UNDERWATERSURVEY 133
reliably position to within 30 mm. The object being posi-
tioned has to be connected to the surface by a cable, as
the transceiver needs to communicate with the computer
system on the surface. This is less of a limitation than it
may appear because divers using these systems need to be
in voice communication with the surface and this usually
requires a cable.
A USBL positioning system can position divers and
ROVs relative to a boat on the surface. An acoustic trans-
ceiver is fitted to the end of a rigid pole and deployed in
the water over the side of the boat. The transceiver on the
pole receives acoustic signals from a transponder beacon
on a diver and these can be used to calculate the distance
and direction of the diver relative to the boat. By com-
bining these measurements with the boat’s position, pro-
vided by a GPS receiver, the position of the diver in the
real world can be calculated. The position accuracy of USBL
systems is dependent on the distance between boat and
diver – the positions get less accurate the further the
diver is from the boat. As the USBL system is attached to
the boat, there is no limit on the area that can be covered
because the boat can simply be moved. Often these
systems are used to track ROVs, allowing the boat to
follow the ROV anywhere, similar to having a dog on
a lead. Because the position accuracy is dependent on
the distance between boat and diver, accuracy is usually
specified in terms of percent slant range (distance); how-
ever, a typical working figure would be 1 m accuracy at
100 m. Unfortunately, the accuracy of the system is very
much dependent on the quality of the GPS receiver and
compass and motion reference unit, which also have
to be fitted to the vessel. USBL systems are seen as being
easier to use than LBL systems, but they require careful
installation and calibration to get the best results.Positioning the Site in the Real World
For simplicity, sites are often recorded using local co-
ordinates that are not referenced to points in the real
world. For projects where the site itself is associated with
features on land, or where remote-sensing data have
been collected, it becomes necessary to determine where
the site is in the real world so as to relate the survey dataFigure 14.19 Surveying a submerged site in shallow water using a shore-based total station. (Drawing by Graham Scott;
after Morrison, 1985, fig. 5.2)