[Therion] Problem with Depth

[Torsten Schnitter]

Hi all
I have a problem (again) with water depth in the map (elevated view).
 
Exporting the xvi file it looks all good and all points are at the expected depth.
(as the grid in xtherion should be 1m per square)
This point in the following picture is around 1.2km inside the cave and is at a water depth of 29m:
So counting grid squares the line is at a waterdepth of 29m (as it should be).
These points are 29 squares down related to the start point at the beginning of the cave and at a depth of 0m.
 
In the map export I use a grid output for a depth line every 10m.
  grid-size 1000 1000 10 m
  grid-origin 0 0 100 m
So the grid is with one line exactly at 0m water depth at the start of the cave:
 
But at a point of around 1.2km inside the cave (which is the part of the xvi shown above) the cave is not at the right level:
The real depth at this point is 29m and not around 35m as drawn on the map.
 
There is only one GPS point at the start of the cave and no loops (and therefore no loop closures).
Why is the map morphed somehow or changed in any other way with a difference in depth over distance?
 
Does anyone have a hint what's going on and what is wrong?
Any help is welcome.
Thanks, Torsten
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