That’s an interesting anecdote, thanks Andrew. Conversely I had a surveyor complete a traverse around my house, a distance of about 60m from about 6 legs, with a loop closure error of 3mm. He was reasonably pleased with that! Also, mines are surveyed very accurately, eg consider the precision drilling described in the book Bad Ground by Tony Wright. So it would be interesting to understand why the Wookey Hole survey was so bad!
I picked up an optical Theodolite from eBay for about £100. It’s a beautiful instrument, and I am interested in using it underground in the right setting. So it would be useful to be able to incorporate Theodolite angular measurements into the Therion or Survex network with appropriately small standard deviations, notwithstanding Andrew’s observation in Wookey, but they would have to be treated differently to magnetic bearings. Can anyone suggest the mathematical approach for doing that? Apologies that’s very lazy of me to ask! I did consider using an approach similar to Andrew’s, ie to calibrate the Theodolite against a magnetic leg, but intuitively that seems to tie the measurement to the inaccuracy of a single leg losing the benefit of error reduction across multiple measurements of the magnetic field (which is what happens in a magnetic traverse). It feels that somehow we need to use both techniques at the same time, using the Theodolite to “lock” the angle between legs while using the compass to align the survey to North. Andrew you might be able to see it more clearly than me, am I missing something? Sent from my iPhone > On 14 Dec 2020, at 20:38, Tarquin Wilton-Jones via Therion > <therion@speleo.sk> wrote: > > >> >> Theodolites are not really set up for doing traverses, for a loop of a few >> hundred metres we have an mis-closure of about 1m. I would be disappointed >> if I was using a disto X. It seems to be par for the course. The show cave >> owners paid for a traverse when they where tunnelling. They claimed >> millimetre accuracy however were metres out. Lead to a bit of an arguement >> between the caver surveyors and the paid surveyors about Which direction to >> dig the tunnel. It was resolved when the tunnel broke through. Luckily the >> tunnelers listen to the cavers. 😁 > > Well, that saves me buying one (as if I could afford one). I was > expecting them to be super-accurate, if calibrated properly. I'm > assuming this is not a calibration issue though. Theodolite grades were > always given as being so much higher than regular surveys, so this is > quite eye-opening. > > Presumably the existing "theodolite" mode you were referring to in > Survex/Therion is designed for use with separate positioning and > levelling, since levelling gives extremely high accuracy vertical > position, so all you need then is the horizontal. (This is similar to > how OS land surveys were traditionally done, with the horizontal and > vertical surveys being almost entirely unrelated, and only tied in a few > places for convenience.) > > I will add a note that yes, out-of-range crazy numbers is actually > normal with theodolite-style devices. Not just for inclination. > Certainly there are some other surveys done with thodolites (both > professional and home-made) where the compass bearing is over 360, with > full wrap-around. > > One other things I just learned from this thread; > degrees:minutes:seconds is a supported format. Cute. Useful for optical > theodolites which are actually accurate enough to measure in such tiny > increments. Never seen it in a cave survey before. > _______________________________________________ > Therion mailing list > Therion@speleo.sk > https://mailman.speleo.sk/listinfo/therion _______________________________________________ Therion mailing list Therion@speleo.sk https://mailman.speleo.sk/listinfo/therion
