Hi Bill, et al, I’ve always thought that backsites didn’t need specific handling as effectively they form a small loop so would be handled by the loop averaging.
I don’t often do backsites, and get reasonable accuracy without (as good as 0.5%, depending on conditions). But when I do I always do a full reading, indeed it would be hard not to with a DistoX-PDA “total station” as all the data is collected automatically. In fact for this setup, there is no way to mark a backsite as a backsite (or if there is, I’ve never used it!) and instead Toporobot just generates a new station for the backsite. I then manually “equate” these in the .th file, creating a loop, together with any actual loops. I usually do this after compiling the survey for the first time, as it’s fun to see on the 3d how close any two measured stations appear, and hence get a visual feel for the accuracy and where potential blunders might (eg mislabelling a station) But now I’m wondering how Survex handles all these loops because if every leg has a backsite, then the number of loops increases, crumbs, very quickly. Hmm, I can't even begin to work out how to count that mathematically. It’s very much smaller than nCr because the loops can only be made up of adjacent legs, eg for 5 legs, the loops generated between any three are only 1-2-3, 2-3-4, and 3-4-5; nCr also counts 1-2-4, 1-2-5, etc, which are not loops. So while nCr quickly becomes uncomputable for even modest sizes of n, I don’t know if the number of loops also becomes uncomputable? What does Survex actually do for all these loops? Best regards Ben Sent from my iPhone > On 25 Dec 2021, at 14:51, Bill Gee <[email protected]> wrote: > > Hello everyone - > > I have a holiday question for the group. We have for years done our survey > shots both directions, obtaining a forward and backward reading for both > compass and inclinometer. The two readings are averaged by Survex and > Therion in an effort to reduce errors. The tape reading, however, has only > been a single number. > > Now that the use of DistoX2 devices is becoming common, it is easy to get a > distance measurement both forward and backward. I have been ignoring the > backward distance reading in the field. It does not get into my notes. I > might need to change that practice. > > I looked at the Survex manual yesterday and see that it supports both TAPE > and BACKTAPE (also called LENGTH and BACKLENGTH) in the survey data. That > brings up two questions: > > 1) Does Therion also recognize the BACKTAPE data type? > > 2) What does Survex do if both TAPE and BACKTAPE are given? Does it average > the two readings? Does Therion do the same thing? > > I understand that Therion will use Survex to reduce the centerline data - if > Survex is installed. In case Survex is NOT installed, then Therion reduces > the centerline itself. As a result they might handle this situation > differently. > > As I write this, another related question occurred to me. When either > Therion or Survex averages a forward and backward reading, how many > significant digits does it carry in the calculation? Can the significant > digits be changed? > > And the same question applies to loop closure calculations. How many > significant digits are carried through the calculations? > > Thanks! > ======== > Bill Gee > > > > > _______________________________________________ > Therion mailing list > [email protected] > https://mailman.speleo.sk/listinfo/therion _______________________________________________ Therion mailing list [email protected] https://mailman.speleo.sk/listinfo/therion
