Accuracy Questions

[graham]

For compass and clino doing a leg between two fixed points (e.g. 2 trees 20m apart) then shooting back the other way will indicate if your instruments are ok in relative terms. Compass bearings should be 180 degrees different and clinos the same angle just positive rather than negative or vice versa.

I have old calibration data from RAF astro-compasses used in the 1950s which shows different errors in different quadrants, so something more complex than that may be required. I do not know how the collected that data nor what they calibrated the instruments against, but I can see that certain systemic faults, such as a pivot point which is not quite central, will give a variable error around the card.

 

[cavermark]

For compass and clino doing a leg between two fixed points (e.g. 2 trees 20m apart) then shooting back the other way will indicate if your instruments are ok in relative terms. Compass bearings should be 180 degrees different and clinos the same angle just positive rather than negative or vice versa.

I have old calibration data from RAF astro-compasses used in the 1950s which shows different errors in different quadrants, so something more complex than that may be required. I do not know how the collected that data nor what they calibrated the instruments against, but I can see that certain systemic faults, such as a pivot point which is not quite central, will give a variable error around the card.

I wonder what amount of error you are talking about there Graham - is it likely to make a significant impact on cave surveying (e.g. less than half a degree)?

Would things like off centre pivot points not be due to manufacturing and within a tolerance to be sold? Or would it occur during use?

 

[Cave_Troll]

I have old calibration data from RAF astro-compasses used in the 1950s which shows different errors in different quadrants, so something more complex than that may be required. I do not know how the collected that data nor what they calibrated the instruments against, but I can see that certain systemic faults, such as a pivot point which is not quite central, will give a variable error around the card.

Over what distance were those differences observed?
I'd expect a greater chance of different errors if you're looking at the errors at two points 10km apart.

 

[graham]

I wonder what amount of error you are talking about there Graham - is it likely to make a significant impact on cave surveying (e.g. less than half a degree)?

Would things like off centre pivot points not be due to manufacturing and within a tolerance to be sold? Or would it occur during use?

I'm afraid I don't have the stuff to hand, it's in our Library. Don't forget that these were ex-WD instruments and should have been of good quality.

 

[graham]

Over what distance were those differences observed?
I'd expect a greater chance of different errors if you're looking at the errors at two points 10km apart.

IIRC the checks were done from the top of a tower & were looking at points about 1 km away in each direction.

 

[jarvist]

There's a very interesting article somewhere (I can't remember where - I think Compass Points, but I couldn't find it yesterday reading the back archive), looking at sources of errors in mechanical instruments. Essentially they came down to offset errors such as the disk slipping, but also much more complex error caused by the jewelled pivot wearing out a (quite possibly non circular) hole in the card (particularly with the heavy clino - I seem to recall it mentioned that you have to do varying inclination readings when calibrating for this reason).

Personally I have not been able to detect any reproducible measurable difference (i.e. any calibration to apply) between the modern Suunto instruments held by Imperial, except for when the instrument is knackered to the point where it is retired anyway. As such I now just take multiple cross-checked measurements between them all to see if any of the compass cards are sticking etc.

However, we wear those poor little things out quickly taking them caving, so they're all pretty young - decades old club instruments may have been oddly magnetised while sitting in the library, balanced for the wrong hemisphere, used to bang in nails, badly jolted rattling in someone's car etc.

Graham - your WD compass certainly sounds like the effect of having some part of the enclosure becoming magnetised (I guess they didn't have such massive stocks of Alu or otherwise non-iron / ferromagnetic based metals back then?) - this happens inevitably to steel boats, and is calibrated by physically placing offsetting magnets in the binnacle.

 

[jarvist]

Found the one on Suunto Compass errors with a suitably long google search... Can't find the info on the Clino - perhaps I imagined it?

"Errors In The Suunto Compass Used For Cave Surveying - Lang Brod", Compass Points 21
http://utter.chaos.org.uk/survex/cp/CP21/CPoint21.htm#art2

Unfortunately missing the figures...

 

[kdxn]

You might also want to check the NSS Compass and Tape archive:

http://www.caves.org/section/sacs/SACS/C_%26_T_Back_Issues.html

Some articles on Suunto readings, accuracy, bias and statistics.

 

[wookey]

And here is the smk system looplist from CUCC's austria dataset, including a lot of ARGE date. This is 822 loops in 97km of cave. Mostly done with compass+clino+tape, but a fair number of SAP, laser tape and distoX surveys in there now too. http://wookware.org/files/smk-system-loops.txt

It's a lot noisier than my earlier post suggested, with quite a lot of loops worse than 2%, even longer ones. An analysis by instrument type, caving club, surveyor would be interesting. I'd expect ARGE's data to be better and distoX data to be better. But that would require a bit more work than:

cavern smk-system.svx; grep Original smk-system.err | sort -n -k5 > smk-system-loops.txt; scp smk-system-loops.tx wookware:wookware/files

plenty of room for survex to provide stats in a more useful and malleable form.