Environment, Ocean Science and the Natural World

Hydrography, tides, waves, bathymetry and marine geology glossary

The measurement of the sea surface, water column, and seafloor: tidal harmonic analysis and the principal constituents, chart datum and Lowest Astronomical Tide, wave spectra (Pierson-Moskowitz, JONSWAP), multibeam bathymetry compiled into the GEBCO grid and Seabed 2030, and marine geology of margins, ridges, seamounts, vents, and methane hydrates. Includes IHO standards S-44 and S-100, GLOSS, PSMSL, and the IODP.

278 defined terms.

Showing 250 on this page (page 1 of 2).

A

Abyssal Hill: Small elevations on the abyssal plain formed at mid-ocean ridges.
Abyssal Plain: Flat region of the deep ocean floor, typically below 4,000 meters.
Accretionary Prism: Wedge of sediment scraped off a subducting plate at a convergent margin.
Active Margin: Continental margin coinciding with a plate boundary.
Admiralty Chart: Nautical chart issued by the UK Hydrographic Office.
Aliasing: Misinterpretation of high-frequency tidal signals due to insufficient sampling.
Amphidromic Point: Location where tidal amplitude is zero and tidal phase rotates around it.
Amphidromic System: Pattern of co-tidal and co-range lines around an amphidromic point.
Andesite Line: Petrologic boundary marking the change from basaltic to andesitic volcanism around the Pacific.
Apogee Tide: Tide occurring when the Moon is at its farthest point from Earth.
Argo Float Network for Hydrography: Subset of float observations contributing to global hydrography.
Astronomical Tide: Tide produced solely by gravitational forcing of the Sun and Moon.
Authalic Sphere: Sphere of equal area to a reference ellipsoid, used in cartography.

B

Backarc Basin: Basin behind an island arc formed by extension above a subduction zone.
Backshore: Beach zone above the normal high tide reach.
Barchan Dune (Subaqueous): Crescent-shaped sand bedform on the seabed.
Barotropic Tide: Tide treated as depth-uniform in a single-layer ocean.
Basement Rock: Crystalline rock underlying sedimentary basins.
Bathymetric Contour: Line of equal depth on a bathymetric chart.
Bathymetry: Measurement and mapping of seafloor depth.
Beach nourishment: Coastal protection technique.
Beach Profile: Cross-shore elevation profile of a beach.
Bedform: Repetitive seabed feature such as ripples, dunes, or megaripples.
Benthic Boundary Layer: Near-bottom layer where bottom friction influences flow and sediment transport.
Berm: Nearly horizontal beach feature deposited by wave action.
Bioclastic Sediment: Sediment composed mainly of biological skeletal fragments.
Black Smoker: Hydrothermal vent emitting hot, sulfide-rich fluid that precipitates dark minerals.
Bottom Echo: Acoustic return from the seabed used in echo sounding.
Bottom Stress: Tangential force exerted by water on the seabed.
Bottom Water: Water occupying the lowest layer of the ocean above the seabed.
British Admiralty Tide Tables: Annual publication of predicted tidal heights worldwide.

C

Calcareous Ooze: Pelagic sediment dominated by calcareous microfossils.
Carbonate Platform: Shallow marine area where carbonate sediments accumulate.
Channel Thalweg: Line of greatest depth along a channel.
Chart datum (CD): Reference for sounding depths, usually LAT.
Co-range Line: Line of equal tidal range.
Co-tidal Line: Line connecting points where high tide occurs simultaneously.
Coastal Plain: Flat lowland adjacent to a coast.
Continental Crust: Outer layer of continents, typically granitic in composition.
Continental Margin: Zone between continental crust and ocean basin, including shelf, slope, and rise.
Continental Rise: Gentle slope at the base of the continental slope.
Continental Shelf: Seabed and subsoil beyond territorial sea, up to the outer edge of the continental margin or 200 nm.
Continental Slope: Steep slope from the shelf edge to the deep ocean.
Crest: Highest point of a wave or ridge.
Crossover Analysis: Comparison of bathymetric or altimetric measurements at intersecting tracks.
Crust: Outermost solid layer of the Earth.
Current Ellipse: Geometric representation of tidal current variation over a tidal cycle.

D

Datum Transformation: Conversion between different geodetic datums.
Debris Flow: Gravity-driven sediment flow on the seafloor.
Decca Navigator: Historic radio navigation system used in hydrographic surveys.
Declinational Tide: Tidal constituent linked to lunar or solar declination.
Deep Sea Drilling Project (DSDP): Predecessor of ODP and IODP that recovered seafloor cores.
Deep-Sea Fan: Sediment fan built at the mouth of a submarine canyon.
Diatomaceous Ooze: Siliceous sediment dominated by diatom frustules.
Differential GPS (DGPS): GPS technique improving accuracy used in hydrographic surveys.
Diffraction: Bending of waves around obstacles.
Direction of Wave Propagation: Mean direction along which wave energy travels.
Diurnal Inequality: Difference in heights of successive high or low tides on the same day.
Diurnal Tide: Tide with one high and one low water per tidal day.
Dredging: Excavation of seabed sediment, often for navigation or reclamation.
Drift Sediment: Sediment transported by currents and deposited as a drift body.

E

Earthquake Magnitude: Logarithmic measure of seismic energy release.
East Pacific Rise: Fast-spreading mid-ocean ridge in the eastern Pacific.
Ebb Tide: Outgoing or falling tide.
Echosounder: Acoustic instrument that measures water depth.
Edge Wave: Trapped wave that travels along a coastline.
Ellipsoidal Height: Height above a reference ellipsoid.
EM Multibeam: Kongsberg multibeam echosounder family used in hydrography.
EPSG Code: Identifier in the EPSG geodetic parameter dataset.
Equilibrium Tide: Theoretical tide assuming instantaneous adjustment to tide-generating forces.
Equinoctial Tide: Larger spring tide occurring near the equinoxes.
ETOPO: NOAA global relief model combining topography and bathymetry.
Eustatic Sea Level: Sea level change due to changes in ocean volume.

F

Falling Tide: Period when tidal level is decreasing.
Fault Scarp: Cliff or step produced by faulting.
Fetch: Distance over water across which the wind blows generating waves.
Fjord: Long narrow glacially carved inlet with steep sides.
Flood Tide: Incoming or rising tide.
Foreshore: Beach zone between high and low tide marks.
Form Number: Ratio used to classify tide as diurnal, semidiurnal, or mixed.
Fracture Zone: Linear feature offsetting mid-ocean ridge segments.

G

Galaxy Tide: Not used; placeholder removed.
GEBCO: General Bathymetric Chart of the Oceans, providing global bathymetric grids.
Geocentric Datum: Datum referenced to Earth's center of mass, such as WGS 84.
Geodesy: Science of measuring Earth's shape, gravity, and rotation.
Geoid: Equipotential surface of Earth's gravity field approximating mean sea level.
Geoidal Undulation: Difference between geoid and reference ellipsoid heights.
GIS for Hydrography: Geographic information systems applied to maritime data.
Global Sea Level Observing System (GLOSS): IOC program coordinating global tide gauge observations.
GPS RTK: Real-time kinematic GPS positioning at centimeter accuracy.
Gravimeter: Instrument measuring gravitational acceleration, used in marine geophysics.
Great Diurnal Range (Gt): Difference between Mean Higher High Water and Mean Lower Low Water.
Greenwich Mean Time: Time standard at the prime meridian, formerly used for tidal predictions.
Group Velocity: Speed at which wave energy or a wave group travels.
Guyot: Flat-topped seamount.

H

Harmonic Analysis: Decomposition of a tidal record into sinusoidal constituents.
Harmonic Constituent: Sinusoidal component of the tide such as M2 or S2.
HAT (Highest Astronomical Tide): Highest tidal level predicted under average meteorological conditions.
Heave Compensation: Correction for vertical platform motion in multibeam surveying.
High Water Full and Change: Time of high water at full and new moon, related to tidal phase.
Higher High Water (HHW): Higher of two successive high waters of a tidal day.
Higher Low Water (HLW): Higher of two successive low waters of a tidal day.
Hot Spot: Mantle plume location producing intraplate volcanism.
Hugin AUV: Kongsberg autonomous underwater vehicle used in survey work.
Hydroacoustic Positioning: Positioning of underwater assets using acoustics.
Hydrographic survey: IHO S-44 compliant bathymetric survey.
Hydrothermal Vent: Seafloor opening discharging geothermally heated water.

I

IHO: International Hydrographic Organization, intergovernmental body for hydrography.
IHO S-100: Universal Hydrographic Data Model.
IHO S-44: IHO standard for hydrographic surveys, defining order categories.
IHO S-57: ENC product specification, edition 3.1.
Intertidal Zone: Area exposed at low tide and submerged at high tide.
Inverse Barometer Effect: Sea level adjustment to atmospheric pressure changes.
IODP: International Ocean Discovery Program for scientific ocean drilling.
Iridium Tide Gauge Telemetry: Use of Iridium satellites to relay tide gauge data.
Island Arc: Curved chain of volcanic islands above a subduction zone.

J

JCOMMOPS: Joint Technical Commission for Oceanography and Marine Meteorology in situ Observations Programme Support Centre.
Jellyfish Sediment Pile: Misuse term; not standard.
JONSWAP: Joint North Sea Wave Project, source of a widely used wind sea spectrum.
Junctional Tide Gauge: Tide gauge sited at a basin junction; not a formal standard.

K

K1: Lunar-solar declinational diurnal tidal constituent with period 23.93 hours.
K2: Lunar-solar semi-diurnal tidal constituent with period 11.97 hours.
Karst Coast: Coastline developed on soluble carbonate rock.
Kelvin Wave (Coastal): Trapped wave traveling along a coastline with rotation effects.
Knoll: Small isolated rounded elevation on the seabed.

L

Lacustrine Sediment: Sediment deposited in lakes; sometimes preserved in coastal sequences.
Landsat Coastal Mapping: Use of Landsat imagery for coastal change analysis.
LAT (Lowest Astronomical Tide): Modern chart datum.
Levee (Submarine): Raised bank along a submarine channel.
Lidar Bathymetry: Airborne or vessel-based laser scanning of shallow seabed.
Lithosphere: Rigid outer layer of Earth comprising crust and uppermost mantle.
Lower High Water (LHW): Lower of two successive high waters of a tidal day.
Lower Low Water (LLW): Lower of two successive low waters of a tidal day.
LOWESS Smoothing: Locally weighted regression used to smooth bathymetric or tidal data.
Lunar Day: 24.84-hour period of Earth rotation relative to the Moon.

M

M2: Principal lunar semi-diurnal tidal constituent with period 12.42 hours.
M4: Higher harmonic of M2 caused by shallow water tidal distortion.
MAREL Network: Operational French coastal observation network.
Mean High Water (MHW): Average of all high waters over a tidal datum epoch.
Mean High Water Springs (MHWS): Average level of all high waters at spring tides.
Mean Higher High Water (MHHW): Average of higher high waters over a tidal datum epoch.
Mean Low Water (MLW): Average of all low waters over a tidal datum epoch.
Mean Low Water Springs (MLWS): Average level of all low waters at spring tides.
Mean Lower Low Water (MLLW): Average of lower low waters, common datum in US waters.
Mean sea level (MSL): Average level of the sea surface.
Megaripple: Large bedform produced by strong tidal currents.
Mercator projection: Conformal chart projection with rhumb lines as straight.
Meteotsunami: Tsunami-like wave generated by atmospheric pressure disturbances.
Methane Hydrate: Ice-like solid containing methane in a water lattice, common on continental slopes.
Mid-Atlantic Ridge: Slow-spreading mid-ocean ridge bisecting the Atlantic.
Mid-Ocean Ridge: Submarine mountain range where new oceanic crust forms.
Mixed Tide: Tide with significant diurnal and semidiurnal components.
MN4: Shallow water tidal constituent from interaction of M2 and N2.
Moho: Mohorovicic discontinuity between crust and mantle.
Multibeam Echo Sounder (MBES): Sonar system mapping a swath of seabed depths.

N

N2: Larger lunar elliptic semi-diurnal tidal constituent with period 12.66 hours.
National Tidal Datum Epoch: 19-year period used to compute tidal datums.
Neap Tide: Tide with smallest range, near first and third lunar quarters.
Nektonic Pile: Not standard.
Nodal Modulation: 18.6-year cycle in tidal amplitudes from lunar nodal precession.
Notch (Coastal): Erosional indentation at the base of a cliff.
Nucleus IODP Core: Scientific drill core archived from IODP expeditions.

O

Ocean Bottom Seismometer (OBS): Seafloor instrument recording earthquake waves.
Oceanic Crust: Outer layer of ocean basins, dominantly basaltic.
Oceanic Plateau: Large submarine elevation formed by voluminous basaltic volcanism.
OOI: Ocean Observatories Initiative providing cabled and mooring observatories.
Ophiolite: Fragment of ancient oceanic crust and upper mantle exposed on land.
Orbital Wave Velocity: Particle motion under surface gravity waves.
Order 1a, 1b, 2 Survey: IHO S-44 categories of hydrographic survey accuracy.
Outer Continental Shelf: Distal portion of the continental shelf approaching the shelf break.
Overtide: Higher harmonic of a tidal constituent produced by shallow water nonlinearities.

P

Passive Margin: Continental margin not coinciding with a plate boundary.
Pelagic Sediment: Fine sediment deposited slowly far from land.
Perigean Spring Tide: Larger spring tide when the Moon is near perigee.
Phase Velocity: Speed at which a single wave crest moves.
Pierson-Moskowitz spectrum: Fully developed sea spectrum.
Pillow Lava: Pillow-shaped basalt formed during underwater eruptions.
Plate Boundary: Edge between tectonic plates, divergent, convergent, or transform.
Plate Tectonics: Theory describing lithospheric plate motion and interactions.
POL (Permanent Service for Oceanographic Levels): Historical predecessor of PSMSL.
Polynesian Wayfinding: Traditional non-instrument navigation using waves and stars.
Pore Pressure: Fluid pressure within sediment pores, important to seafloor stability.
Port Reference Tide Gauge: Tide gauge providing tidal data for a specific port.
Pressure Tide Gauge: Tide gauge measuring water pressure to infer sea level.
Prime Meridian: Reference meridian for longitude, historically Greenwich.
Principal Lunar Tide: M2 tidal constituent.
PSMSL: Permanent Service for Mean Sea Level archive of tide gauge data.

Q

Q1: Lunar elliptic diurnal tidal constituent with period 26.87 hours.
Quartz Sand: Common siliciclastic sediment on temperate beaches.
Quaternary Sediment: Sediment deposited during the Quaternary period, last 2.6 million years.

R

Radar Altimeter: Satellite instrument measuring sea surface height by radar.
Range Pole: Survey accessory for shoreline positioning.
Real Time Tide Gauge: Tide gauge transmitting data in near real time.
Reduced Tide: Tide adjusted for atmospheric and meteorological effects.
Reefs (Geomorphic): Wave-resistant constructional features built mostly by corals.
Refraction: Bending of waves as they enter shallower water.
Relict Sediment: Sediment formed under conditions different from current ones.
Resurvey Interval: Period after which an area should be re-surveyed under IHO standards.
Reverse Polarity: Period when Earth's magnetic field was opposite to today, recorded in oceanic crust.
Ridge Push: Force from elevated mid-ocean ridges driving plate motion.
Rip Channel: Channel through a longshore bar created by a rip current.
Rise (Mid-Ocean): Broad axial uplift along a fast-spreading ridge.
Rising Tide: Period when tidal level is increasing.
Roaring Forties Sediment Drift: Contourite drift built by strong Southern Ocean currents.
RTK Tide Gauge: Real-time kinematic positioning used to support tide gauge benchmarks.

S

S1: Solar diurnal tidal constituent with period 24 hours.
S2: Principal solar semi-diurnal tidal constituent with period 12 hours.
Sa: Solar annual tidal constituent.
Sa1: Variant of solar annual constituent in some catalogs.
Salinity Stratification: Vertical variation of salinity affecting acoustic propagation in surveys.
Sandwave: Large subaqueous dune in tidally swept seas.
Satellite Bathymetry: Bathymetric estimation from optical satellite imagery.
Scarp: Steep slope on the seafloor or coast.
Sea Mountain (Seamount): Submerged volcanic mountain rising at least 1,000 meters from the seabed.
Sea State Code: WMO scale relating significant wave height to qualitative description.
Seabed 2030: Nippon Foundation-GEBCO initiative to map the entire seafloor by 2030.
Seafloor Spreading: Process by which new oceanic crust forms at mid-ocean ridges.
Sediment Core: Cylindrical sample of seabed sediment recovered for analysis.
Seiche: Standing wave in a partially enclosed body of water.
Self Tide: Tidal contribution from self-attraction and loading of the ocean.
Semi-Diurnal Tide: Tide with two high and two low waters per tidal day.
Shelf Break: Edge of the continental shelf where the slope steepens.
Shoaling: Increase in wave height as a wave moves into shallower water.
Siliceous Ooze: Pelagic sediment dominated by siliceous microfossils such as diatoms and radiolaria.
Single Beam Echo Sounder: Sonar producing a single depth value per ping.
Slope Failure: Mass wasting on a submarine slope.
SLR (Satellite Laser Ranging): Geodetic technique used in maintaining the terrestrial reference frame.
Spreading Rate: Velocity at which new crust is formed at a mid-ocean ridge.
Spring Tide: Tide with largest range, near new and full moon.
SRTM: Shuttle Radar Topography Mission, providing global land elevation data.
Subduction Zone: Convergent plate boundary where one plate descends beneath another.
Submarine Canyon: Steep valley incised into the continental slope.
Submarine Landslide: Mass movement of seafloor sediment downslope.
Sweep Width: Cross-track distance ensonified by a swath sonar.

T

Tidal Bore: Wave that travels upstream against the river current with a rising tide.
Tidal Constituent: Sinusoidal component of the tide such as M2, S2, K1, or O1.
Tidal Current: Horizontal water motion associated with the tide.
Tidal Datum: Reference plane defined by a phase of the tide such as MLLW.
Tidal range: Difference between mean high and low tide.
Tide gauge: Instrument recording sea-surface elevation.
Tide Prediction: Calculation of future tides using harmonic analysis.
Tide Producing Force: Differential gravitational pull of Sun and Moon on the ocean.
TideRack Network: Generic term for distributed tide stations.
Tilt Meter: Instrument measuring ground tilt, used in geodesy.
Topographic Slope: Gradient of the seabed.
Total Sediment Load: Sum of bed load and suspended load.
Trans-Atlantic Tidal Wave: Free Kelvin and Poincare waves crossing the Atlantic basin.
Transform Fault: Strike-slip boundary, especially along mid-ocean ridges.
Trench: Deep linear depression at a subduction zone.
Tsunami: Long period gravity wave generated by seafloor displacement.

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