[Therion] Problem with elevation view

Torsten Schnitter torsten.schnitter at netcologne.de
Thu Nov 1 16:09:00 CET 2018


Hi

I figuered out the problem and may be there is a bug in loop closure... or somewhere else.

The cave does start at 0m of depth and with a fixed point (fix 0 x y z) with z=118.4m.
After roughly 800m of underwater cave survey (which does result in absolute z values for every station) the cave does surface again at a depth of 0m.
I fixed this point as well (fix 0 x y z) with z=118.4m (to correct the x and y error).

So far so good.
But further (around 1km more) into the cave it does surface again and I realized in elevation view that 0m is not at the same level than the first two points are.
As soon as I deleted one of the two fixed points the z elevation was as expected and the 0m point into the cave was at the same level than the first two surface points.

After some tests I realized the start of the cave was the problem.
You can see this part on the attached picture.
As you can see there is a vertical start of the cave.

Here the data from that first part:
data diving station depth newline tape compass
0   0.0
            4.9   0
1   4.9
            9.1   0
2   14.0
          28.1   78
3   28.2
            8.9   88
4   30.3

As soon as I add 0.01m to the first and second (vertical) tape measurement (4.9 = 4.91 and 9.1 = 9.11) all is good and the problem with loopclosure is gone. Both fixed points are added again and the eleveation is as expected (0m level is at the same level at all points).
So there is some more distance needed for the vertical parts. But I can't explain why.

Is this a problem with the loop closure algorithm?
May be some of the experts around can post a explanation for this behaviour!?

regards,
Torsten

> Bruce Mutton via Therion <therion at speleo.sk> hat am 30. Oktober 2018 um 08:34 geschrieben:
> 
> 
>     Torsten
> 
>     The image of the xvi file makes it look as though there is a loop (not sure if it is a plan or elevation view – presume it is elevation).  Also if you have a fixed point at each end, then this could also cause ‘a loop’ which might give differing elevations at each end.
> 
>     Also, if the water is flowing, then the water levels at each end will actually differ.
> 
>      
> 
>     But I assume you have correctly assessed these effects as negligible, or at least not detected by your survey method.
> 
>      
> 
>     I think you are implying that the 0m line is a raster background.  If that is the case, it may be distorted by Therion.  One way to check would be to turn on gridlines for the layout.  That will show you what Therion really thinks is a horizontal line.
> 
>      
> 
>     Also check that Therion has the water surface survey stations at the same altitude.  You can check this easily by interrogating a .3d output with Aven.
> 
>      
> 
>     Bruce
> 
>      
> 
>     From: Therion <therion-bounces at speleo.sk> On Behalf Of Torsten Schnitter via Therion
>     Sent: Tuesday, 30 October 2018 04:36
>     To: therion at speleo.sk
>     Cc: Torsten Schnitter <torsten.schnitter at netcologne.de>
>     Subject: [Therion] ***UNCHECKED*** Problem with elevation view
> 
>      
> 
>     Hi all
> 
>     I have a under water cave survey and all the data is done with depth measurements.
>     So the Z coordinates are absolute and the survey is not with clino data.
>     The cave starts at one point and after roughly 800m you have another surface point.
> 
>     When I compile the data I get a strange behaviour.
>     Although it's not a WYSIWIG map editor I would expect points which have the same depth are at the same "depth"/level on the xvi file. But they are not (see attached screenshot "elevation problem").
>     The 2 points within the red marks do have the same depth measurements.
>     But if you have a closer look to it you can see they are not at the same level.
>     (They differ somehow in x/y way and therefore the points are not on top of each other.)
>     There are no loop closures inbetween which could be a reason for different levels.
> 
>     Not having the same level with these points (I assume it is this point) leads to a bigger problem later on.
>     When drawing the elevation scraps and putting a water surface (depth = 0m) to the scraps and having a raster in the background, the surface at the beginning of the cave is not at the same level than at the next surface point of the cave. But it should be.
>     See pictures "elevation problem-start" and "elevation problem-end".
> 
>     What's wrong?
>     Can anybody help?
>     If more info is needed please let me know.
> 
>     Thanks,
>     Torsten
> 


 

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