Glossary & formula sheet
62 terms and 34 formulas, each linked to its chapter.
Abort point
Ch 25The last position from which a vessel can safely abandon an approach and remain clear of danger.
AIS
Ch 28Automatic Identification System. Transmits vessel identity, position and voyage data; Class A for SOLAS ships, Class B for smaller craft.
Amplitude
Ch 17The angular distance north or south of the prime vertical at which a body rises or sets; used to determine compass error.
ARPA
Ch 13Automatic Radar Plotting Aid. Tracks targets and computes CPA and TCPA, subject to tracking lag and target swap.
Buys Ballot's Law
Ch 34Northern hemisphere: with your back to the wind, low pressure lies to your left.
CATZOC
Ch 29Category Zone of Confidence. ENC attribute rating survey quality from A1 (best) to D and U (unassessed).
Chart datum
Ch 2The vertical reference for charted soundings, typically MLLW or Lowest Astronomical Tide.
Compass error
Ch 6The algebraic sum of variation and deviation; the total correction from compass to true.
CPA / TCPA
Ch 13Closest Point of Approach and Time to CPA, derived from relative motion.
Danger bearing
Ch 8A limiting bearing on a charted object which, if not crossed, keeps the vessel clear of a hazard.
Dangerous semicircle
Ch 35The half of a tropical cyclone, right of track in the northern hemisphere, where wind and storm motion combine and drive a vessel into the path.
Dead reckoning
Ch 7Advancing a position by course steered and speed through the water only, without current or wind.
Declination
Ch 15Angular distance of a celestial body north or south of the celestial equator; the analogue of latitude.
Departure
Ch 24The east-west distance made good, in nautical miles, along a parallel.
Deviation
Ch 6Compass error caused by the vessel's own magnetism; varies with heading and is tabulated on a deviation card.
DGPS
Ch 11Differential GPS. Broadcast corrections from a fixed reference station reducing error to a few metres.
Dip
Ch 16The correction for the observer's height of eye, always subtracted from sextant altitude.
Drift
Ch 7The speed of a current, in knots.
ECDIS
Ch 14Electronic Chart Display and Information System; satisfies SOLAS chart carriage when used with official ENCs and adequate back-up.
ENC
Ch 14Electronic Navigational Chart; official vector chart data to S-57 or S-101.
EPIRB
Ch 28Emergency Position-Indicating Radio Beacon transmitting distress alerts on 406 MHz.
Equation of time
Ch 18The difference between apparent solar time and mean solar time, up to about 16 minutes.
Estimated position
Ch 7A DR position adjusted for current, leeway or a single LOP; plotted as a square.
Fetch
Ch 32The distance over water that a wind blows in a constant direction, a primary control on wave height.
Geoid
Ch 2The equipotential gravity surface approximating mean sea level; irregular, unlike an ellipsoid.
GHA
Ch 15Greenwich Hour Angle; angular distance west of the Greenwich meridian, 0°–360°.
GMDSS
Ch 28Global Maritime Distress and Safety System; equipment requirements vary by sea area A1–A4.
Gnomonic projection
Ch 3A projection on which great circles appear as straight lines; used for great-circle planning charts.
Great circle
Ch 24The shortest track between two points on a sphere; its course changes continuously.
Growler
Ch 33A small, nearly awash piece of glacial ice; a poor radar target and a severe hazard.
HDOP
Ch 11Horizontal Dilution of Precision; the factor by which satellite geometry multiplies ranging error.
Ho / Hc
Ch 20Observed altitude and computed altitude; their difference is the intercept.
IALA
Ch 5International Association of Marine Aids to Navigation and Lighthouse Authorities; defines buoyage Regions A and B.
Index error
Ch 16Sextant error found against the horizon; on the arc subtract, off the arc add.
Intercept
Ch 20a = Ho − Hc, in minutes of arc equal to nautical miles; plotted toward the body if Ho is greater.
Leeway
Ch 7Downwind displacement caused by wind acting on the hull and superstructure.
LHA
Ch 15Local Hour Angle; GHA corrected for the observer's longitude.
Line of position
Ch 8A line on which the vessel is known to lie; two or more crossing LOPs give a fix.
Loran-C / eLoran
Ch 12Terrestrial hyperbolic radionavigation using time differences at 100 kHz; eLoran is a resilient GNSS backup.
Mercator projection
Ch 3Conformal cylindrical projection on which rhumb lines are straight and scale expands as sec of latitude.
Meridional parts
Ch 24Tabulated values used in Mercator sailing to compute an exact rhumb-line course.
Multipath
Ch 10Signal reflections that corrupt radio and GNSS measurements near structures or a reflective sea surface.
NAVTEX
Ch 28Automatic narrow-band direct printing of navigational and meteorological warnings on 518 kHz.
Nominal range
Ch 5The luminous range of a light in 10 nautical miles meteorological visibility.
Parallel indexing
Ch 13A radar technique using a pre-drawn offset line to monitor cross-track error against a fixed object.
Polar Code
Ch 33IMO mandatory code for ships in polar waters; requires a Polar Ship Certificate and Polar Water Operational Manual.
Range (natural)
Ch 8Two charted objects observed in line, giving the most accurate line of position available.
Rhumb line
Ch 3A track of constant course, crossing all meridians at the same angle; straight on a Mercator chart.
Rule of Twelfths
Ch 9Approximation dividing tidal range over six hours as 1, 2, 3, 3, 2, 1 twelfths.
Running fix
Ch 8A fix from LOPs at different times, one advanced along the DR track; carries DR error.
Safety contour
Ch 14The ECDIS depth contour used to generate anti-grounding alarms; set deliberately for each passage.
SART
Ch 28Search and Rescue Transponder; produces twelve blips on a 9 GHz radar display.
Set
Ch 7The true direction toward which a current flows.
SHA
Ch 15Sidereal Hour Angle; a star's angular distance west of the first point of Aries.
Significant wave height
Ch 32The average height of the highest one-third of waves; maxima may approach twice this value.
Thermohaline circulation
Ch 31Deep global circulation driven by density differences from temperature and salinity.
Traffic separation scheme
Ch 27A routeing measure under COLREGS Rule 10; proceed with the flow and cross at right angles.
TVMDC
Ch 6True, Variation, Magnetic, Deviation, Compass — the sequence for converting between true and compass directions.
Variation
Ch 6The angle between true and magnetic north at a place; shown on the compass rose.
Vertex
Ch 24The point of highest latitude on a great circle, where the course is 090° or 270°.
WGS 84
Ch 2The global geodetic datum used by GNSS; charts may use a different datum requiring a shift.
Zone description
Ch 18The whole hours applied to zone time to obtain UT; positive in west longitude.