[Therion] Multiple fixes for the same point

Bruce Mutton bruce at tomo.co.nz
Sat Sep 9 23:35:49 CEST 2017

Survex manual https://survex.com/docs/manual/datafile.htm 

fix fixes the position of <station> at the given coordinates. … The standard errors default to zero (fix station exactly).  Cavern will give an error if you attempt to fix the same survey station twice at different coordinates, or a warning if you fix it twice with matching coordinates.

Further reading about fix and cs in the survex manual explains some reasoning and how you can use the implemented behaviour to best effect.


Therion manual

fix <station> [<x> <y> <z> [<std x> <std y> <std z>]]  fix station coordinates (with specified errors—only the units transformation, not calibration, is applied to them).


1.	As a beginner, years ago, I took it as self-evident (after reading only the Therion manual) that omitting standard errors was tantamount to the user telling the software, ‘I want you to fix this position with no corrections or adjustments’.  I expect this would be the usual assumption,  for a non-technical user, and the easiest input arrangement.  They should expect an error if they accidentally apply two fixes to the same station.  This means all the distortion due to loop closure occurs in survey legs.  And it means that a user can be confident that they more or less have control over what the software is doing.


2.	The next step is to realise that applying standard errors to fixed stations allows the distortion to be shared between the survey leg network and the fixed stations.  That means fixes for positions of your cave with at least two entrances (and at least 2 fixed stations) can self-adjust to what is perhaps some sort of best fit. 


3.	For users who want to add another level of complication, as I eventually did, the concept of applying multiple coordinates to a single fixed station can be used.  Requiring standard errors to be defined explicitly case by case protects newer users from what would usually be unintentional duplication.  There are tricks to getting this to work in Therion, as the old forum posts I linked to allude to.  (I am not even sure if they are working in my projects, as I have not focused on those areas for a few years – perhaps I should have another look!)


My vote is squarely with the status quo (with any bugs that may remain in step 3 repaired, of course).



-----Original Message-----
From: Therion [mailto:therion-bounces at speleo.sk] On Behalf Of Andrew Atkinson via Therion
On 08/09/17 20:47, Bruce Mutton via Therion wrote:

> As above, I think Therion already provides users with straight forward control.  Aside from a few quirks and maybe bugs of course.


Yep I'm sure this is a quirk, maybe even a bug. The default of no variance means that multiple entrance co-ordinates cannot be entered in the most obvious manor, as currently Therion will only take one of them.

There is a way round it, that takes experience, it would be better if the most obvious way of entering it, did what you might assume, which is what I and probably others, especially beginners did/do.






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