Diving data loop closures altering depths

[Footleg]

Yes that was a typo. Should have been:
Station entrance.030 is at depth -5.20

I follow the argument you make Olly, but there are a couple of things
which do not make sense.
1) Why does having data in diving format change the positions of
stations with a loop closure compared to data for exactly the same
loop in normal format?
2) If diving data is dealt with differently, then it is not a valid
assumption to treat errors in depth readings in the same way as errors
in tape and compass.
The tape and compass errors are cumulative, so it is valid to adjust
each station position with respect to the last station in adjusting
for loop closure errors. But the depth readings are absolute, so
errors to not accumulate. If you adjust the depth of each station
relative to the depth of the last station then the depths will change
well outside the relative errors of their measurements the longer the
loop.

To deal with depths in loop closures of diving data, the depths should
be adjusted relative to the two ends of a section where depths are
fixed by other data, so it would not be possible for raising one end
of a section of diving data to result in stations getting deeper along
that section. Once the depths have been adjusted to deal with errors
in the closure, the other dimensions should be adjusted while
constraining the depths to the adjusted values they were assigned in
the depth adjustments step previously.

Footleg

On 27 June 2013 06:13, Olly Betts <olly at survex.com> wrote:
> On Mon, Jun 17, 2013 at 02:38:04PM +0100, Footleg wrote:
>> Trying to track down why some underwater data was several metres higher
>> than a sump water surface, I discovered that closing loops between stations
>> at the same depth in diving data alters the depth of surrounding data. This
>> was not expected!
>>
>> I've attached a fragment of the data which illustrates this. Without the
>> loop closed:
>>
>> Station entrance.030 is at depth -5.30
>
> Is this a typo?  I get -5.20 as the depth for this station.


> I've had a look - I think this is something that can reasonably happen.
>
> Each leg in a diving survey gives the slope length, the vertical change,
> and a bearing.  Although the two ends are the same height, when you tell
> survex to "pull them together", the diving legs in between don't simply
> have the constraints of the depth readings, they also have the
> constraints of the bearings and slope lengths.
>
> If you take this individual underwater leg with a slope length of 5m and
> a depth change of 4m (and so the horizontal distance is 3m, by
> Pythagoras' Theorem):
>
>      ---->
>       B   ^
>      /    4m
>     /     |
>    A      v
>    <3m>
>
> (best viewed in a monospace font...)
>
> The leg says that the length "wants" to be 5m and the vertical change
> "wants" to be 4m.
>
> If you close a loop such that the horizontal distance needs to increase,
> then simply moving B horizontally preserves the vertical reading but
> forces the length to change.  But there's no reason to suppose that the
> depth gauge is totally precise and the tape measure is lousy, so it's
> more reasonable change the length by less but also change the depth a
> bit (so rotate the leg, rather than shearing it).  If you think of
> simulating the readings and their errors as a system of springs, then
> pulling B to the right stretches the length spring, causing B to also
> be pulled down.  B moving down compresses the depth spring, which tends
> to push B back up.  B will end up in some equilibrium position,
> depending on the strength of the two springs (which corresponds to what
> the standard deviations given for the readings are) and the force
> applied to move B right.
>
> In your example it's a lot more complex as there are many legs involved,
> but I think this is probably the effect behind what you are seeing.
>
> I tried playing with some simpler examples, which show similar effects,
> but I did find something odd that happens that I can't explain.  For
> example, this survey lies entirely in an E-W vertical plane, but closing
> the loop causes the survey to rotate out of that plane:
>
> ;*equate sump2dry sump2wet
>
> *fix sump1 0 0 0
>
> ; Dry route between the sumps:
> sump1 2 10.00 090 045
> 2 sump2dry 10.00 090 -045
>
> ; Wet route between the sumps:
> *data diving from to length bearing fromdepth todepth
> sump1 3 8.00 090 0 -6.00
> 3 sump2wet 8.00 090 -6.00 0
>
> ; These are the unclosed positions of the stations:
> *fix 2_        reference  7.07 0.00  7.07
> *fix sump2dry_ reference 14.14 0.00  0.00
> *fix 3_        reference  5.29 0.00 -6.00
> *fix sump2wet_ reference 10.58 0.00  0.00
>
> While there's no reason to trust the compass readings as being perfect,
> I'd expect the symmetry of the situation to mean that the survey stayed
> in the plane.
>
> Cheers,
>     Olly