GT » Feb 1, 2011 1:06 pm

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.
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Re: Possible to collect good survey data without backsights?

Postby Chads93GT » Feb 1, 2011 1:11 pm

I have to say, having your book man, be the lead guy seems rather odd. Afterall, he walks through the passage not yet drawn, to set up a lead station, pull tape and hold a light on station (in your case im assuming this is how you do it) then he draws the passage from the end station without ever walking back through the passage to accurately sketch in floor ledges, ceiling ledges, draw cross sections in higly complex areas in the middle of the shot, accurately drawing the position of columns and other features.

I am at the back of the crew, as I have to sketch as I go through. Seems like a half ass way to sketch if you walk through the passage first, stand at the station you just set and eyeball everything in from there.....

this is just me interpreting it though, i could be wrong.
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Re: Possible to collect good survey data without backsights?

Postby Extremeophile » Feb 1, 2011 1:41 pm

I would never make a blanket statement like "all surveys without back sights are meaningless" or "all surveys with back sights are comletely trustworthy". Obviously there are exceptions to both statements. I think it has been shown that with traditional survey tools and techniques (e.g. Suuntos, fiber glass tape, team of 3, etc.) a blunder is made about once every 20 shots. A blunder can be in the compass, clino or tape reading. It could be in the reading of the instrument, recording of the data, or in data entry. I'm sure many surveyors feel they are very conscientious when it comes to reading instruments and therefore commit fewer blunders. I've thought this about myself. But I've been on too many surveys with extremely experienced surveyors while taking back sights and I've found the one blunder every 20 shots to be about right, even for my own readings. I'm very aware of the many types of blunders that are possible and yet I make them anyway. Back sights won't catch all of them, but it will catch most, and you can then correct on the spot. If you have loops in your survey, then data reduction will help identify that a blunder exists, and can provide clues as to where the blunder occurred, but wouldn't it be better to avoid the blunder in the first place, and how much survey within the cave is not part of a loop?

I don't buy the excuse that taking back sights slows down the survey process. On rare occasions as the sketcher I've had to wait on the other members of the survey team, but this is usually because of uncertainty in which passage to take or where to set point, not because it takes a lot of time to read the instruments. When you have one set of instruments the sketcher can illuminate the back sight, but it's not necessary for the instrument and point person to run back and forth, as some people seem to think.

Given the fact that blunders are inevitable, and back sights are easily taken, I can see very few excuses for not taking them. I have omitted them on specific shots because it was at the edge of a drop, or in a location that was difficult to reach, or a dead-end (aka terminal) shot, but often this can be avoided by better placement of the station.

Back sights are one of the key criteria that go into setting the Grade of a survey, and for good reason. I said it in the other thread, but I think it's important to emphasize that surveying takes a lot of time and effort. The data needs to be accurate and reliable or else why bother. If someone will need to resurvey what you've done in the near future then you are wasting your time and you might as well just scoop.

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.

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Re: Possible to collect good survey data without backsights?

Postby Extremeophile » Feb 1, 2011 1:51 pm

When surveying with two-person teams then 2 sets of instruments are needed to take back sights and avoid a lot of running back and forth. In this case the sketcher needs to take time out to get a reading, and this does slow down the process. I can't see how putting the sketcher on point would make things more efficient.
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Re: Possible to collect good survey data without backsights?

Postby jlillest » Feb 1, 2011 2:24 pm

derekbristol wrote:Back sights are one of the key criteria that go into setting the Grade of a survey, and for good reason.


The Backsight dogma on here amazes me, mainly because of the insinuation that the survey that the rest of do is total crap.

But, I will say that adding backsights to a Grade 5 survey doesn't increase the survey grade, only more precise instruments will do so. Adding backsights is a form of error detection, not a way to add precision.

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Re: Possible to collect good survey data without backsights?

Postby Extremeophile » Feb 1, 2011 2:57 pm

I'll have to do a little more research, but I believe both precision and accuracy are factors in determining the grade of a survey. You can use a Disto X and get measurements to the nearest 0.01 feet and 0.1 degree (high precision), but if the instrument isn't calibrated and the book person writes down the wrong numbers, then it's not a very accurate survey and therefore a lower grade. The issue with back sights relates to accuracy. Some people use front sights only when compiling the data, and the back sights are for error checking only. Others use an average of the front sight and corrected back sight. I'm not sure this affects precision.

People seem to be taking this debate very personally. The past is the past. Evaluate the information and arguments presented in this thread and make your own decision about how to survey in the future.

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Re: Possible to collect good survey data without backsights?

Postby NZcaver » Feb 1, 2011 3:20 pm

Sure it's possible to collect good survey data without backsights, and sometimes it's the only practical option.

But from what I've seen from my sporadic survey project involvement in several regions around the US, an absence of backsights for an entire survey is clearly the exception rather than the rule. I know of an experienced survey crew in one region who routinely do foresights only, and seem to produce acceptable stand-alone surveys. But - justified or not - others in the region consider these no-backsight surveys a quick-and-dirty method of maximizing footage in minimal time at the expense of a little accuracy.
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Re: Possible to collect good survey data without backsights?

Postby John Lovaas » Feb 1, 2011 3:26 pm

jlillest wrote:The Backsight dogma on here amazes me, mainly because of the insinuation that the survey that the rest of do is total crap.


Dogma, or personal experience.

My favorite "we don't need any stinkin' backsights" story is this- I was working on a survey in southern IL, and some 'experienced cave surveyors' were going to show me how pointless backsighting- particularly with clino- was.

After running the instruments on our calibration course, we set up the first shot- a very short shot, around 8 feet. The 'experienced surveyor' took the first clino shot. I asked him to humor me and do a backsight. Turns out there was a five degree difference.

Using other readers and instruments, we determined his frontsight was, in fact, 5 degrees off.

A 5 degree error on an 8 foot shot might just make water run uphill- just sayin'... ;-)
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Re: Possible to collect good survey data without backsights?

Postby Chads93GT » Feb 1, 2011 4:02 pm

jlillest wrote:
derekbristol wrote:Back sights are one of the key criteria that go into setting the Grade of a survey, and for good reason.


The Backsight dogma on here amazes me, mainly because of the insinuation that the survey that the rest of do is total crap.

-Jon


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
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Re: Possible to collect good survey data without backsights?

Postby jlillest » Feb 1, 2011 4:41 pm

Chads93GT wrote: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
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Re: Possible to collect good survey data without backsights?

Postby Aaron Addison » Feb 1, 2011 5:04 pm

This thread reminds me of the civil engineers creed. "There's never enough time or money to do a project right, but there is always time and money to do a project over".

More seriously, it has me thinking of another issue with compass courses. I am forever seeing folks "calibrate" their compass at places like Hamilton Valley. There dutifully read compass and clino to verify that the two sets of instruments read within a couple of degrees of each other and head off to the cave. Rarely to I see folks inquire as to what the readings should have been. The end result is one that addresses relative error, but not the possibility of absolute error in the survey.

I am willing to admit that there are times when I do not use backsights. This is usually on small caves that have no hope of connecting to anything else, and only after the instrument person has read backsights on the first couple of stations. Call it a sanity check on whether or not they actually know what they are doing.

In the end it is simple risk analysis. The surveyor(s) must take stock of the risk of errors in the data collection vs doing over vs doing a poor job. Many of the places I seem to be surveying these days are very difficult to gain access to. The option of going back and correcting mistakes may take years, or may never happen. For me the risk is simply too high not to take backsights. Everyone on the team is like minded and works efficiently.

Your mileage may vary.

Cheers,

AA
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