Hydrography & Charting

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Outer beams have been smiling since lunch, drop a cast before the next line.

  TranslationThe edges of the multibeam swath curl upward because the sound speed profile is out of date, so take a new profile before continuing.

• Two holidays at the north end, add infills and run splits over that 4.1.

  TranslationThere are two gaps in coverage at the north end; fill them with short extra lines and run lines between the existing ones over a suspicious shoal sounding.

• We dropped to float under the bridge, flag those lines and apply the SBETs tonight.

  TranslationRTK positioning lost its centimetre fix near the bridge, so those lines need post-processed position and attitude applied overnight.

• Crosslines bust by 15 centimetres on the afternoon lines, smells like the tide zoning.

  TranslationThe check lines disagree with the main lines by 15 cm for afternoon data, probably because the tide corrections for that zone are wrong.

• Contact 12 stands about 2 metres proud off the shadow, develop it and get a least depth for the DtoN.

  TranslationMeasuring the side-scan shadow puts a target about 2 m above the seabed; survey it in detail to find its shallowest point and report it urgently as a danger to navigation.

• Patch came out at minus 0.1 roll, the pole mount must have shifted when we hit the pontoon.

  TranslationThe calibration shows a new roll bias, probably because the over-the-side sonar mount moved after a collision.

• Weathered out on the bar, so it's cleaning fliers and building the CUBE surface.

  TranslationThe sea is too rough to survey, so the crew is editing bad soundings and gridding the data instead.

Survey orders, uncertainty limits, line plans and the coverage rules that decide when a survey is good enough.

• Bathymetryalso bathy, seabed topography
  The measured depth and shape of the seabed, or the work of measuring it.
• Coveragealso bottom coverage, full coverage
  The portion of the seabed actually measured or imaged by a survey, expressed as a percentage of the area.
• Crosslinesalso check lines, cross lines
  Survey lines run across the mainscheme lines so the two data sets can be compared as an independent check.
• Exclusive Order
  The strictest survey order, used for berths and critical channels where under-keel clearance is minimal.
• Feature detectionalso object detection
  The ability of a survey system to find seabed objects of a stated minimum size, usually given as a cube edge length.
• Hydrographic surveyalso hydro survey, hydrographic surveying
  A systematic measurement of depths, seabed features and shoreline, made so the results can be charted and trusted by mariners.
• Infillalso infill line, fill-in line
  A short extra line run to fill a gap in coverage, usually to clear holidays found during processing. Shop talk
• Line planalso survey lines, line file
  The set of pre-planned survey lines, with spacing and orientation, that a vessel follows to achieve the required coverage.
• Mainscheme linesalso mainscheme, main scheme
  The primary set of parallel survey lines that delivers the required depth coverage over a survey area.
• Order 1a
  A survey order for coastal waters and fairways where features matter but under-keel clearance is less critical.
• Overlapalso sidelap, swath overlap
  The share of each swath that is also covered by the neighbouring line, planned in to guard against gaps.
• S-44also IHO S-44
  The international minimum standard for hydrographic surveys, setting uncertainty, detection and coverage limits by order.
• Sheetalso survey sheet
  Crew shorthand for one registered survey area and its data set, worked and reported as a single unit. Shop talk
• Special Orderalso IHO Special Order
  A survey order for harbours and channels where under-keel clearance is critical, with full coverage and 1 m feature detection.
• Splitsalso split lines, half-spacing lines
  Extra survey lines run halfway between existing lines to tighten spacing over a shoal or suspicious spot. Shop talk
• THUalso total horizontal uncertainty
  Total horizontal uncertainty: how far a sounding's true position could be from where it was recorded, at 95 percent confidence.
• TPUalso total propagated uncertainty, uncertainty budget
  Total propagated uncertainty: the combined estimate of how far each sounding could be from the truth, at 95 percent confidence.
• TVUalso total vertical uncertainty
  Total vertical uncertainty: the depth part of a sounding's uncertainty, judged against a depth-dependent limit.

Echo sounders, side-scan, towfish and the acoustic behaviour that turns pings into depths and images.

• Acoustic shadowalso shadow
  The dark area behind an object in a side-scan image where the object blocked sound from reaching the seabed.
• Aerationalso bubble sweepdown, bubble wash
  Air bubbles under the hull or across the transducer face that block sound and cause lost pings.
• ALBalso airborne lidar bathymetry, bathymetric lidar
  Airborne lidar bathymetry: depth measured from an aircraft with green laser pulses that pass through the water.
• Backscatteralso BS, acoustic reflectivity
  The strength of sound returned from the seabed, used to map bottom type alongside depth.
• Bottom detectionalso bottom detect, detection
  The method a multibeam uses to pick the instant the seabed echo arrives in each beam and turn it into a depth.
• Bottom pickalso digitized depth, bottom digitizing
  The depth chosen from an echo trace as the true seabed, either by the sounder's tracker or by an operator. Shop talk
• Dropoutalso drop-out, data dropout
  Missing pings or beams where the sonar failed to record a valid depth, leaving gaps in the data. Shop talk
• Flying the fishalso fish height, towfish altitude
  Controlling a towfish's height above the seabed by adjusting cable length and boat speed. Shop talk
• Laybackalso cable layback, fish layback
  How far astern of its tow point a towed sensor actually rides, measured horizontally.
• Lost bottomalso losing bottom, lost lock
  Said when an echo sounder stops tracking the real seabed and reports noise, a second echo or nothing. Shop talk
• MBESalso multibeam, multibeam echo sounder
  Multibeam echo sounder: a sonar that measures a fan of depths across the track on every ping, covering a wide swath.
• Nadiralso nadir beam
  The point on the seabed directly beneath the sonar, where the beam looks straight down.
• Nadir gapalso nadir zone, water column gap
  The unimaged strip directly under a side-scan towfish, roughly twice as wide as the fish's height above the seabed.
• Pingalso ping rate, pinging
  One transmit-and-listen cycle of a sonar; as a verb, to run the sonar.
• Pole mountalso over-the-side mount, pole-mounted sonar
  A sonar transducer fixed on a rigid pole over the side or bow of a boat instead of built into the hull. Shop talk
• Side-scan sonaralso sidescan, SSS
  A sonar that images the seabed to either side of its track, revealing objects and textures by their echoes and shadows.
• Singlebeam echo sounderalso SBES, singlebeam
  An echo sounder that measures one depth directly beneath the vessel on each ping, using a single vertical beam.
• Sub-bottom profileralso SBP, chirp
  A low-frequency sonar that penetrates the seabed to image buried layers and objects beneath the bottom.
• Swathalso swath width
  The strip of seabed measured or imaged across the track on one line of a multibeam or side-scan survey.
• Towfishalso fish, towbody
  A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.
• Transduceralso sonar head, head
  The device that turns electrical pulses into sound in the water and converts returning echoes back into signals.
• Wet endalso wet side
  The underwater parts of a survey system, such as the sonar head or towfish, as opposed to the topside electronics. Shop talk

Satellite positioning, motion sensors, calibration and the vessel geometry behind every sounding.

• Bar checkalso bar check calibration, plate check
  A singlebeam calibration that hangs a plate on marked lines under the transducer and compares the sounder with each known depth.
• Fixalso position fix, RTK fix
  A measured position of the survey vessel at a recorded instant, and the act of determining it.
• Floatalso RTK float, float solution
  Describes an RTK solution whose carrier-phase ambiguities are not yet fixed, giving only decimetre-level accuracy. Shop talk
• GNSSalso GPS, satellite positioning
  Global navigation satellite systems: satellite positioning such as GPS, GLONASS, Galileo and BeiDou, used together.
• Heavealso vertical motion
  The vertical up-and-down motion of a vessel caused by waves, measured and removed from sonar depths.
• Latencyalso time delay, timing offset
  A time delay between when a sensor measures something and when that measurement gets its time stamp.
• Lead linealso leadline, hand lead
  A marked line with a lead weight on the end, used to measure depth by hand and to check echo sounders.
• Lever armsalso offsets, vessel offsets
  The measured distances between sensors on a survey vessel, used to carry positions and motion to each sonar.
• MRUalso motion reference unit, motion sensor
  Motion reference unit: the sensor that measures a vessel's roll, pitch and heave to correct sonar data.
• Navalso navigation, nav data
  Shorthand for the vessel's positioning data stream, or for the positioning system as a whole. Shop talk
• Patch testalso calibration, patch
  A set of calibration lines over flat ground and slopes that measures sonar alignment biases and timing error.
• Pitchalso pitch angle
  Bow-up or bow-down rotation of a vessel, which shifts sonar footprints forward or aft along the track.
• PPKalso post-processed kinematic
  Post-processed kinematic: GNSS positions computed after the survey from logged rover and base data, at centimetre level.
• Rollalso roll angle
  Rotation of a vessel about its fore-and-aft axis, measured to correct the angle of every sonar beam.
• RTKalso real-time kinematic, RTK GNSS
  Real-time kinematic: GNSS positioning that uses carrier-phase corrections from a base station to reach centimetres live.
• Run-inalso run in, lead-in
  The stretch before a survey line where the boat settles on course and speed before logging starts. Shop talk
• SBETalso smoothed best estimate of trajectory, post-processed nav
  Smoothed best estimate of trajectory: the post-processed position and attitude file applied to multibeam data. Shop talk
• Squatalso settlement and squat, dynamic draft
  The lowering of a vessel in the water as it moves, which must be added to sonar depths at survey speed.
• Static draftalso draft, draught
  The vertical distance from the waterline down to the transducer face while the vessel is at rest.

Sound speed profiles, refraction, tides and the datums that give depths their meaning.

• Castalso SV cast, dip
  One lowering of a sound speed or CTD probe through the water column to measure a profile. Shop talk
• Chart datumalso CD, sounding datum
  The low-water level from which charted depths are measured, set so actual depth is seldom much less than shown.
• ERSalso ellipsoidally referenced survey, ellipsoid-referenced survey
  Ellipsoidally referenced survey: depths taken from GNSS ellipsoid heights and converted to chart datum with a separation model.
• Refractionalso refraction error, sound speed error
  The bending of sonar beams as they cross water layers with different sound speeds, which distorts depths if uncorrected.
• Separation modelalso separation surface, datum separation model
  A grid giving the height difference between the GNSS ellipsoid and chart datum across an area.
• Smiles and frownsalso smiley, frowny
  Cross-track curling of multibeam swaths, up or down at the edges, caused by sound speed errors. Shop talk
• Surface sound speedalso SSV, surface sound velocity
  The speed of sound right at the sonar face, measured continuously so the multibeam can steer its beams.
• SVPalso sound velocity profile, sound speed profile
  Sound velocity profile: how the speed of sound changes with depth through the water, measured to correct sonar depths.
• Tide correctionalso tide reduction, tide reducers
  The adjustment that removes the height of the tide from measured depths so they refer to chart datum.
• Tide zoningalso tidal zoning, zoning
  Dividing a survey area into zones, each with time and height adjustments for transferring tides from a reference gauge.

Cleaning, gridding and the artefacts processors hunt down before data leaves the survey office.

• Bustalso tide bust, heave bust
  A vertical or horizontal mismatch between overlapping lines or surveys that exposes an error somewhere. Shop talk
• Cleaningalso data cleaning, editing
  Reviewing survey data to reject spikes, noise and false bottoms before building final products. Shop talk
• CUBEalso CUBE surface
  Combined Uncertainty and Bathymetry Estimator: an algorithm that builds a depth surface by weighting soundings by uncertainty.
• Flieralso flyer, spike
  A spurious sounding that sticks out shoaler or deeper than the real seabed, usually from noise or a false detection. Shop talk
• Griddingalso surfacing, grid
  Turning millions of scattered soundings into a regular grid of depth values at a chosen cell size.
• Holidayalso data gap, coverage gap
  A gap in survey coverage where no data was collected, which must be filled before the survey is complete. Shop talk
• Junctionalso junction analysis, junction comparison
  An overlap between a new survey and an adjoining or older survey, compared to check consistency.
• Soundingalso depth, spot depth
  A single measured depth at a known position, or a depth figure printed on a chart.
• Wobblealso motion artefact, roll artefact
  A rocking or rippling artefact in multibeam data that follows the vessel's motion, caused by attitude or timing errors. Shop talk

Charts, ENCs, data standards and the cartographic rules that keep shoal water on the chart.

• Aids to navigationalso AtoN, ATON
  Buoys, beacons, lights and electronic marks placed to guide vessels and warn of hazards.
• CATZOCalso zone of confidence, ZOC
  Category of zone of confidence: the ENC quality code that tells mariners how far to trust the depth data.
• Depth contouralso isobath, depth curve
  A chart line that follows a single depth below chart datum across the seabed.
• Drying heightalso drying heights, dries
  Height above chart datum of a rock, bank or flat that dries at low tide, shown on charts as an underlined figure.
• ECDIS
  Electronic Chart Display and Information System: the type-approved chart system a ship can carry in place of paper charts.
• ENCalso electronic navigational chart, vector chart
  Electronic navigational chart: an official vector chart database used in ECDIS, built from objects with attributes.
• Nautical chartalso chart, navigational chart
  A map of waters and coasts made for mariners, showing depths, hazards, aids to navigation and landmarks.
• Notice to Marinersalso NtM, NM
  An official bulletin of chart corrections and navigational warnings, issued regularly to keep charts current.
• S-100also IHO S-100
  The international data framework behind a family of marine products, including vector charts, gridded bathymetry and water levels.
• S-57also S57, IHO S-57
  The international data transfer standard behind most electronic navigational charts, built on objects and attributes.
• Shoal-biasedalso shoal bias
  Favouring the shallower value whenever depths are generalised, so a chart never shows more water than exists.
• Sounding selectionalso sounding thinning, selected soundings
  Choosing which surveyed depths appear on a chart so the shoalest water is always shown.

Wrecks, rocks, shoals and the investigations that pin down their least depths.

• Contactalso target, sonar contact
  A target picked out of side-scan or multibeam data that may be an object worth investigating.
• Developmentalso feature development, developing
  Extra survey work over a feature or shoal to pin down its least depth, shape and extent.
• Disprovalalso disprove, disproval search
  Survey work proving that a charted feature does not exist, so it can be removed from the chart.
• DtoNalso danger to navigation, DTON
  Danger to navigation: a newly found hazard reported urgently so charts and warnings change before the survey ends.
• Fluffalso fluid mud, soft bottom
  A soft, fluid mud layer over the firm seabed that gives different depths at different sonar frequencies. Shop talk
• Hangalso drag hang, wire hang
  In a wire drag sweep, the moment the wire catches on an obstruction, and the obstruction that caused it. Shop talk
• Least depthalso shoalest depth, minimum depth
  The shallowest depth over a wreck, rock or other feature, which is the value that gets charted.
• Obstructionalso obstn
  A submerged object that hinders vessels or fishing and is not a wreck or natural rock, such as piles or debris.
• Rock awashalso rock awash at chart datum
  A rock whose top lies at about the level of chart datum, breaking the surface or just under it at low water.
• Shoalalso shoaling
  A seabed area shallower than its surroundings that can endanger vessels; as a verb, to become shallower.
• Sweepalso wire drag, wire sweep
  A search that drags a wire or bar at a set depth to prove an area clear of obstructions to that depth.
• Wreckalso wreckage
  The remains of a sunken vessel, charted as a hazard or a feature depending on its depth and size.

Nicknames and boat talk heard on survey launches and in the processing room.

• Bathyalso bathy data, bathy surface
  Clipped form of bathymetry, used for depth data, depth surfaces or the depth-measuring part of a job. Shop talk
• Droggyalso droggie
  Royal Navy slang for a hydrographic surveyor, from a clipped form of hydrographer. Shop talk
• Hydroalso hydrography, hydrographic surveying
  Clipped insider word for hydrography, hydrographic surveying, or the survey team itself. Shop talk
• Mowing the lawnalso lawnmower pattern, lawnmowing
  Running long parallel survey lines back and forth across an area, like cutting grass in strips. Shop talk
• Onlinealso on line, logging
  On a planned survey line and logging data; going offline means ending the line and closing the file. Shop talk
• Survey launchalso launch, hydro launch
  A small survey boat fitted with sonar and positioning gear, often lowered from a mother ship for inshore work.
• Weathered outalso weather day, weathered in
  Unable to survey because wind, sea or swell would ruin data quality or make boat work unsafe. Shop talk

Frequently asked questions

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