Nautical terms
48 of 469 terms in Navigation & charts. Show all
A
- Air draft Navigation & charts
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The height of the highest point of your boat above the waterline, including mast, antennas, radar and lights. You need it to judge whether you fit under bridges and overhead power cables.
The Coast Pilot advises boats with masts or antennas to leave extra room under power lines, since electricity can arc across without contact.
B
- Bearing Navigation & charts
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The direction of one point from another, measured clockwise from 000 degrees, usually from north. You take a bearing on a landmark to fix your position, or on another boat to judge risk of collision.
C
- Can buoy (Can) Navigation & charts
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An unlighted buoy whose top is shaped like a cylinder. In the US system cans are green with odd numbers and mark the port (left) side of a channel when returning from sea.
- Chart datum (Sounding datum) Navigation & charts
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The reference water level that chart depths are measured from. On NOAA charts of tidal waters it is Mean Lower Low Water, so most of the time there is more water than charted. The Great Lakes and non-tidal inland waters use a separate Low Water Datum.
Lower-than-average low tides, shown as minus tides in the tide tables, leave less water than the chart shows.
Sources disagree on year mean lower low water became chart datum for all US marine waters — left out rather than guessed. Check your local authority.
nauticalcharts.noaa.gov · tidesandcurrents.noaa.gov · tidesandcurrents.noaa.gov
- Chart scale (Scale) Navigation & charts
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The ratio between a distance on the chart and the real distance: on a 1:10,000 chart, one inch equals 10,000 inches. Large-scale charts show a small area such as a harbor in great detail; small-scale charts cover more area with less detail.
NOAA advises using the largest-scale chart available near shore, because smaller-scale charts may leave out some hazards.
- Coast Pilot (United States Coast Pilot) Navigation & charts
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NOAA's series of books that add what charts cannot show: navigation regulations, landmarks, channel and anchorage details, dangers, weather, currents, bridges and port facilities for US coastal waters and the Great Lakes.
- Compass rose Navigation & charts
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The circle printed on a chart, marked in degrees clockwise from 000, used to measure directions. Each rose also shows the local magnetic variation, the year it applies to, and its annual change.
dco.uscg.mil · oceanservice.noaa.gov · nauticalcharts.noaa.gov
- Course Navigation & charts
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The direction a boat is steered, or intended to be steered, through the water, measured in degrees from north (true or magnetic, so say which).
- Course over ground (COG, Track) Navigation & charts
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The direction you are actually moving over the bottom, as shown by GPS, after wind and current have had their effect. It can differ from both your heading and your steered course.
- Cross-track error (XTE, XTK) Navigation & charts
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How far you are to port or starboard of the straight line between two waypoints, as shown by a GPS, plotter or autopilot, usually with an arrow showing which way to steer back.
Starting a new "Go To" redraws the line from your present position, so a zero cross-track error may no longer mean you are on the safe track you planned.
D
- Daybeacon (Daymark, Dayboard) Navigation & charts
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An unlighted aid fixed on a post or piling, carrying a dayboard whose shape and color show which side to pass: green squares with odd numbers mark the left side and red triangles with even numbers the right side when returning from sea.
nauticalcharts.noaa.gov · pdept.cgaux.org · dco.uscg.mil · uscgboating.org
- Dead reckoning (DR) Navigation & charts
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Working out your position from your last known position plus the course steered, speed and time run, without outside references. Comparing a DR position with a fix shows how much current and wind have moved you.
- Deviation (Compass deviation) Navigation & charts
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The error in your own boat's compass caused by magnetic influences close to it. It changes as the boat turns, so after the compass is adjusted the boat is "swung" through different headings to record the leftover error in a deviation table.
Total compass error is variation and deviation added together.
E
F
- Fathom Navigation & charts
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A unit of depth equal to six feet. Some charts give soundings in fathoms, so always check which unit a chart uses.
- Fix (Position fix) Navigation & charts
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A position found without relying on a previous one, for example where two or more lines of position cross, or a latitude and longitude from GPS.
G
- Global Positioning System (GPS) Navigation & charts
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The US satellite system that gives a receiver its position, speed and time anywhere, in any weather. Chartplotters use it to show your position, and a DSC radio connected to GPS sends your position with a distress alert.
nauticalcharts.noaa.gov · oceanservice.noaa.gov · icdept.cgaux.org · uscgboating.org
H
- Heading Navigation & charts
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The direction the bow is actually pointing at a given moment, in degrees. Wind and current can make your heading differ from the path you actually travel over the bottom.
I
- Information and regulatory markers (Regulatory markers, State waterway markers) Navigation & charts
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White markers or buoys with orange shapes, kept by the states: a diamond warns of danger (rock, shoal), a crossed diamond marks an area boats must keep out of, a circle marks controlled areas (no wake, speed limit) and a square gives information.
- Intracoastal Waterway (ICW, Intracoastal) Navigation & charts
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An inland waterway for small craft and small commercial vessels along the Atlantic and Gulf coasts, in several separate segments. Its aids carry yellow symbols: a yellow triangle means keep the aid on your starboard side, a yellow square on your port side.
- Isolated danger mark Navigation & charts
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A black mark with one or more broad red bands and two black balls on top, placed on or next to a small danger that has navigable water all around it. If lit, it shows a white light flashing in groups of two.
The Coast Guard Auxiliary advises not approaching these marks closely without special caution.
K
- Knot (kt, kn) Navigation & charts
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A unit of speed: one nautical mile per hour, or about 1.15 statute miles per hour. Boat speed, wind speed and current speed are all given in knots on the water.
L
- Lateral marks (Lateral aids, Channel markers) Navigation & charts
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Buoys and beacons that mark the sides of a channel. Returning from sea, green marks with odd numbers are on your port side and red marks with even numbers on your starboard side, with numbers rising as you go in.
Channel marks do not always sit exactly on the channel edge; some are inside or outside it, and the Light List often gives the offsets.
uscgboating.org · navcen.uscg.gov · dco.uscg.mil · nauticalcharts.noaa.gov
- Latitude (Lat) Navigation & charts
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How far north or south of the equator a place is, measured in degrees and minutes.
- Light characteristic (Characteristic) Navigation & charts
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The color and rhythm that identify a lighted aid, written on the chart as, for example, "Fl G 4s" (a green flash every 4 seconds) or "Iso R 6s". Rhythms include fixed, flashing, quick flashing, occulting (on longer than off), isophase (equal on and off) and Morse code.
Sources disagree on flash rate that counts as quick flashing — left out rather than guessed. Check your local authority.
dco.uscg.mil · pdept.cgaux.org · uscgboating.org · navcen.uscg.gov
- Light List Navigation & charts
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The Coast Guard publication, updated weekly on the Navigation Center website, describing the aids in a region: light characteristics and visibility, light structures, buoys, sound signals and electronic aids.
- Line of position (LOP) Navigation & charts
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A line on the chart that your boat is somewhere on, drawn from a single observation such as a compass bearing to a charted lighthouse. Two or more lines crossing give a fix.
- Local Notice to Mariners (LNM) Navigation & charts
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A notice published by each Coast Guard District listing changes to aids to navigation, channel depths, new charts, bridge work, marine events and other safety information for local waters.
- Longitude (Long, Lon) Navigation & charts
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How far east or west of the prime meridian at Greenwich, England a place is, measured in degrees and minutes.
M
- Magnetic north Navigation & charts
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The direction a compass needle points under the earth's magnetic field alone. It differs from true north by the local variation and slowly shifts over the years.
- Mean Lower Low Water (MLLW) Navigation & charts
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A tidal datum: the average height of the lower of each day's two low tides, taken over NOAA's 19-year National Tidal Datum Epoch. It is the chart datum for US tidal waters, and tide-table heights are measured from it too.
A minus tide in the tide tables means the water will drop below MLLW, giving less depth than the chart shows.
tidesandcurrents.noaa.gov · oceanservice.noaa.gov · tidesandcurrents.noaa.gov
N
- Nautical chart (Chart) Navigation & charts
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A map made for navigation on the water. It shows water depths, the shape of the shore, hazards, the bottom type, aids to navigation, landmarks and port facilities. In US waters NOAA makes them, in print and electronic form.
The Coast Guard stresses keeping charts current: a GPS tells you where you are, not what is around or under the boat.
uscgboating.org · oceanservice.noaa.gov · nauticalcharts.noaa.gov
- Nautical mile (NM, Sea mile) Navigation & charts
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The unit of distance used on the water: about the length of one minute of latitude, fixed internationally at 1,852 meters (about 6,076 feet, or roughly 1.15 statute miles).
Because a minute of latitude is about a nautical mile, you can measure distance on a chart against its latitude scale.
dco.uscg.mil · weather.gov · discoverboating.com · ionianmarinesurveys.com · higherlogicdownload.s3.amazonaws.com
- Nun buoy (Nun) Navigation & charts
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An unlighted buoy with a cone-shaped top. In the US system nuns are red with even numbers and mark the starboard (right) side of a channel when returning from sea.
P
- Preferred channel mark (Junction buoy, Junction mark) Navigation & charts
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A red-and-green banded mark, lettered rather than numbered, where a channel splits. The top band shows the preferred channel: green on top means treat it as a green (port-side) mark; red on top, as a red one.
R
- Range (Range lights, Leading lights) Navigation & charts
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Two marks or lights at different heights and distances, with the rear one higher. When you see them lined up one above the other you are on the charted line, usually the center of a channel.
- Red right returning (Three R rule, 3-R rule) Navigation & charts
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The memory aid for US lateral marks: when returning from seaward, or heading upstream, keep the red marks on your right (starboard) side.
S
- Safe water mark (Fairway buoy, Mid-channel buoy) Navigation & charts
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A mark with red and white vertical stripes showing navigable water all around it, used for fairways, mid-channels and sea approaches. It is never numbered, and if lit it shows a white light in Morse code "A" (short-long).
- Sounding (Depth sounding) Navigation & charts
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A measured or charted depth of water. The small numbers scattered over the water on a chart are soundings, in feet, fathoms or meters (check the chart title) below chart datum.
dco.uscg.mil · oceanservice.noaa.gov · nauticalcharts.noaa.gov
- Special mark (Yellow buoy) Navigation & charts
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A solid yellow buoy or beacon that marks a special area or feature rather than a channel side, such as an anchorage, a cable or pipeline area, a military exercise zone or an ocean data buoy. If lit, it shows a yellow light. Many are private aids.
- Speed over ground (SOG) Navigation & charts
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Your actual speed over the bottom, which is what GPS shows. With a current against you it is less than your speed through the water; with a fair current it is more.
T
- True north Navigation & charts
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The direction to the geographic North Pole. Chart meridians run true north-south, and bearings and courses in the Coast Pilot are given in degrees true.
dco.uscg.mil · oceanservice.noaa.gov · nauticalcharts.noaa.gov
U
- U.S. Chart No. 1 (Chart No. 1, Chart 1) Navigation & charts
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The official key to the symbols, abbreviations and terms used on US paper and electronic nautical charts, kept jointly by NOAA and the National Geospatial-Intelligence Agency and free online.
V
- Variation (Magnetic variation, Declination) Navigation & charts
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The angle between true north and magnetic north at a given place, in degrees east or west. It is printed on the chart's compass rose with its yearly change, and it is the same for every boat in that area.
dco.uscg.mil · oceanservice.noaa.gov · nauticalcharts.noaa.gov
- Vertical clearance (Bridge clearance) Navigation & charts
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The charted height of open space under a bridge or overhead cable. In tidal waters it is measured from Mean High Water, so at higher tides there is less room than charted. Some bridges have gauges showing the actual space at the present water level.
Know your boat's air draft and compare it with the clearance and the current water level before approaching.
W
- Waypoint (WPT) Navigation & charts
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A position you save in a GPS or chartplotter, such as a harbor entrance, a turn or a destination. Two or more waypoints joined into legs make a route.
The plotter draws straight lines between waypoints and does not know about hazards on them; check the whole route on the chart.
simrad-yachting.com · nauticalcharts.noaa.gov · oceanservice.noaa.gov
General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.