[Therion] Diving LRUD dxf export - Vertical floor/ceiling alignment at scrap joins

[Stephen Clark]

I am new to Therion, so please forgive me if I am making a simple error.

I am experiencing a problem when exporting 3d .dxf model data (to AutoCAD)

Background :
I have a centreline of diving data with LRUD's. Approx 1000m / 0-70m 
depth  / 125stations. Simple linear passage (The St Georges Resurgence, 
Montvalent, France)
I have used the .xvi export of plan and elevation into the map editor.
I have drawn 12 plan scraps and 12 elevation scraps.
These are joined and have exported correctly as conventional pdf plan 
and elevation maps.

Problem:
When I export the data as a dxf model using the '-wall-source maps' 
option, there appear to be vertical steps in the passage floor and 
ceiling at the start and end of each (plan) scrap. Within a single scrap 
there are no steps. Typically the dimensions relative to the centreline 
in the scrap 'before' the join are correct. The first up/down dimensions 
in the 'next' scrap are incorrect. The horizontal alignment of the walls 
is perfect.

Typical image illustrating the problem here :

http://www.steveclark.demon.co.uk/dxf_error.png

It does not appear to be a problem with the centreline data, because if 
I re-draw two scraps as a single scrap with only 2 wall lines there is 
no step in the ceiling or floor.

There is no step in the pdf elevation export of the same scraps.

Joining all the individual wall lines in both plan and elevation does 
not solve the problem.

The export does not appear to use the elevation scraps to generate 
passage heights. I get identical output if these are deleted or adjusted.

If I use the export '-wall-source centreline' option, there are no 
steps. This, however, does not allow me to export/render any features 
not recorded in the simple LRUD data.

I am relucant to draw the whole survey as a single scrap with just 2 
walls, but it appears this may solve the problem.

Any thoughts, or is this just a quirk of the dxf export?

Regards,

Steve Clark