Basic Piloting Terms For GPS and Radar
You can’t get to where you’re going if you don’t first know where you are.
Navigating at sea bears little resemblance to terrestrial navigation. Finding your way through a fluid sea is akin to flying an airplane, thus marine navigation and aviation share many unique terms that may not be familiar to everyone.
Below is a quick primer to explain some of the more common "nav" terms encountered at sea.
Fix is a known position attained by a reliable source, such as GPS or other electronic navigation device, or from known points. The lat/lon readout on your GPS is a fix. GPS devices commonly have a “mark” or “MOB” button which allows you to record your present position, making it a permanent waypoint. This is a valuable resource in Man Overboard (MOB) situations, providing an instant “fix” to that exact spot.
Heading is the direction which the boat is pointed. This three digit compass reading is either from the magnetic compass (using magnetic north) or true direction (direction in reference to true north). In an ideal static environment this is the way the boat will travel, but current, wind, leeway and other factors often cause the boat to drift while on this heading, altering its path.
Course is the intended or planned direction of travel to get your vessel from point A to point B. You plot your “course” to a destination or next waypoint. The pilot will then factor current and leeway, to calculate a “heading” or “course to steer”.
A cross wind and/or current will mean your heading may be many degrees different from the desired direction of travel to compensate for these outside effects.
Track or Course Over Ground (COG) is the actual path the vessel has made from A to B, again referred to in 3 number direction. This factors in the effects of set and drift from wind and current which is often different from your heading. (May also be referred to as Course Made Good.)
Speed refers to the rate of a vessels motion over water, i.e. the readout on your boats speedometer. If there is a head current coming at you at a rate of 5 knots and you are traveling at a rate of 15 knots through the water, your actual rate of advancement over the sea bottom is 10 knots.
Therefore, speed over ground or speed made good is another term used to describe this net effect. A GPS calculates your speed made good, not speed over water.
Bearing vs. Relative Bearing
Bearing is the compass direction from your vessel to an object or vessel in your vicinity. It is often confused with heading, yet bearing and heading are only ever the same when the bearing to something is directly off the bow or 12 o’clock position, i.e. the same as your heading.
Relative bearing is a term you won’t find on your GPS but will hear. It simply means the bearing is relative to your boat, with directly off the bow being 0 degrees (regardless of heading), abeam to starboard is 90 degrees, dead astern is 180 degrees and abeam to port is 270 degrees.
Some Common Radar Terms
Range is the distance of a something from your vessel. You hear this term in the Navigation Rules, “risk of collision is deemed to exist when another vessel is on a constant relative bearing with decreasing range.” Elsewhere, a radar uses range rings to graphically display the distance of a detected target to you.
Target is any object detected in your vicinity.
Line of Position is simply a line radiating out from some point along which an observer is presumed to be located. Commonly it’s a line plotted from an observed landmark to your vessel.
An example of this in practice, you sight a landmark that bears 100 degrees off your starboard. You could plot a line on a chart radiating out from that mark and know your vessel is somewhere along that line. (Must account for difference between magnetic and true).
Article courtesy of Michael Reardon