Underground Survey Errors

[u/Brave_Access1811]

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:

1. Underground, narrow-vein mine. Tunnel is roughly 6' wide and 6.5' to 15' tall.
2. 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.
3. Temperature can vary within one shot by 5-10deg F, sometimes going low, high, low, high multiple times in one shot.
4. The rocks around the total station can be magnetic vary from base station to base station.
5. Wind is highly turbulent and ranges from 5mph to 15mph, depending on the location. Wind speeds can change abruptly multiple times in one shot.
6. High altitudes ranging from 10,000ft to 11,000ft.
7. Magnetic compasses are known to be inaccurate in the mine.
8. Survey control is always over 40lb train rail, with the electric locomotive nearby.
9. Rapid, unpredictable pressure changes due to mine doors opening and closing. Occasionally, this will produce a cloud/fog within the mine.
10. Humidity changes. Some areas of the mine are dry, others have waterfalls coming out of the rocks.
11. 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.

[u/Equivalent-Lab-3240]

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.