The chosen site contained a number of fixed and rigid
structures suitable for recording. These
included two large concrete blocks, the wall of the Fort itself (Fig 1) and a 7
m long ex-Pilot cutter called Tavy.
A network of 21 control points was installed on the structures (Fig 2),
the shape was designed to give a large amount of redundancy and minimal
sensitivity to depth errors. The
control points installed on the structures were 5mm galvanised coach bolts
cemented into pre-drilled holes. The diameter of the control point bolts was
accounted for within the processing program.
period of a year many teams of divers had recorded to 1mm resolution a
pre-defined set of measurements between the control points.
The same set of tape measures was used for each exercise. As a
check for systematic errors, all tape measures were calibrated against a
steel tape measure at 5 m, 10 m and 20 m distances and any tape measures
with more than 5 mm in error were not used.
transcription errors, standard recording forms were used and the data was
transferred from the form straight into a computer spreadsheet for
analysis. Measurements were
exported from the spreadsheet directly into the processing program.
A set of
tape measurements was made on a single baseline on land for comparison.
Data from other sites was collected and compared with the results
from the test site.