[Therion] Fixing a point using standard deviation in different centrelines

[Bruce Mutton]

Freaky.  I have done this a number of times, but of course never looked critically at the results.  Too trusting.
So I can replicate the effect for both the scenarios you describe using my usual 'loop-closure survex' in therion.ini.
If I switch to 'loop-closure therion' then both scenarios give co-incident stations 1a and 1b, which I believe is what you are after.

So all these years I've been using the 'better' survex loop closure to avoid one bug, and inadvertently falling for another!

Bruce

-----Original Message-----
From: Therion <therion-bounces at speleo.sk> On Behalf Of Tarquin Wilton-Jones via Therion
Sent: Wednesday, 16 September 2020 23:48
To: Therion <therion at speleo.sk>
Cc: Tarquin Wilton-Jones <tarquin.wilton-jones at ntlworld.com>
Subject: [Therion] Fixing a point using standard deviation in different centrelines

Hi folks,

Our survey has a location fix. The location has been taken 4 times, on 2 separate dates, with separate teams and equipment. As a result, I would like to have the fix data in 2 centreline sections, each with its own date, team and instrument. Each fix has its standard deviation values set, so that Therion can calculate the weighted position.

When I do that, Therion miscalculates the position. If I put the fix statements all in the same centreline section, Therion calculates the position correctly. It is definitely using all four "fix" statements, as the positions are different if I remove the second centreline block completely. But when they are split over separate centreline blocks, it seems to reduce the weighting that it applies to the fixes in the second block. The specified standard deviations don't seem to affect the bug.

I have attached a demo that shows the problem, along with .3d and .kml outputs that show the incorrect positions.

Am I doing something wrong, or is this a Therion bug?

Cheers,

Tarquin