Diving data loop closures altering depths

[Footleg]

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