was a commercially obtainable ring flash. The unit was bulky and difficult 
to handle but proved the viability of the idea, and indicated that the 
required order of accuracy was attainable. Serious development was 
instigated at the beginning of 1981, and the first complete prototype 
system was tested at Stanton-on-the-Wold on 13th February, 1981. The 
light plane generator built for this test was less than 0.5 metres in 
diameter, and very much more portable than the original model. The light 
source was stilla ring flash. This unit was christened FITS (Flash 
Illuminated Tunnel Section) but the light output left something to be desired. 
This led to the development of HITS (Hallogen Illuminated Tunnel Section). 
HITS had the advantage that the tunnel section delineated was permanently 
displayed. This has proved to be a powerful tool for the tunnel engineer, 
enabling him to observe bulges, flattenings and other distortions in the 
section with remarkable clarity. Development has been continuous, and 
the Current unit employs a much more powerful ring flash which provides 
sufficient output from a single flash for use in sooty railway tunnels. This 
unit is a combination of FITS and HITS and thus has the advantages of both 
systems. 
The Mono Photogrammetric Tunnel Profiling System 
The heart of the system is the light plane generator which delineates the 
section being recorded by a line of light. The generator is mounted on a 
portable railway trolley, which is connected by a draw bar to a second 
trolley on which a non-metric camera is mounted (see diagram). The 
camera records the light line onto film which is then processed to obtain 
maximum contrast. The section is digitised by a line following raster 
scanning laser direct from the photographic negative. The digitised 
section is then corrected for lens distortion, by applying predetermined 
parameters, and scale, using "scale points" recorded in the 
photograph. 
The relative accuracy of the system is high when compared with stereometric 
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