Navigation concepts
Quick refresher for terms frequently used in maritime warnings and plotted coordinates.
Bearing
Bearing is the horizontal direction from one point to another, typically measured clockwise from true north or magnetic north.
Variation
Variation is the angle between true north and magnetic north for a specific location and date.
Deviation
Deviation is compass error caused by local magnetic influences on a vessel or platform.
Coordinate formats
Messages can include decimal degrees, degrees-minutes, or formatted coordinate strings. Always confirm datum and hemisphere before plotting. On this site a position may be typed in any of these forms: 55-23.9N 003-48.7E, 55°23.9'N 3°48.7'E, 55°23'54"N 3°48'42"E or 55.398, 3.811.
Nautical mile and knot
One nautical mile is 1,852 m, close to one minute of latitude, so the latitude scale on the side of a chart is the distance scale. A cable is a tenth of a mile. A knot is one nautical mile per hour: at 12 knots a ship runs 288 miles a day.
Rhumb line and great circle
A rhumb line keeps one true course from waypoint to waypoint; it is a straight line on a Mercator chart and the way an ECDIS draws a normal leg. A great circle is the shortest track over the earth; its course changes along the way, and on a Mercator chart it bows towards the pole. From New York to the English Channel the great circle saves about 120 miles on 3,100. The difference matters on long ocean legs at high latitudes; on a coastal leg it is nothing. Route files mark great-circle legs (RTZ Orthodrome) and the passage tools here keep that.
Ship's time and UTC
Warnings and forecasts are in UTC. Ship's time is UTC plus the zone kept on board: zone +3 means clocks are three hours ahead of UTC, written as zone description -3 in the log because you subtract three hours to get UTC. Ports keep their own civil time, with daylight saving in some seasons, which is why an ETA at a port is worth checking in both.