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Slightly niche compass question….

JAA

Active member
Pondering…

Has anyone with a “modern” computer with an onboard compass made any comparison between it and a ‘traditional’ dive compass?
Suunto describes theirs as “accurate to +/- 15 degrees” which seems a fairly large margin, especially if surveying with one?

It does seem semi useful that they give the ability to lock a bearing in whilst swimming and adjust the declination etc but I wonder if I’m still going to have to wear a standard compass for survey work.

Not that I have miles of passage to survey I hasten to add!
 

Steve Clark

Well-known member
Never bothered to try with a digital compass, other than someone trying to use a MNemo tool, which I believe has a digital compass built in and records at the start and end of a shot.

We've found this compass setup exceptionally useful for compass and line work. You don't have to put down or store the compass to write down the bearings.

Recycled plastic pencils. The pencil is very easy to hold above the line and align by looking straight down, even in poor vis. (Poor-good vis obvs, you may have a different scale to work with on that!)

It helps to get the cable tie at exactly the right tightness to allow you to swap the pencil underwater if it snaps but not flop about. You can store the compass and spare pencils in the pocket in the back of wetnotes.

10155901_774556165910170_2135246133979646327_n.jpg
 

Pitlamp

Well-known member
That's a neat idea Steve.

One possible advantage of a digital compas (probably; I've not tried it 'cos they're expensive and rely on a battery) is the ability to get a numerical readout rather than squinting at small graduations on a traditional compass whilst trying to hold it level. This feature of a digital compass would be really useful in low bedding planes, where it's difficult to focus on the graduations with the mask so close to the instrument.

That quoted degree of (in)accuracy is surprising; I've played with the digital compass on my mobile (above water) and it seems far more precise than that. Perhaps Suunto's quoted +/- 15 degrees allows for magnetic interference from steel rtanks and bright lamps etc? In which case good for them for being up front with the limitations.

JAA; if you end up experimenting with an underwater digital compass I'd be interested to learn how you get on.
 

JAA

Active member
That's a neat idea Steve.

One possible advantage of a digital compas (probably; I've not tried it 'cos they're expensive and rely on a battery) is the ability to get a numerical readout rather than squinting at small graduations on a traditional compass whilst trying to hold it level. This feature of a digital compass would be really useful in low bedding planes, where it's difficult to focus on the graduations with the mask so close to the instrument.

That quoted degree of (in)accuracy is surprising; I've played with the digital compass on my mobile (above water) and it seems far more precise than that. Perhaps Suunto's quoted +/- 15 degrees allows for magnetic interference from steel rtanks and bright lamps etc? In which case good for them for being up front with the limitations.

JAA; if you end up experimenting with an underwater digital compass I'd be interested to learn how you get on.
Take my computer for a swim if you like @Pitlamp and see what you make of it?
 

Pitlamp

Well-known member
That's a kind offer; thanks. But I'll let you pursue the experimentation. Doubt I'll get much chance to get my head wet over the next few weeks as I have a busy period looming.
 
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alastairgott

Well-known member
That's a kind offer; thanks. But I'll let you pursue the experimentation. Doubt I'll get much chance to get my head wet over the next few weeks as I have a busy period looming.
I see a flourishing acting career ahead of you. At least until a couple of weeks before the panto season finishes.
 

Steve Clark

Well-known member
One possible advantage of a digital compas (probably; I've not tried it 'cos they're expensive and rely on a battery) is the ability to get a numerical readout rather than squinting at small graduations on a traditional compass whilst trying to hold it level. This feature of a digital compass would be really useful in low bedding planes, where it's difficult to focus on the graduations with the mask so close to the instrument.

We've refined the compass reading technique to avoid the need to squint, but there is a bit of subtlety to it. You've only got two hands and can't hold anything in your teeth! It goes something like this for a right-handed person with the computer on the right wrist :

  • Get yourself to the start point, check gas, check everything well, give yourself a mental 'leave' time & gas before you start.
  • Get the wetnotes and compass+pencil out. Notes in left hand, compass+pencil in right.
  • Arrive at the 'station'
  • Write down any description - "eg. silt screw"
  • Write down estimates of the LRUD dimensions - in this case "L 2m, R 1m, U 1.5m, D 0.2m". (We always do this relative to the direction of travel, so it works in survex/therion. This is not always into the cave).
  • Put dive computer (right hand) next to the station, mental note of the depth.
  • The use the right hand to write it in the notes - "6.1m"
  • Put the compass pencil over the line. Adjust the bezel so the arrow outline is over the needle.
  • Then lift it away from the line to a comfortable position, read off the bearing it is set to. Then use pencil to write it in the book. - say 078 deg
  • Tuck pencil into back of wetnotes, get out the string, put wetnotes in pocket. Because we are usually surveying someone else's unknotted line, we use a 1m length of cord with a loop in each end and a knot at 0.5m. Pass it hand over hand down the line until we reach the next station, counting as we go. Estimate the last fraction of a metre, which quite easy with the knot in the middle.
  • Get the wetnotes and pencil back out, tuck away the string. Record the shot length (slope distance), say "7.6m"
  • Then take the compass, which is already set, place it over the string and the needle should still be in the arrow outline if the shot was straight. If not, find the mistake or take an average.
  • Record the end station description & depth as before - say "6.8m", triangle shaped boulder.
  • AT THIS POINT DO A GAS, TIME AND SANITY CHECK BEFORE STARTING THE PROCESS AGAIN.
We practiced and used this technique for hundreds of survey shots. Initially in Capernwray laying a long line course that was a loop so we could check the closure. Then in cave, generally in teams of two. One person doing the bulk of the work and the other person doing some lighting off the line to estimate sidewall dimensions. It works solo though, you don't need to put anything down or have someone hold it. In easy-ish cave you can get it down to a few mins per shot, maybe 15 stations an hour.

We record using the template from Mexico which is quite neat. The data above would look like this for one shot :

2024-10-27 19.16.27.jpg
 

Pitlamp

Well-known member
Some interesting suggestions there Steve. What sort of % misclosures does the method give?
 

Steve Clark

Well-known member
In a loop of about 100m of line in Capernwray it was in the 1-2m range on plan. It’s hard to judge too much though because there could be a systematic error in the length (say all a bit short) and it would probably cancel out. I guess on an average shot of 8m, we were getting length within 0.2-0.3m compared to a tape, so maybe 3%? For an angle of +/- 2deg, that’s about 2.5% or 0.3m on 8m too.

From memory, the error was angle dominated. Open like a horseshoe rather than significantly off for length.

We only managed one loop in St Georges. Half wet and half dry between p.750 & p.900m. Two passages in a big diagonal fault. It was about 3m out over 150m out, 150m back.

However, that’s only part of the story. We had two major errors that existed in the survey for the first 5 years or so! One was a 2.1m shot that was put in therion as 21m. We found that when we went back to sketch it in detail and the centreline didn’t match the cave. Another was a 64m depth that looked like 54m scribbled on the survey sheet. That put a weird kink in the shaft down to 75m that again didn’t actually exist when we tried to sketch the elevation. There may be more! Without doing it all twice there’s no redundancy to be 100% sure.

On thing we did notice introduced errors was the difference in calibration/salinity display from using different dive computers. Up to 1m at 30m depth. We didn’t mark survey stations, just relied on depth and description initially. That caused some confusion when a different team went back and the depth was out by a fair bit.

Another thing we noticed was the error in using handheld GPS coordinates of entrances and then using therion to overlay on Google earth. I was expecting 5-10m but it was more than that in practice. Either in gps accuracy or the Google projection / alignment. In the end, we adjusted the coordinates so the overlay matched the actual entrance position visible on Google earth.

We also had a different primary objective to most cave divers. It wasn’t really exploration of new cave or trying to find leads etc. We were focussed on producing a high quality survey that would be used by lots of folks in the future, so we could concentrate on getting good at just that part of the job.

An interesting aspect is that I’ve never been beyond about p1000m, but from the data provided by others and the video I know it a lot better than many who’ve actually dived the deeper bit of sump 2.
 

Steve Clark

Well-known member
To do the Google earth adjustment we surveyed from the cave entrance / start of the line back out to a known object visible on Google earth - in our case the corner of a bridge. Then used google’s coordinates for that point as the fix station in therion. When it was processed again, it shifts the lines to align with Google earth.
 

Pitlamp

Well-known member
Thanks Steve, those are pretty good misclosures! Best I ever managed, using an A4 Formica board with the compass screwed to it (to improve alignment) and a non stretch measuring tape was 0.7% (Keld to KMC). There is a bit of info about this in the CDG manual, from memory.

But that was done very carefully over several dives. The standard CDG method, which evolved such that standard instruments routinely carried are used (unmodified) is a necessary compromise between speed and accuracy. It often makes the difference between getting at least a reasonable survey and getting nothing. The thinking is that if a simple method is available, divers who are laying line are more likely to survey back along them each time. Using this method, 2% misclosure is regarded as about the best that can be hoped for. 5% wouldn't be unusual.

I agree that the main reason for misclosure is compass errors. It's difficult to eliminate these altogether.

Some time ago (30 years?) I remember a French or Swiss cave diver doing experients in this area. He laid two steel cylinders on a wooden turntable and a compass (in the sort of position it would be relative to the tanks on a dive). He then recorded the bearings from the compass as the set up was rotated and got some interesting results. The error varied with different alignments relative to the Earth's magnetic field. There is a note about this (with, from memory) a graph somewhere in a CDG Newsletter.
 
It should be relatively simple to find one’s compass error. If all survey shots are taken with the same equipment configuration, the error should be a magnetic bias. This can be found by experiments conducted before or after the cave survey.

To find the bias, you would lay out radial lines at known intervals (say 45 degrees) and take shots along each. Then plotting a constant distance along each measured azimuth should produce a circle, whose centre offset from the origin indicates the bias. This in turn can be used to determine corrections for any measured azimuth.

The only difficulty here is laying out the lines at known angles without bias. Perhaps the easiest way would be to find an open water location, shallow enough to lay out the lines without dive gear, yet deep enough to wear your cave rig. With an assistant, all the sighting to lay out lines could be done above water.
 
It should be relatively simple to find one’s compass error. If all survey shots are taken with the same equipment configuration, the error should be a magnetic bias. This can be found by experiments conducted before or after the cave survey.

To find the bias, you would lay out radial lines at known intervals (say 45 degrees) and take shots along each. Then plotting a constant distance along each measured azimuth should produce a circle, whose centre offset from the origin indicates the bias. This in turn can be used to determine corrections for any measured azimuth.

The only difficulty here is laying out the lines at known angles without bias. Perhaps the easiest way would be to find an open water location, shallow enough to lay out the lines without dive gear, yet deep enough to wear your cave rig. With an assistant, all the sighting to lay out lines could be done above water.
Sorry - it's not simply the centre offset, that would need horizontal intensity which would require an electronic compass. But those measurements will certainly suffice (indeed, you probably only need points 90 degrees apart). Will post the correct procedure shortly if it's of interest, assuming nobody beats me to it.
 

Steve Clark

Well-known member
I'm interested in the understanding procedure. I have a background in engineering surveying (buildings mainly), so am quite used to instrument checks, workflow that causes error cancellation and the like. Obviously, we don't use compasses or the earth's magnetic field for anything remotely accurate. Compass is +/- a few degrees, but it is absolute so errors (particularly gross errors & typos) don't propagate in the same way to relative angles measured with theodolites & total stations. We can measure angles to 1 arc sec (~0.5mm at 100m), but if just one shot is wrong the whole thing is miles off and knackered. Relies on loops and redundancy through repetition and triangulation checks with length.

I don't know for sure, but I would suspect that other effects would be larger than a systematic magnetic bias on a compass used underwater, or that the bias wouldn't be constant every time you took a reading.

As Pitlamp mentioned above, you can't always position yourself to read the compass from a consistent position. The line can be against the left or right wall, so you have to use different hands and the angles to your eyes are different. The line can be quite steeply inclined so you have to hover above and sight through (the pencil in our case). If it's on the floor and going downslope, you have to turn yourself around and use the compass rotated 180 or do some maths on the fly.

There's also the possibility of interaction with electronics & lighting. We would normally dive with a goodman handle style light in our left hand, but we clip it off on the shoulder when doing this kind of survey. There is a ballast with a coil in the HID lamps. There's also 40kgs of steel tanks moving about.

Overall, it helps to understand the principles so you can make some judgements. For a line dropping 20m almost vertically down a shaft, an error of 10deg in its plan bearing will make no noticeable difference at all - that's good because its almost impossible to measure accurately. For a horizontal 20m shot it's worth putting in the effort to get it accurate, 3 degrees is a full metre sideways at the other end.
 
All good points. My analysis was based on the primary interference coming from steel tanks and on surveying straight horizontal cave. The lamp coil should behave in a similar way if its relative position is constant (shoulder sounds better than wrist). As you point out, the effects depend on one's attitude, but worst case is likely to be horizontal. Actually horizontal east or west is worse than north or south if the error field is aligned with the tanks. If left and right walls occur with similar frequency, they may tend to cancel out in calculating final position. It's possible that other effects may be similar or even greater at times, but the key to getting the best final position is to reduce systematic errors as much as possible.
I now have a procedure to reduce biases using the Solver add-in in Excel 2013 (I'm a cheapskate). I'd love to play around taking measurements, but I haven't owned any steel tanks in a long time.
I'm now retired, but my background was in oilwell surveying where magnetic surveys were used quite a bit for real-time navigation, as they are more robust than high-accuracy gyros at present. We had to deal with similar issues due to the massive steel drilling equipment, but we had the advantage of using 3D magnetometers.
I'll tidy up the spreadsheet a bit and see if I can post it here.
 
It seems I can't attach a .xlsx file, however I have zipped it and perhaps that will work. Hopefully the notes in the spreadsheet are sufficient to explain the math.
The idea is to remove those errors which have the greatest effect on final position; next to gross errors, the worst are likely to be systematic azimuth errors which peak in horizontal attitudes. That is what I am trying to correct for here. At least this could be used to estimate the significance of errors caused by steel tanks and light coils, and thus see whether corrections are worthwhile.
 

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Pitlamp

Well-known member
I suspect the problems of actually reading graduations on a compass accurately enough underwater shouldn't be underestimated. Not all of the compass error is due to errors directly caused by the compass itself.

Steve does lot of surveying in French conditions (and the results of his team's surveying are utterly superb).

Here in the Dales the conditions mean that accuracy must be a more rough and ready compromise.

When I first began cave diving very few people surveyed underwater at all. I was inspired to do it by people like Geoff Yeadon. It's far better to surface with a less than ideal survey than with no numbers at all. These days there are ways of dragging the survey around to reduce errors, such as radio location. This is particularly useful when cave divers break ground above water beyond sumps. This approach has proved very useful over the years; the pinpointing of Mincemeat Aven in Notts II and the aven which eventually gave the Shuttleworth Pot dry entrance to Witches Cave II spring to mind.
 
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Cantclimbtom

Well-known member
Apologies to intrude.. but if you dive with an "ordinary" compass (i.e. the sort of thing someone might have for D of E) rather than Sunto SK8 (or some other dive intended compass) does it get broken by diving? I mean are the tilt compensation and retractor/bungee mount the major physical differences or is the capsule sealed more robustly on a diving one? (ignoring the different marking styles)
 

Steve Clark

Well-known member
So…we’ve had a bit to history with this. Generally, a silva compass is filled with incompressible mineral oil and it works fine underwater. However, they occasionally develop an ‘air’ bubble within the capsule. If it’s small this doesn’t really affect the compass operation at the surface. However, when used underwater at depth this allows the plastic face of the capsule to compress against the central needle mount and stops the needle rotating.

We are not really sure where the ‘air’ comes from. Possibly dissolved gas in the mineral oil. It could be that diving with them causes some kind of rapid change in pressure / nucleation event that creates the bubble. They never seem to go away and re-dissolve though.

It effectively knackers the compass for underwater use after that. We drilled one of these to let the oil drain out and we could again use this underwater, it just partially fills with water. Lacking the oil makes the needle damping much worse so it’s harder to use.

If you’re buying a silva compass to use underwater ensure it has no bubble in it whatsoever when you buy it. From a shop, not online.
 
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