APPENDIX IV: Offset Measurements
10 APPENDIX IV: Offset Measurements
10.1 Lever Arm Measurement – Offsets
Each component or sensor that produces information, unique to its position, will have a point that is
considered the reference point of that sensor. The Sonic 2024/2022, the motion sensor, and the
GPS antenna will have a documented point from which to measure. The gyrocompass’ data is not
dependent on its position on the vessel so, therefore, does not require an offset measurement.
10.2 Vessel Reference System
When all equipment (Sonic 2024/2022 sonar head, motion sensor, gyrocompass and GPS) have been
permanently mounted, the physical offsets to a central reference point (CRP) must be measured.
The central reference point (CRP) or vessel reference point (VRP) is that point that the surveyor
chooses to be the origin for the X and Y grid that will define the horizontal relationship between all
of the sensors. The vertical or Z reference can be the water line or other logical vertical reference.
Generally, the CRP corresponds to the centre of gravity or rotation of the vessel. All of the sensors
must have their physical relationship to each other measured and entered into the data collection
software or the processing software.
All offsets, between sensors, are defined by an X, Y and Z offset from a reference (CRP or VRP) point.
The X axis runs athwartship, i.e. from the port side to the starboard side. The Y axis runs alongship
from the bow to the stern. The Z axis runs perpendicular through the reference. The origin can be
any point; the origin will remain the same for all sensors. Some surveyors take the GPS antenna as
the origin for all measurements, others take the sonar head itself, while others might take the
motion sensor (especially if it on the centre of rotation for the vessel). The sign convention is
standard for a Cartesian plane, translated to a vessel: starboard of the reference point is positive,
forward of the reference point is positive. The sign for Z may differ, depending on the data
collection or processing software.
Page 127 of 210
Version 5.0 REV r002
Date 05-08-2014
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