[Therion] Handling of tape and backtape in survey data

[Ben Cooper]

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 <bgee at campercaver.net> 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
> 
> 
> 
> 
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