[Therion] equate two (or more) fixed stations

[Bruce]

>That's how Survex handles this case.  It'll only give an error if you
>omit the standard deviations (the "4 4 12" in this case) from both fixed
>points and the coordinates aren't the same).

Thanks Olly
That makes perfect sense. I was hoping Therion would do the same.  In my
mind there is little conceptual difference between closing a loop on a pair
of fixed stations and a pair of stations attached to survey legs.

More importantly, if Therion could do this, then from a data organisational
point of view I could treat multiple survey trips to a fixed surface station
in the same way as I treat multiple survey trips to an underground station.

And Xavier, 
> My solution to this problem is to equates the two fixed stations to a
third one which is not fixed.

That is a good tip.  Therion can indeed equate fixed points, so with a
slightly messy hack as below, I can organise my surface surveys in the same
way as I organise underground surveys.

I tried;

  #Connect Surveys
  #===============  
 centreline
  equate 807 at NSpots M15 at m.Hawkes #a conventional equate, fixed to floating
station, it works fine
  
  equate  809d 809 at NSpots A1 at a.Kairuru #equate of dummy THEN two fixed
stations
  #set up dummy third station to trick therion into allowing equate of the
above fixed stations
  flags duplicate
  mark temporary  
  date 2012.01.01  
  data normal from to tape compass clino
  809d 809d1 0.5 0 -90
 endcentreline

and it works!

Some interesting observations though.
1. The dummy survey station must come before the fixed stations.
    equate  809 at NSpots A1 at a.Kairuru 809d
still causes error -- equate of two fixed stations -- 809 at NSpots.NgaruaArea
and A1 at a.Kairuru.NgaruaArea

2. I would have expected that therion and survex's cavern (when called from
within therion) would report this equate to fixed coordinates as a loop, but
they do not, so there are no loop closure statistics reported.  Of course
the % error would make no sense, but the absolute error would provide the
user useful warning that they have equated fixed points (loop length =0),
and the absolute error distance between the points.  Are there reasons why
these are not reported?

The lack of loop closure reporting made me wonder what in fact was been
done. I changed one of my fixed cords by 500m (with identical standard
deviations) and the fixed station and cave attached to it were moved by 250m
in the output.  Changing the relative standard deviations affects the
displacement as I would expect so I am satisfied that the 'loop' is indeed
being resolved properly.  I really do think that these 'loop' statistics
need to be reported however.

Incidentally, I have been using the survex loop closure option from within
therion for all of these trials.

Cheers
Bruce