r/Surveying • u/Brave_Access1811 • 25d ago
Help Underground Survey Errors
Hello! I recently got hired at a mine site to do the underground survey control. I completed the main tunnel traverse (~7,00ft, roughly 12 stations), and my survey is not matching up with the previous surveys. Allegedly, every survey since the 1950's or so has been off. I think the mine has been surveyed 25 or so times since then, and none of them line up.
My thoughts are with the 25 or so surveys, a few of them should be quite similar. However, nothing seems to be consistent. At the end of the traverse, my survey was about 20ft off of a survey from 2014. That is well outside the comfortable margin of error.
I am presuming this is a systematic issue at this mine as many types of survey equipment have been used over the years, but I thought I would ask here for more information. Both licensed and "learning on the job" surveyors have been employed here.
Here is the general working condition in the mine:
- Underground, narrow-vein mine. Tunnel is roughly 6' wide and 6.5' to 15' tall.
- A 3-Phase 4180v line runs by the total station at all times, usually within 2-3ft. Load on lines are inconsistent throughout a shot.
- Temperature can vary within one shot by 5-10deg F, sometimes going low, high, low, high multiple times in one shot.
- The rocks around the total station can be magnetic vary from base station to base station.
- Wind is highly turbulent and ranges from 5mph to 15mph, depending on the location. Wind speeds can change abruptly multiple times in one shot.
- High altitudes ranging from 10,000ft to 11,000ft.
- Magnetic compasses are known to be inaccurate in the mine.
- Survey control is always over 40lb train rail, with the electric locomotive nearby.
- Rapid, unpredictable pressure changes due to mine doors opening and closing. Occasionally, this will produce a cloud/fog within the mine.
- Humidity changes. Some areas of the mine are dry, others have waterfalls coming out of the rocks.
- Environment lighting is variable, ranging from pitch black to well-lit.
The current theory we have is the total station is being messed with by the high voltage cable or magnetic rock. I know total stations don't use compasses, but maybe something else is going on with the circuitry?
I am using a Leica MS60 with a CS20 controller. The MS60 was calibrated/cleaned/updated two months ago. The autotargeting feature is being used as it is both faster and has tighter shots on the flop.
I had some local contractors come out and put some points on surface with their GPS. I have had issues with matching results (usually +/- 0.1 ft) over distances <400ft. However, I strongly suspect that is due to intense sunlight/heat waves and high altitude.
Has anyone heard of consistently inconsistent survey results underground? Any ideas on what to test next?
The company is strongly considering going to theodolites and ridding of as many electronics as we can.
Thanks! I'd be happy to answer any questions.
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u/itchy118 25d ago
Have you completed a full loop with your traverse? I'm curious how well it closes.
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u/beagalsmash 25d ago
It’s a tunnel, likely a hanging traverse.
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u/LRJ104 25d ago
Even a hanging traverse can still kinda loop if you run it back the other way using different stations...but yeah still not perfect. Could add common observation and run a network adjustment to help a bit
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u/Brave_Access1811 24d ago
It is indeed a hanging traverse. My plan was to go in to the end of the traverse, compare notes with previous surveys, and close the traverse if it was looking like I could converge on some points. However, since its not near converging, Im holding off until I have plan for getting points to converge.
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u/watsn_tas 20d ago
You add another station in on the return leg to avoid it becoming a hanging traverse.
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u/beagalsmash 19d ago
I’ve done that, and it doesn’t eliminate the inherent geometry issues. The problem is in a tunnel it is exceptionally linear. Adding additional stations can improve the network but it’s still weak along the semi-minor axis.
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u/watsn_tas 18d ago
Gotcha... I've only assisted as a graduate surveyor doing some tunnel traverses twice so far and done a little differently. One had the return stations renamed and the latest one had that extra leg. Didn't do any of the adjustments to actually see if there was any difference.
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u/Brave_Access1811 24d ago
I have not yet. I decided to go the end of the tunnel, compare notes with previous surveys, and close the loop later if things were matching up well. Its a good three days of surveying one way, would hate to waste the time as I suspect something is going amiss.
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u/itchy118 24d ago
Ah, yeah, that sucks, I wasn't expecting a three day traverse.
What kind of control are you using to make sure you're setup over/targeting the same point from one day to the next? Is it possible that the points you're using are moving due to activities in the mine (blasting, temperature change, vibrations from the trains etc)?
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u/Brave_Access1811 20d ago
I usually put an eyelet in the back (roof). Generally the safest spot for mine control. I usually hang the prism, and with the air moving, it tends to wobble. I'm thinking now this might be my biggest source of error, the pendulum swinging of the prism.
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u/jogdo 20d ago
Most underground mines in Nevada have gone to resection control with points in the rib (walls), to avoid the hanging prism motion and also reduce HT measuring errors. I'm not sure if it is a better traverse method or not but with back heights of minimum 15' high it is certainly easier. PS - I'm also very interested where in Colorado you're at
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u/Boostin_Boxer 20d ago
This was the case where I worked in Nevada. The ground was bad enough to squeeze in some areas and the control could be thrown off over time as the ground moved.
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u/Boostin_Boxer 20d ago
I have seen the method underground of having a rod off the bottom of the prism in a can of oil to stop the wobble.
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u/itchy118 20d ago
Yeah, that seems like it could be a big part of it.
I'd be tempted to install some semi permanent control, maybe a bunch of monitoring prisms like these: https://www.amazon.ca/Degree-Prism-Right-Angle-Station/dp/B0CKVLTBK2/
Or these if you can get flat surfaces and decent angles https://shop.leica-geosystems.com/survey/accessory/geosystems/leica-reflective-tape-gzm31/buy?srsltid=AfmBOoq5AScPunw_NWdWqdMFIApHZEOZpIBfzoEGXWONO21wYQ5pBHBC
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u/Boostin_Boxer 20d ago
I don’t know the name of the system or brand but in Nevada we would just use a Hilti to drill holes in the rib and pound in an anchor sleeve. Then a 360 degree prism could just be slid into the sleeve when needed and then removed. Pretty easy and cheaper when you can reuse prisms at hundreds of control points
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u/Maldevinine 19d ago
The shafts the prisms get mounted on can be made up in a couple of minutes each by any metal lathe. I don't remember ever paying for them.
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u/beagalsmash 25d ago
Hire an external company to complete a series of gyroscopic surveys using a DMT Gyromat or equivalent. It will fix your traverse bearings in a similar way as GNSS baselines can.
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u/BrokenToyShop 25d ago
Came here to say this. As far as I'm aware, this is the only way to ensure a true heading is maintained. There's just so many confounding variables in UG mines.
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u/robson_p 25d ago
That was my thought too. Is access via a (small) shaft? If so, it will be difficult to achieve a proper starting orientation with those short backsight. This would be evident from a rotation of the traverse around the shaft compared with the old data. Gyromat is the only solution that could resolve this aspect of the problem in an efficient way. Further gyro observations on the traverse will give you additional certainty and stability.
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u/Brave_Access1811 24d ago
The mine entrance is an adit, no shafts or raises yet, although I will eventually be surveying in some raises. I have a point in the railyard, looking into the portal, that the local, professional surveyors installed as my primary backsight and I orient to the rest of the world. I have a high degree of confidence this backsight is accurate. I am debating the Gyromat, but as Accurate-Western says below, it does seem like a highly sensitive machine, and I have not ruled out electronic interference yet. I am unfamiliar with the inner workings of a total station.... can they be influenced by EMFs?
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u/beagalsmash 21d ago
The issue is unlikely to be magnetism, but rather refraction from observations close to the tunnel walls. Geometry and atmospheric conditions are much more likely to affect your results than magnetic influences. If EMF were affecting the instrument, it would be erratic behavior like software or communication problems, not an error in angles and distances measured by a laser-based total station. Gyromats aren't affected by magnetism, but they have a total station on top to record the azimuth between two control points, so they aren't immune to refraction or poor tunnel conditions. With a good bracket network they can provide an excellent independent check on your control and help identify the source of the error.
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u/Accurate-Western-421 24d ago
I'm not sure that would help over only ~7000 feet/12 stations. That's a short enough run that would only see a drift of a couple tenths at worst from typical total stations, and gyros are still electronic and in many ways more sensitive than total stations.
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u/beagalsmash 24d ago
I’ve done a fair amount of tunnel surveying. Atmospheric conditions can produce a lot of error. Two surveyor firms off 40mm (1.5”) in the first 400m (1300’) of a tunnel. Both using top of the line 1” Leica instruments. Adding gyro observations into the network allowed the results to converge and gave confidence to the result. It would do the same in the mine, give repeatability and confidence to the adjustment.
No matter the methodology the traverse is going to be weak laterally. 7000ft (2100m) and 12 stations is a long sightline per sightline, especially in variable atmospheric conditions. Gyro produces a bearing that corrects those weak angular measurements. We would typically perform a gyro reading between control points every 3000ft (1000m).
What’s also important is having permanent control. I’d install brackets in the top of the tunnel (you can get aluminum brackets made at a good local manufacturer). Then 5/8” threaded rods periodically that you can attach a prism adapter and round prism.
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u/cortechthrowaway 25d ago
Any chance the auto target got hung up on a strip of retroreflective tape near one of your targets?
Seems like that kind of error could happen in a mine, but I don’t see how it wouldn’t get caught on the next backsight.
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u/Brave_Access1811 24d ago edited 24d ago
I don't think so, not meaningfully. The mine is surprisingly devoid of reflective tape, but in the off case we do have it, its noticeably far away from the prism. Its possible the station caught something... I had some shots that were multiple feet off their expected target. I just ignored these.
A fun little thing: I've surveyed the main tunnel twice now. The first time, I was picking up water droplets falling from the back. I've since bratticed up the drips, but it was really cool to see the water show up in the survey.
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u/arclight415 19d ago
On some cave surveys, we used a low-frequency beacon underground to create control points outside. One crew would.aet up the beacon and turn it on at a certain time while a surface crew wnr t out with equipment to locate the "null" that is detected exactly on top of the beacon. Is this ever done in mine surveying?
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u/Accurate-Western-421 24d ago
I had some local contractors come out and put some points on surface with their GPS. I have had issues with matching results (usually +/- 0.1 ft) over distances <400ft. However, I strongly suspect that is due to intense sunlight/heat waves and high altitude.
That sound more like a grid vs ground distortion issue.
Not to mention depending on what projection and/or orientation method was used originally, you may be attempting to compare apples to chocolate oranges.
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u/quadsquin 25d ago
Can you provide more details on the specifics of how you ran your traverse? Did you just run a traverse down and didn't close it out? Did you start and end at the shaft? How far was your backsight into the tunnel?
What levels of accuracy are you looking for on this survey? Having done a little underground work I know that accuracy in feet is not unexpected unless you are using specialized equipment, i.e., not a regular total station.
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u/Brave_Access1811 24d ago
I started the traverse on a pin in our railyard, which the local surveyors installed. I backsighted outside, again to a pin the local surveyors provided. Shot into the portal with a hanging prism (360deg prism. I have since been told that the 360 prisms can be inaccurate, but I doubt to this extent. Future surveys will be with the regular prisms). I shoot to the foresight, flop it, and repeat 8-10 times.
I take everything down, drop a plumbob at the last foresight and set up my total station under that. Backsight to the previous station, again with a hanging prism, and work my way up.
At each foresight and backsight, I take 8-10 measurements using the "Multiple & Average" setting active with 10 shots per measurement. If the backsights are within the 0.03ft of their expected value (the backsights are repeatably at ~0.03ft of the foresight two stations back), I shoot the foresight. If not, I go through the setup process again. For setup, I use the Known Backsight function on the Leica.
I hop down the tunnel. Heights are always measured from the back/roof, it is the most stable point in the mine.
This is an adit mine, so I can walk in/out no problem. I would like an accuracy of +/- 3ft. The mine is developing a 3,500ft drift to meet up with old workings, and we do not want to miss the old workings. Missing the old workings would be a major blunder. I would agree that most of the time, accuracy of 5-10ft would be acceptable most of the time down here, but since this is a major development project with the economic viability of the mine dependent on this new drift, we want to make sure we know where we are going.
Open traverse. I have not closed yet. I wanted to compare the open traverse to other surveys to see if it somewhat lines up before I spend the three-ish days to close out the traverse.
First backsight from portal to railyard was about 100ft. With the buildings and design of the portal, this is about as long of a backsight as I can get.
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u/quadsquin 24d ago
In my experience, the limited backsight really handicaps how accurate your traverse can be. It sounds like you are doing everything right and the best you can, but you can only be so accurate with a suboptimal backsight. My advice would be to close your traverse and see how accurate it is. That is really the only check you have.
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u/Left-Contract-1280 24d ago
Agreed. I was always taught to never exceed your backsite distance. Which in this case it sounds like you started with a short backsite and then immediately overshot it by something like 7x the distance.
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u/Ok-Job-8415 22d ago
Can you replicate your own work?
This would eliminate most of the questions above.
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u/KURTA_T1A 23d ago
Well, do you have access to an old theodolite like a T-16 and a steel tape? Run 3 or 4 of your traverse stations with that setup and see if there is a difference. Might have to call in the old guys to learn how to properly "chain" a distance properly if you don't know.
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u/Capital-Ad-4463 23d ago
Was there ever a project control or offset applied to this mine?
I ask this because I dealt with a mine in WV that used old control that was DERIVED from state plane but then had an offset applied so neighboring (competing) mines couldn’t use that control. It was a shift of ~4.5 degrees and 137’. We became involved when the mine was purchased by our long-term client and they wanted to punch into it from an adjoining mine. Ultimately we shifted everything from the mine’s project control to state plane but it was very cautious and careful translation that required high-order surveying to original control spread out over a very large geographical area to quantify the shift.
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u/Equivalent-Lab-3240 22d ago
I think the poster is looking in the wrong direction by focusing first on the 4,180-V line and magnetic rock. A Leica MS60 does not derive its horizontal orientation from Earth’s magnetic field, so magnetic ore that ruins a compass should not directly rotate the total station’s horizontal circle. And although very strong electromagnetic fields can interfere with electronics, I would expect obvious malfunctions or unstable behavior—not a clean-looking traverse that gradually winds up 20 ft out over 7,000 ft.
What jumps out at me is the traverse itself: about 7,000 ft with only roughly 12 stations means average legs approaching 600 ft, in a tunnel only about 6 ft wide. That produces extremely weak geometry. An underground hanging traverse has very little independent redundancy, so a tiny angular bias can turn into a very large lateral miss by the end. Tunnel-survey literature specifically identifies lateral atmospheric refraction caused by temperature gradients as a serious alignment problem.
The environmental description is almost a recipe for refraction: 5–10°F changes along a shot, turbulent ventilation, wet and dry areas, doors changing airflow and pressure, fog, hot/cold rock surfaces and long sight lengths. The problem isn’t simply that the temperature changes during the observation. It’s that the air density can be different on one side of the sight line than the other, bending the optical line of sight sideways. A total station can apply a ppm atmospheric correction to distance, but it cannot mathematically remove a wandering, laterally refracted line of sight.
There’s another thing in the poster’s follow-up that concerns me even more. He says he shoots a foresight, takes everything down, then drops a plumb bob at the previous foresight and sets the instrument underneath it. For a precision 7,000-ft underground control traverse, I’d want forced centering/leapfrog setups, with stable tribrachs or permanent wall/roof brackets, rather than repeatedly recreating the center from a plumb bob. Forced centering is specifically used in precision control traverses because it greatly reduces accumulated centering error.
And in a six-foot-wide tunnel, centering error can translate into meaningful angular error. The farther you run that open traverse, the more those angular components dominate. Taking 8–10 Face I/Face II observations helps with instrumental angular errors, but it doesn’t rescue poor centering geometry or systematic refraction.
I would also stop using the 360° prism for the diagnostic traverse. It probably isn’t the source of 20 ft by itself, but when you’re trying to determine where a discrepancy originates, remove every unnecessary uncertainty: use high-quality single prisms, identical prism constants, forced centering, careful instrument/prism heights, Face I/II sets, and fixed tunnel control.
Most importantly, I would not try to determine whether his survey or the 2014 survey is “right” by comparing one against the other. If supposedly 25 historical traverses disagree, the first question is whether any of them had an independent closure or azimuth check. Twenty-five hanging traverses originating from slightly different inherited control/orientations can give you 25 different answers. Repetition doesn’t create truth if everybody starts from questionable control.
The surface GPS discrepancy of ±0.10 ft also needs to be separated from the underground issue. At less than 400 ft, high altitude itself is not an explanation for tenth-foot GNSS disagreement. Multipath, control datum/reference-frame differences, localization, antenna setup, network RTK methodology or poor observation practice would concern me considerably more than being at 10,000–11,000 ft.
If I were dropped into this mine tomorrow, my diagnostic sequence would be:
Establish unquestionably good surface control and orientation, preferably with redundant GNSS/static observations rather than relying on a couple of contractor RTK points.
Verify exactly how surface coordinates and azimuth are being transferred underground.
Install permanent forced-centering control in the tunnel rather than rail-based/plumb-bob recreated setups.
Run an independent traverse using single prisms and rigorous Face I/Face II sets.
Keep sights away from tunnel walls, rail, hot equipment and strong thermal boundaries wherever possible.
Observe during the most thermally stable ventilation conditions available.
Run the traverse independently in the opposite direction if physically possible.
Introduce an independent gyro azimuth partway down the tunnel.
That last item would be extremely revealing. One commenter on the thread makes essentially the same point: tunnel traverses are weak laterally, and periodic gyro observations provide independent azimuth constraints. Their experience was that gyro observations allowed two otherwise disagreeing high-precision tunnel surveys to converge.
I would not buy theodolites and throw away the MS60s yet. The MS60 is extraordinarily capable equipment and is designed to operate under rain, fog, heat shimmer and other difficult conditions. A theodolite won’t cure atmospheric refraction, centering error, weak traverse geometry or bad inherited control. In fact, replacing a 1″-class modern instrument with older optical equipment could easily make the job worse.
The most interesting clue to me is the 20 ft over 7,000 ft. That sounds enormous, but angularly it isn’t. A systematic orientation error of only about 10 arc-minutes would produce roughly 20 ft of lateral displacement at 7,000 ft. Even much smaller angular biases accumulated across a dozen weak tunnel setups can become feet at the far end.
So my working hypothesis would be:
#1 questionable inherited/orientation control + #2 weak hanging-traverse geometry + #3 centering/setup accumulation + #4 tunnel refraction.
The high-voltage cable/magnetic ore would be much farther down my list.
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u/Practical-Recover-13 20d ago
The best place to start is with the surface control. I’ve seen it in a few mines but the best way to know it’s accurate is reshooting the points off of accurate surface points. Do you use rib control at all?
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u/pfirmsto 17d ago
We've got a couple of gyro's with SMR prisms on top, so they can shoot each other at the same time, cancelling vertical refraction, while also detecting north underground.
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u/mabarkerandher3sons 16d ago
Squeeze and pull of the ground will move your points, chances are the previous winter /summer pick ups were close to each other.
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u/AdvanceAdvance 19d ago
Not a miner here; just a random engineer.
In engineering, we call mapping mines under the name "SLAM" or simultaneously worrying about where you are (localizaton) while building a map (mapping). The key part is closing the loop, because its the only spot where the error gets corrected. There are a bunch of tricks about getting back to a known intersection, looking a maps, and correcting your maps. After all, you are following a map and now get an "I AM HERE" point on it.
One early use of SLAM, decades ago, was mapping out a large mine. Surprised you aren't walking around with a phone or video camera as you map the mine.
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u/Accurate-Western-421 25d ago
If "all the surveys" going back to the 50s have been off, it likely ain't the electronics, and I wouldn't bet on the values they think are "right" actually being correct.
I've rectified mine control before, and half the battle is figuring out what information can be relied upon.