124 UNDERWATERSURVEY
work in pairs where one diver takes measurements while
the other ensures that tapes are not snagged. Avoid
pulling tapes to free them from snags as this can damage
them. Each tape being used for recording should be
checked against an unused tape-measure at 15 m and
30 m (49 and 98 ft) lengths under normal tension. Any
difference larger than the required accuracy for the
survey indicates a problem and the tape should not be
used. The tape-measure used as the standard should be
steel-cored rather than fibreglass and ideally should not
be used under water to avoid damage.Vertical Control (Height/Depth)
The other component of a survey position is height or
depth. In survey terms, this is called vertical control and
the principles described here are similar to those used
for levelling on land (figure 14.7). The most common
tools for measuring depth or height are the digital depth-
gauge and the dive-computer. These instruments measure
the depth of water and display it on a screen, usually to
a resolution of 0.1 m (4 in). A dive-computer takes time
to settle to the correct measured value, so allow time for
this before recording each depth measurement. The
depth-sensor reading is affected by large changes in tem-
perature, so measurements will change as the computer
cools or warms up during a dive. If the temperature dif-
ference between air and water is very different, keep the
computer out of the sun or put it in a bucket of sea-
water for about 30 minutes before the dive. Some are more
accurate than others, so try to use the same computer for
all depth measurements. Waves and tides affect any
instrument that measures depth. Underwater surveys are
frequently done relative to a point on the sea-bed
because this cancels out the effects of tides. A permanent
feature on the site is nominated to be the depth reference
or temporary benchmark (TBM) for all depth measure-
ments (figure 14.8).
Other tools which may be useful in underwater survey
include:- rigid 1 m (3 ft) long rules and plastic folding rules
for recording detail; - a diver’s compass for finding magnetic north;
Figure 14.7 The principle of levelling to establish relative
heights/depths. For example, if the benchmark is at 6 m:
top image shows the level is 6 + 1 = 7 m above chart datum;
bottom image shows the level is 2 m above timber; there-
fore, top of protruding timber is 7 −2 = 5 m above chart
datum. (Drawing by Graham Scott)
Figure 14.8 Depth of cannon (= B – A) relative to a tem-
porary benchmark. (Drawing by Graham Scott)