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@NSpots M15@m.Hawkes #a conventional equate, fixed to floating station, it works fine equate 809d 809@NSpots A1@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@NSpots A1@a.Kairuru 809d still causes error -- equate of two fixed stations -- 809@NSpots.NgaruaArea and A1@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