Diving data loop closures altering depths

[Graham Mullan]

I would guess from Footleg's comments that Survex is not treating the
depth in dive data as absolute in that it is independent of the other
readings. If it is treating it purely geometrically, then surely this
sort of problem will arise. The algorithm that covers dive data ought to
treat the horizontal and vertical data sets utterly separately & clearly
doesn't.

Graham
--
graham.mullan at coly.org.uk

0117 9502556 or 07887 637064 (or 0844 2327803)

On 26/06/2013 15:20, Footleg wrote:
> The seriousness of this diving data loop closure issue is illustrated
> by the Keld Head sump survey in the cave registry. Currently the
> upstream end of the sump is pulled 13.8m higher than the resurgence
> level due to the survey data from the Valley Entrance down to the
> sump. You would think this would make the sump survey appear up to 13m
> higher than it should be. But in fact Survex generates it with the
> deepest points 25m deeper than the diving depths in the input data. If
> you look at it in elevation it has plotted the sump descending to -45m
> before curving back up to the resurgence level. But the deepest point
> in the dive is only -20m. If you break one side of all the loops it
> does not bow the sump like this. Be aware then that diving depths
> should not be trusted from Survex models if any loops are involved.
> 
> Footleg
> 
> On 18 June 2013 13:45, Footleg <drfootleg at gmail.com> wrote:
>> A simple example data file follows. Two lengths of diving data forming two
>> sides of a loop. Both end at the same depth. Joining the ends to close the
>> loop causes the depth of the stations 4 and 'd' to change from -5.8m to
>> -5.95m. Creating the same series of legs in normal tape,compass,clino format
>> and closing the loop causes the depth of the stations 4 and 'd' to change
>> from -5.8m to -5.78m. Clearly something different (and undesirable) appears
>> to be happening in loop closures on diving data.
>>
>> -----------------------------------------
>>
>> *begin divingtest
>>
>> *fix Ent 0 0 0
>> *entrance Ent
>>
>> *equate Ent dive_open.0
>> *equate Ent dive_closed.0
>> *equate Ent normal_open.0
>> *equate Ent normal_closed.0
>>
>> *begin dive_open
>>
>> *data diving from to bearing length fromdepth todepth
>>
>> 0 1 330 17.40 0 -02.00
>> 1 2 350 03.20 -02.00 -03.30
>> 2 3 020 10.70 -03.30 -05.90
>> 3 4 070 12.90 -05.90 -05.80
>>
>> 0 a 045 13.80 0 -02.90
>> a b 021 07.80 -02.90 -03.90
>> b c 340 15.00 -03.90 -04.20
>> c d 350 04.35 -04.20 -05.80
>>
>> *end dive_open
>>
>> *begin dive_closed
>>
>> *equate 4 d
>>
>> *data diving from to bearing length fromdepth todepth
>>
>> 0 1 330 17.40 0 -02.00
>> 1 2 350 03.20 -02.00 -03.30
>> 2 3 020 10.70 -03.30 -05.90
>> 3 4 070 12.90 -05.90 -05.80
>>
>> 0 a 045 13.80 0 -02.90
>> a b 021 07.80 -02.90 -03.90
>> b c 340 15.00 -03.90 -04.20
>> c d 350 04.35 -04.20 -05.80
>>
>> *end dive_closed
>>
>> *BEGIN normal_open
>>
>> 0 1 17.40 330 -6.60
>> 1 2 3.20 350 -23.96
>> 2 3 10.70 020 -14.07
>> 3 4 12.90 070  0.44
>>
>> 0 a 13.80 045 -12.13
>> a b 7.80 021  -7.37
>> b c 15.00 340  -1.15
>> c d 4.35 350 -21.55
>>
>> *END normal_open
>>
>> *BEGIN normal_closed
>>
>> *equate 4 d
>>
>> 0 1 17.40 330 -6.60
>> 1 2 3.20 350 -23.96
>> 2 3 10.70 020 -14.07
>> 3 4 12.90 070  0.44
>>
>> 0 a 13.80 045 -12.13
>> a b 7.80 021  -7.37
>> b c 15.00 340  -1.15
>> c d 4.35 350 -21.55
>>
>> *END normal_closed
>>
>> *end divingtest
>