Sonic 2022, Operation Manual

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reader. Starting at either the reference point or the sensor, the distance will be measured. When
either the reference point or the sensor is reached, the two people will reverse roles: the holder is
now the reader and the reader is the holder, the transverse is made back to the point of beginning,
but not using the same path. If reference marks were made on the first leg, they should not be used
on the second leg back. If the measurement from the sensor to the reference point, in one direction,
agrees with the measurement in the opposite direction, made by a different reader and holder, then
the offset is good. If there is a small disagreement in measurements, the two measurements can be
averaged. If there is a large disagreement then the process should be repeated. What is a small
disagreement? A few centimetres can be expected.
10.3.3 Vertical Measurement
To measure elevations or the Z offset, the use of a plumb bob is required. This can be something as
simple as a spanner tied to a length of line and lowered from one deck to the next. The plumb bob
will also allow for accurate measurements in the X and Y direction when transposing them from one
deck to the other.
The plumb bob works, of course, by gravity so generally points to the centre of the earth. This being
the case, if the vessel is not in good trim, i.e. has a list, the resting position of the plumb bob may not
be at the true vertical point under the place from which it is being held. This is very critical when
transposing X and Y measurements from one deck to another.
The draft of a vessel will not be constant. Prior to going out on a survey, the fuel and water may be
filled up, causing the vessel to settle lower in the water. Possibly less people are on board causing
the vessel to rise higher in the water. The main concept here is that the draft of the sonar head
changes. All X and Y offsets remain the same as long as the sensors are not moved, but the Z offset
changes constantly depending on the draft of the vessel.
If possible, the pole should be marked to show the depth of the head. Measuring up from the sonar
head’s acoustical reference, rings can be painted on the pole in 10 cm (or other) increments, with 2
cm hatching between rings. The surveyor may have to observe the pole over the course of a few
minutes to determine where the water line is and would then estimate the depth by interpolating
between the 10 cm depth rings.
Another method would be for the surveyor to initially measure from the sonar head’s acoustical
reference to the top of the hydrophone pole. This is the total pole measurement. At the start of a
survey day, the surveyor will go to the pole and measure from the top of the pole to the water line
(using the tape measure and plumb bob or similar weight), this is called the dry measurement.
Taking the dry measurement from the total pole measurement yields the wet measurement, which
is the draft of the sonar head. Due to wave motion, the surveyor may have to take a series of
measurements to ensure an accurate reading.
When the draft or Z of the sonar head is determined the Z for the GPS antenna and the motion
sensor can be adjusted accordingly, if the Z reference is the water line. In most data collection
software a Z shift, in relation to the water surface, can be entered in for the CRP, which will do the
vertical adjustment for all offsets
It is very important that when measuring the draft on small vessels that the person taking the
measurement does not unduly cause the vessel to list towards that side. Having someone counter
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Version 5.0 Rev r002
Date 05-08-2014