11.7.3
Water depth
The deeper the water, the better the result. In shallow water, DGPS wobble (as noted in the
Brissette paper), creates more relatively severe position errors. A corollary to this is that the
subtended angle is larger in shallow water, which can blur the definition of the object used, be
it a
feature or slope. The shallower the water, the larger the subtended angle; the deeper the
water,
the smaller the subtended angle and, therefore, the better the definition of the object
or slope.
11.7.4
Use predefined survey lines
The most important positioning issue is having the sonar head pass over the same exact
location in
both of the survey data collections. This is especially true when using a highly
variable slope. One
way to assist the helmsman is to give the helmsman a defined line to
navigate by. Just trying to go
over the same track, without a line reference, does not work, as
it is the sonar head that has to pass
over the same exact point; this accuracy cannot be
obtained just by using the grid display to steer
the vessel.
When setting up the survey software, make sure that the sonar head is the steered reference
for all
offline measurements. It does no good to have the vessel on the survey line, if the sonar
is mounted
on the side of the vessel; it is the sonar that should be on the survey line.
11.7.5
Speed
When doing the latency data collection, the fast run should be at survey speed where, if there is
squat or settlement, it should have been previously measured and can be applied. Many times,
the
fast run survey line is at a speed that is greater than the normal survey speed and induces
unknown
squat and settlement errors into the computation.
11.7.6
Vessel line up
In order for the angular measurement to be accurate, the vessel should have sufficient time to
come
on line and allow the motion sensor to ‘settle down’. Sufficient lead/run in should also
be allowed in
order for the helmsman to find the proper heading so that vessel can maintain as
straight a course
as possible.
11.7.7
Pole variability
The other issue, which is often overlooked, is the variability in the repeat position of a
deployable
hydrophone pole. With any moveable mounting arrangement the pole should be
recovered and
redeployed a few times, during data collection, to determine if it does, indeed,
go back into the
same aspect every time that it is deployed. (It is a good idea, after redeploying
the head, to do a few
figure 8 manoeuvres.)
Page 138 of 210
Version 5.0 Rev r002
Date 05-08-2014
Part No. 96000001
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