[Therion] Joining old and new surveys

Graham Mullan graham.mullan at coly.org.uk
Tue Feb 17 08:50:54 CET 2015

On Tue, 17 Feb 2015 08:15:16 +0200, Steven Tucker wrote


I have an old survey from the 80s and I have new sections that I want to add with
Therion. The old survey has no centerline data.

I have redrawn the old survey in a th2 file and allocated 3 points which correspond
to 3 different entrances. In the th file I have given coordinates to all 3 of those
points. This works perfectly.

I am struggling to connect the new section with this old section. The new section
starts in a deep section of the cave and is far away from any of my currently defined

What is the easiest way to connect the new and old sections?


As other contributors have noted, you need centreline data from your entrance fixed
points to the start of your new section. There are only two ways to do this:

Run a rapid centreline survey through the cave to the start of the new sections &
hang everything off that. Or

Digitise the old survey. I have successfully used Larry Fish's "Map to Dat" utility
from "Compass" to do this, though you then have the irritation of converting Compass
data to Therion (not difficult, just irritating). 

Both of these approaches work OK when producing 3D models, but I have found them
rather less satisfactory when producing drawings as far various reasons the old
drawings never seem quite 'right'. A third approach is to hunt down the original
surveyors and see if the notes really have been lost or just left in a drawer
somewhere. If the latter, rejoice, if the former then, well, smearing them with honey
& lowering them into a pit of tiger ants seems about right.

Alternatively, you could just bite the bullet & resurvey the whole system from
scratch, we've had to do this once or twice & some of the largest systems in the UK
have been/are being completely redone for these reasons. If you do take this path
then please just ensure that your data is archived properly so no-one else is faced
with the same problem in another 30 years or so.


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