Defining common GPS terms

Words and acronyms all boaters should know

You might have noticed — I certainly did — that more and more devices are using the GPS satellite system for a variety of gadgets.

To get the most out of these devices, you should know some of the terms that apply to GPS:

Almanac data (satellite) — Specific satellite constellation information transmitted to your GPS receiver from each GPS satellite. This MUST be received before GPS navigation can begin.

Bearing — The direction of one point with respect to another, or the compass direction from your position to a destination (usually to the nearest degree).

Chart (Nautical) — A representation of geological features, mainly of areas of water as an aid for marine navigation. Chart grids are normally listed in latitude and longitude.

Course — The direction of movement from a beginning position to a destination point.

Course made good — The bearing from your starting point to your present position. The speed at which you are approaching your destination.

Cross track error — The distance you are off a desired course in either direction.

Datum (with reference to mapping) — A way and year a map or chart was measured and plotted. Maritime interests use charts such as NOAA, NOS, normally use the World Geodetic Survey Datum, (WGS-84) and/or North American Datum 83 (NAD 83); United States Geological Survey maps commonly called quadrangle of coverage, “quads”, “sections” or TOPOS, normally use North American Datum of 1927 (NAD 27) or of 1983 (NAD 83).

Desired track (DTK) — The compass course between the “from” and “to” waypoints.

Differential GPS (DGPS) — An extension of the GPS system that uses land-based radio beacons to transmit position corrections to “differential receivers” designed to receive corrections and communicate with a differential ready GPS unit via an interface. This system requires a separate receiver.

EGNOS — European Geostationary Navigation Overlay System, Europe’s WAAS differential.

Estimate position error — An estimate of horizontal position error.

Estimate time of arrival (ETA) — The time of day of your arrival at a destination.

Estimate time enroute (ETE) — The time needed to reach your destination at your present speed.

Fix — An intersection of two or more lines of position, obtained simultaneously. A position is usually in a horizontal plane. (Lat/long, etc)

GoTo — A straight line route consisting of one leg, with the present position as the start of the route and a single defined waypoint as the destination.

Grid or Position format — A coordinated system that projects the earth on a flat surface, using intersecting lines to form zones for measurement. They are used to help locate and detail specific areas.

Ground speed — The velocity you are traveling relative to a static ground position.

Heading — The direction of intended movement (ay differ from the actual course over ground due to winds, sea conditions).

Initialization — The first time a GPS receiver orients itself to its current location. A receiver should be initialized if the receiver is off for more than three days, and/or if it were moved more than 200 miles from its last position while in an off mode. After initialization, the receiver remembers its location and can acquire a position more quickly.

Latitude (abbreviated “L”) — A system of geometric coordinate parallel lines. Used to determine north and south positions on the earth’s and/or map’s surface. Usually defined in degrees, minutes and seconds or in tenths of minutes. North America, north of the equator is defined as N or North.

Longitude (abbreviated “L.” or “Lo.”) — A system of geometric great circles used to define east and west locations on the surface of the earth, and on maps and charts to designate a location. Meridians of longitude are measured from the earth’s axis and labeled east or west of the Prime Meridian. They are the north-south lines on maps, and are measured in degrees, minutes and seconds to a maximum of 180 degrees East or West from the Prime Usually defined in degrees, minutes and seconds or in tenths of minutes.

Man overboard (MOB) — The most-feared call onboard a ship other than “fire.” On GPS receivers it means the receiver has a control button that, when activated, instantly saves the current position as a waypoint and provides steering guidance back to the position marked. Used for instantly marking a position you have to immediately go back to. The saved waypoint is usually automatically named MOB, and an automatic “Go to” is provided.

Map — A representation of geological features, mainly of land. A map is designed to emphasize land features. Map grids are normally listed in UTM meter grids. A marine chart, by comparison, is almost exclusively concerned about navigation on open water and with Aids To Navigation controlled channels, etc..

Map scale — The scale to which a map or chart is drawn represents the ratio of the distance between two points on the earth and the two corresponding points on the map. The degree to which map or chart measurements are reduced; ratio of proportions on which a map is based (1 inch = one mile ). 1:80,000 in some NOAA charts, 1:24,000 in some TOPO’s. A map in 1:30,000 scale shows greater detail than one in 1:40,000 scale. Inland marsh area maps should be at least 1:40000 scale.

MGRS — Military Grid Reference Systems is a grid system used by the U.S. Military based on meters north and south of the equator and east of a longitude 180 degrees from the prime meridian. Each of the north/south and east/west grid lines are equal anywhere on the earth. A military “click” is the same at the equator and near the poles. A separate scale is used in the immediate vicinity of each pole.

Navigation — The process of knowing where you are while traveling from one place to another in relation to your course.

Position — A unique location, at the current time, based on a geographic coordinate system.

Route — A trip, usually made up of “legs” between two waypoints. Routes generally are made up of several legs.

Time of Arrival (TOA) — The difference between the beginning and the time of arrival at a specific point.

Track (TRK) — Direction of movement based on a ground position.

Universal Transverse Mercator (UTM) — A grid system that projects global sections onto a flat surface to measure position in specific zones. Distances measured in meters north and south of the equator and east of the beginning of each of sixteen zones east of a longitude 180 degrees from the prime meridian. Usually used and listed on USGS “TOPOS” or “quads.” Coordinates are listed as an Easting (meters east of the beginning of one of 60 zones), Northing (meters north of the equator or south of the equator) and a zone number.

Variation — The difference between true and magnetic north. It is different in each longitude and, because of the wobble of the earth on its axis, is constantly changing. Maps and charts usually update the changes yearly.

Velocity Made Good (VMG) — The speed you area traveling in the direction of the destination.

WAAS — Wide Area Augmentation System. Sometimes called a satellite differential system. Developed by the U.S. FAA to supplement GPS as a differential for the air industry. Similar to D-GPS, but instead of land-based transmitting towers it uses satellites. Transmits on frequencies very close to normal GPS signals, therefore it can be configured into a single receiver with only a modest price increase. Accuracy reported to be within 1 meter. Standard consumer models have reported accuracy of within 3 meters. Most of the GPS gadgets do not have this feature.

Waypoint — Coordinates for predetermined positions or locations, expressed in degrees, minutes and thousandths minutes or seconds and/or meters and tenths of meters (UTM). A specific position marked, or saved in a system, for future reference; virtual markers for defining routes and reporting progression along a path.

About Captain Paul Titus 192 Articles
Capt. Paul Titus has been responding to G.P.S questions on since 2000. He has been fishing and hunting in Louisiana since 1957. Titus holds a USCG license and conducts instruction courses in the use of GPS for private individuals and government agencies.

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