49
Scale References
Along with targetting of the ship-halves, the shipyard
also fastened two 50-foot beans lin a vertical direction
on each of the traverse sections to be mated. A target
was attached near each end of each beam. After completion
of the photographic work, the distance between each target
pair along a beam was measured with a flat-wire band chain.
These distances were used later to establish a true scale
for the photogrammetric solution in the manner described
in paragraph 2.4.
While it may seem extreme to have attached beams to the
mating faces, it must be recognized that photogrammetrically
derived coordinates of targets will inherent a scaling
error exactly equal to the error in determining the average
scale factor over the reference distances. Hence, it is
mandatory that very accurate reference distances be
established. in order to obtain these accurate distances
the measurements must be made with a good steel or invar
tape and must not be contaminated by piecemeal measurements,
physical obstructions such as weld beads, cambered surfaces
on long unsupported lengths of the tape.
To establish the accuracy required of the taped
reference distances it must first be appreciated that the
scale factor to be derived will include the error of the
photogrammetrically triangulated coordinates of the targets
defining each reference distance as well as error in the
physical taping of the same distances. For Sun's ship
lone toward the port side and one toward the starboard.
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