1508 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1975 
superimposed onto the design section. This 
plot was drawn by using an automatic 
draughting machine which was programmed 
to exaggerate by a factor of 10X any difference 
between as-built and design coordinates of 
each hardspot in the critical direction (i.e., 
the X coordinate difference for the side shell 
and longitudinal bulkheads and in Y for the 
deck and bottom shell). In this way dis- 
crepancies between the mating faces are 
more readily seen. 
The maximum discrepancies found were of 
the order of 30mm and the RMS error for the 
whole section was + 11mm. Generally the 
larger errors occurred in those parts of the 
structure where some flexibility remained, 
such as along the longitudinal bulkheads. In 
those parts which were submerged during 
the joining operation and where fairing 
would be difficult, the errors were much 
smaller. The shell plate itself is 25 or 30mm 
thick and errors of this order were acceptable. 
Because of the rapidity with which the join- 
ing operation was carried out, due largely to 
the high quality of the match, it was difficult 
to check actual discrepancies against those 
predicted by photogrammetric means. How- 
ever, at those few locations where a check 
was possible the discrepancies were con- 
firmed to within a few millimetres. 
This trial has successfully demonstrated 
the feasibility of using photogrammetry to as- 
sess the equivalence of the mating faces of a 
ship built in two halves. The accuracy of meas- 
urement achieved, whilst not as good as that 
planned for, was acceptable in this instance 
and is probably within the required toler- 
ances for measurements of this nature. Im- 
provements in accuracy could be expected 
with modifications to the technique. More 
detailed accounts of this trial are reported 
elsewhere (Newton, 1974; Jack, 1974). 
THE VALUE OF PHOTOGRAMMETRY 
The advantages to be gained from the use 
of photogrammetry in the measurement of 
marine structures may be summarized. 
(1) Photogrammetry allows separation of the da- 
ta acquisition and data processing phases. 
As a result, the on-site time is reduced 
to a minimum and amounts only to the time 
needed to carry out the photography and 
control survey. In the case of each half-ship 
the on-site time amounted to two days, of 
which one-half day was spent on photog- 
raphy. For a unit of a fixed-base offshore 
platform the on-site time may be as little as 
three hours (two hours preparation and one 
hour photography) whereas direct meas- 
urement of the unit would take at least 
twice that time. 
(2) Because of the reduction in on-site time 
and because photogrammetry is a non- 
contact measuring technique, there is little 
or no disruption of production. Photog- 
raphy is a relatively quick process and 
good use may be made of rest weekends 
and lunch breaks for the purpose. 
(3) Objects difficult to survey by conventional 
techniques may be measured with ease by 
photogrammetry. In comparison, direct 
measurement appears as a time- 
consuming, less accurate and, on occasions, 
physically dangerous activity. 
(4) Considerable savings in erection time 
should be possible if the fit-up of a unit or 
sub-assembly can be guaranteed, by 
photogrammetric measurement, prior to 
lifting it into position. This should result in 
shorter production times and, also, in fi- 
nancial savings. 
(5) Photogrammetry provides a permanent rec- 
ord in the form of photographic plates of 
the as-built dimensions of the structure. 
These plates may be remeasured at any 
time in the future if a need arises. 
Against these advantages must be weighed 
the following disadvantages. 
(1) The measurements of such structures are 
usually required quickly. This creates few 
problems from the photographic point of 
view but inevitably there is then a delay 
whilst the photography is returned to the 
photogrammetric organization for meas- 
urement. If photogrammetry is to be use- 
ful as a quality control aid on individual 
units of marine structures, a maximum 
turn-around time of 24 hours must be con- 
sidered. This virtually means that the meas- 
urements must be carried out on-site. To 
do this, the individual organization must 
set up its own measuring laboratory or the 
photogrammetric organization must be 
equipped with a mobile laboratory. 
In the case of half-ship sections, the 
urgency is not so great. If the measure- 
ments are available within a week of 
photography this should allow ample time 
for any modification. 
(2) The high capital cost of equipment and the 
need for trained personnel to operate it 
make the establishment and running of a 
photogrammetric unit expensive. Savings 
in manpower and production times must be 
possible to justify this expenditure. 
(3) In many applications there is a lack of flexi- 
bility in the time of photography. This may 
mean that photography has to be taken in 
conditions of poor light, high winds, or 
even rain, with detrimental effects. Once 
photography is taken, however, photo- 
grammetric measurement is virtually inde- 
pendent of the weather. 
51-816 M DESIGN MOULDED BEAM 
  
  
  
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