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

[Tarquin Wilton-Jones]

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

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encoding  utf-8
source "multifix.th"
export model -fmt kml -o multifix.kml
export model -fmt survex -o multifix.3d
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survey foo
	centreline
		cs OSGB:SN
		fix 1a 0 0 100 1 2 3
		fix 1a 1 1 101 1 2 3
		fix 1a 2 2 102 1 2 3
		fix 1a 3 3 103 1 2 3

		fix 1b 0 0 100 1 2 3
	endcentreline
	centreline
		cs OSGB:SN
		fix 1b 1 1 101 1 2 3
		fix 1b 2 2 102 1 2 3
		fix 1b 3 3 103 1 2 3
	endcentreline
	centreline
		date 2020.01.01
		1a 2a 5 0 0
		1a 3a 5 45 0
		1a 4a 5 90 0
		1a 5a 5 135 0
		1a 6a 5 180 0
		1a 7a 5 225 0
		1a 8a 5 270 0
		1a 9a 5 315 0
		1b 2b 5 0 0
		1b 3b 5 45 0
		1b 4b 5 90 0
		1b 5b 5 135 0
		1b 6b 5 180 0
		1b 7b 5 225 0
		1b 8b 5 270 0
		1b 9b 5 315 0
	endcentreline
endsurvey