photogrommetry, and changes in the section as small as 2mm can be seen 
by eye in photographic reproductions of the light line, although they cannot 
be practically measured as measurements this small are less than the 
effective accuracy. Absolute accuracy is dependent upon the digitisation 
of the "scale points", which, because they are mounted on the trolley, 
have a much smaller base than the tunnel diameter. Evaluations made 
whilst operating the system show an accuracy of better than plus or minus 
10mm for any digitised point. Further development promises to improve 
this figure significantly. 
Use of the combined FITS/HITS light plane generator enables the photography 
to record the section to be taken using the very rapid flash of the FITS, 
whilst preserving the section display of the HITS. This enables the 
engineer to observe continuously the shape of the tunnel and allows for 
very rapid gathering of the data in the field. A progress rate of 200 metres 
of tunnel per hour, with a section recorded every two metres, is attainable 
in railway tunnels. 
Anomalies in sections can often be introduced by features other than lining 
deformations (e.g. cable ducts). Colour photographs recording a minimum 
of 20m of the tunnel (using high powered electronic flash) and the light line 
at the specific section, are taken at each whole» "chainage" (or wherever 
requested) which gives the engineers an additional and useful record of 
their tunnels. These photographs are taken simultaneously with the 
recording of the light line and do not slow the operation. 
Computer analysis of the digitised sections can be carried out and a 
"signature" produced for each section which can provide evidence of 
deformations. Overhead line equipment and rail positions are also 
included in the digitisation process,: 
27