Error in loop closure algorithm?

[Mike McCombe]

I've been using Survex as a means of generating a reasonably accurate architectural drawing. The technique is to assume the builder got it right and that the walls of a rectangular room are aligned at 0, 90, 180 or 270 degrees. You then just go round each room with a tape measure, producing a loop per room, connected at the doorways. This works really well but seems to demonstrate a possible error in the Survex loop-closure algorithm.

To demonstrate this, take a simple survey of two perfect loops, joined by a single leg:-

*begin test
1 2 10 90 0
2 3 10 180 0
3 4 10 270 0
4 1 10 0 0
4 5 10 270 0
5 6 10 0 0
6 7 10 270 0
7 8 10 180 0
8 5 10 90 0
*end test

Now introduce an error into one of the loops:-

*begin test
1 2 10 90 0
2 3 10 180 0
3 4 10 270 0
4 1 10 0 0
4 5 10 270 0
5 6 10 0 0
6 7 10 270 0
7 8 7 180 0 ; Gross error in this leg only
8 5 10 90 0
*end test

Survex moves not only the survey legs in the "bad" loop but also adjusts the connecting leg (i.e. test.4 - test.5) and moves the station in the "good" loop (i.e. test.4) where the connecting leg joins. The "error" is only 1cm which wouldn't show in most cave surveys (with me using the instruments!) . However, the error is still 1cm if you scale the loops down to, say, 1m per leg - which becomes quite noticeable when the loops are small and generally well-closed. 

Is this an error in the algorithm or just the result of me using Survex in a way that wasn't intended?

Mike
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