Glossary | 121
9 Glossary
Apparent wind – The speed and direction from which
the wind appears to blow with reference to the bow when
the boat is moving (also called relative wind).
Arrival alarm – An alarm signal issued by a GPS/chart
plotter that indicates arrival at or at a predetermined
distance from a waypoint (see arrival circle).
Arrival circle – An artificial boundary placed around the
destination waypoint of the present navigation leg, the
entering of which will signal an arrival alarm.
Bearing – The horizontal direction of one terrestrial point
from another, expressed as the angular distance from a
reference direction, usually measured from 000° at the
reference direction clockwise through 359°.
BTW – Bearing to a specified waypoint from present
position. The correct acronym for the more commonly
used BPW (Bearing Position Waypoint).
BWW – Bearing waypoint to waypoint - Bearing
angle of the line between the “TO” and the “FROM”
waypoint, calculated at the “FROM” waypoint for any two
arbitrary waypoints.
COG - Course Over Ground - The actual direction of
progress of a vessel, between two points, with respect to
the surface of the earth. The vessel’s heading may differ
from the course over ground due to the effects of wind,
tide and currents.
GPS - Global Positioning System - This system is
based on satellites in fixed orbits, circling the earth at an
altitude of approximately 20,200 km. The system will
provide the user with 24 hour a day all weather position
coverage, with an accuracy of 5 to 30 meters.
Magnetic bearing – Bearing relative to magnetic north;
compass bearing corrected for deviation.
Magnetic deviation – A local magnetic field on-board a
vessel. Can interfere with the earth’s magnetic field and
create compass readings that may deviate from the
actual magnetic heading. The deviation will vary with the
actual heading.
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