[Therion] Correcting bad drawings

[John Pybus]

Hi again,

It would be good to have some advise on how people deal with mistakes in 
  sketches. For example:

I have scans of a survey which was drawn up in the field however the 
centreline on this contains a blunder (a leg 180 degrees out).  I can 
trace the survey and give therion the correct centreline data, but it 
produces a scrap with very large distortions around the blunder -- I 
need to redraw a section of the scrap.

I can export a .xvi to draw around but that doesn't contain the outline 
of the rest of the scarp to join in to.  I can export a PDF or SVG but 
that can't be imported as a background into xtherion.  How do people 
deal with this situation?

My current idea is to produce a PDF, render to PNG with ghostscript, 
then import that into xtherion and overlay a .xvi (hoping they'll line 
up) to get station LRUDs to draw round.  Does anyone have an easier way, 
I have a number of such things to correct?

Cheers,
John