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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