Traverse Adjustment
Need a little help and advise guys. Its been years since I have done a traverse adjustment.(more like decades) most of our stuff is done with GPS or after 3-4 good turns checking in with no problems. I have a site that has about 40 (extremely unbalanced) turns. There are some with 20′ back sites and 150′ fore sites. This was through a cave system so we had to stretch the limits of our hardware. The traverse was ran with a Trimble s8 0.5 sec gun. After I ran through the cave I come out on a known point and I miss it about 2′ horizontally but flat vertical. All of my distances where double checked through the traverse with none of them being off more than 0.05′. I’m sure I’m dealing with an angular issue but my question to you guys is what is the best way to adjust this traverse? Any advise?
- 46 Replies
Least Squares definitely and starnet if you can. A first pass through a least squares adjustment will see if the 2′ you are seeing will still be a problem overall. What kind of accuracy are you trying to get to at the outer reaches of your control network?
—Dan MacIsaac, PLS
Thanks DAN…thats what I was thinking but wanted to get some input before I proceeded. Regarding the accuracy….its subject to whatever I feel is necessary. I would like to get it within 0.25′ spatially. We are going to come through and scan the cave after the control and then run aerial LiDAR on top. Should make for quite the map.
I second the least squares recommendation, with reasonable standard error estimates for your conditions. My gut tells me the gun was overkill for the job and the most critical issue is centering error.
If LS isn’t available do an angle adjustment with the error distributed proportional to 1/bs + 1/fs at each point.
A centering error of maybe 0.01 ft on a 20 ft leg is a minute or two angle error and at a few points would easily accumulate 2 ft closure in a traverse of 3000 or 4000 ft.
After whatever adjustment your points are likely to be closer to the truth, but still probably significantly off. LS will tell you what is likely.
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I don’t think this level of error is acceptable to adjust. It needs to be re-run. A point nodal prism for the shorter legs may help and also make sure autolock and or 360 prism is not used on the Trimble.
I also use StarNet – I’m a big fan and proponent. I run virtually everything I do through it. That is more an effort to trap blunders rather than to adjust minor errors, but of course that also happens. It is also a convenient place to fix point numbering issues, descriptors & measure ups, and so on. I doubt that you have saved any time by not analyzing your work in this way. You have just shifted the work to other parts of your process.
How good is that known point? Might that be the issue, worth a double check?
While not good enough for boundary work, if this is topo or something requiring less accurate location, wells, wetflags, etc, it’s acceptable.
I never used Starnet to ‘adjust’ data. Fooling yourself into thinking you make the data better is a completely different thread. What it does do is evaluate data by detecting blunders and validating error estimates.
I would spend some time looking for the error before doing any field work over. More often than not a little office time can eliminate return trips to the field.
The more oddly shaped structure to the traverse will create large closure differences at the closing points. While 2′ is large, what least squares will show is that a small change to a few angles of 5 or 10 seconds or a distance of a hundredth or two may tighten up the control back to his accuracy limits. I have had odd traverses that the crew comes back thinking they had a major error because the EOC was 0.5’+ but when the adjustment was performed through starnet and the small little problems were tweaked the traverse was actually fairly tight. You’re not making the measurements better but you are losing the effects of small error propagation through the network. I go back to these types of odd traverses frequently and have a lot of success doing follow on work that confirms the original adjustment was solid or worst case better than it could have been with another adjustment type.
—Dan MacIsaac, PLSAs you can see there are various philosophies on adjustment. And some people who didn’t read the word CAVE in the OP.
what least squares will show is that a small change to a few angles of 5 or 10 seconds or a distance of a hundredth or two may tighten up
Yes. That was my point above. It will tighten up the closure, but I think it may be open to misinterpretation when you say tighten up the control. Statistically it is better, but you can’t guarantee each point is better.
You’re not making the measurements better but you are losing the effects of small error propagation through the network.
Yes.
.I’m 100 percent sure its purely angular error since the vertical was flat. I double checked all the distances when I was traversing and had nothing over 0.05′ horizontal error.
I’m 100 percent sure its purely angular error since the vertical was flat. I double checked all the distances when I was traversing and had nothing over 0.05′ horizontal error.
To use precise language, while the misclosure may be in the direction most affected by angles, the usual suspect in a case of this type are the centering errors. Same with your 0.05′ distance error. The distances (from gun to glass) are much better than that. Probably no more than 0.005′. It is the centering of the gun and target over the marks that are causing you to read 0.05′ differences. If you are off center as much as that in the direction of the traverse line sometimes, you will be off center perpendicular to the traverse line by as much at other times. A centering error of 0.05′ on the early part of the traverse, unadjusted, can easily propagate into a 2′ misclosure.
If you haven’t done it recently you might check your rod bubbles and tribrachs. But for a traverse through a cave I expect that this thing can be adjusted satisfactorily.
The conditions were not ideal……
Completely off topic.
That looks like a sweet project.
And some people who didn’t read the word CAVE in the OP.
I read it fine and I also read that a 0.5 second gun was used. If a 0.5 second gun was used on this traverse I suspect it may have been used to attain a certain level of accuracy. If that is the case a 2′ closure is probably unacceptable. If it just happens to be their everyday gun then fine. Perhaps if the OP could give an idea of what the survey is for?
And some people who didn’t read the word CAVE in the OP.
I read it fine and I also read that a 0.5 second gun was used. If a 0.5 second gun was used on this traverse I suspect it may have been used to attain a certain level of accuracy. If that is the case a 2′ closure is probably unacceptable. If it just happens to be their everyday gun then fine. Perhaps if the OP could give an idea of what the survey is for?
This “misclosure” doesn’t interest me much. See me once the thing is put together and the residuals determined. Closures are so 20th century.
It is just as likely that they were using a 0.5″ gun because that is their daily driver. But in order to get 0.5″ angles out of a 0.5″ gun you are going to need to be setting it, and the targets, on concrete pillars. Setting it on common tripods, and over chalk marks, as pictured, says more about the required precision that the choice of instrument.
Traverse adjustments are intended to correct for random error. 2′ sounds like more than random error to me. Even with your less then ideal circumstances you should be able to do much better then that. Are you confident in in the coordinates for your start and ending? I would be looking for a blunder or an overlooked systematic error, rather than an adjustment, but running through adjustment procedures might help you find it.
And some people who didn’t read the word CAVE in the OP.
I read it fine and I also read that a 0.5 second gun was used. If a 0.5 second gun was used on this traverse I suspect it may have been used to attain a certain level of accuracy. If that is the case a 2′ closure is probably unacceptable. If it just happens to be their everyday gun then fine. Perhaps if the OP could give an idea of what the survey is for?
This “misclosure” doesn’t interest me much. See me once the thing is put together and the residuals determined. Closures are so 20th century.
It is just as likely that they were using a 0.5″ gun because that is their daily driver. But in order to get 0.5″ angles out of a 0.5″ gun you are going to need to be setting it, and the targets, on concrete pillars. Setting it on common tripods, and over chalk marks, as pictured, says more about the required precision that the choice of instrument.
It would interest me in this case considering the nature of the traverse conditions and the probable limits on redundancy.
He said “I would like to get it within 0.25′ spatially” so we know it isn’t expected to be super good.
They have multiple short legs in the traverse, as small as 20 ft. Centering error, even with McMillan precision, can account for much of the 2 ft closure after propagating azimuths through a long traverse. The calculation I showed in my first post supports that based on centering that is better than can be expected here, as probably indicated by the 0.05 ft discrepancies in repeated distance measurements.
The analysis and any adjustment MUST consider probable centering error if it is to be meaningful, and Star*Net can do that easily. He doesn’t seem to have (or have used) a program to do least squares. It’s too big for the demo mode of Star*Net. Are there other inexpensive options for him to get least squares capability?
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Mutli-track prisms, bipod foresites & Backsites, 20 foot legs, you’ll accumulate error quick. Star net will help you sort it out, if you provide realistic error estimates.
How many sets were you turning? FWIW, I don’t traverse or side tie with a 360 or multi-track.
It would interest me in this case considering the nature of the traverse conditions and the probable limits on redundancy.
It’s not likely a 2′ error. It’s probably a 0.05′ error in a short backsight propagated by a long traverse. The misclosure is meaningless until this data is simultaneously adjusted and analysed.
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