chartTwo water levels both read 1.7 metres
VERTICAL ACCOUNT · ONE COMMON ZERO
The same 1.70 metres is not the same elevation after its datum changes
Attach source, datum, unit and sign to every value. Only depth and tide explicitly sharing CD can be added directly.Average weather; observations may fall lower
Common NOAA chart datum
Usually not sounding zero
| Charted depth | +5.40 m | CD to seabed |
|---|---|---|
| Predicted tide | +1.70 m | CD to surface |
| Nominal depth | =7.10 m | Surface to seabed |
| Static draft | −5.80 m | Waterline to keel |
| Squat | −0.35 m | Underway example |
| Nominal UKC | =0.95 m | Still needs risk allowance |
A datum gives changing depth observations one common zero
A field sounding runs from the water surface existing at the observation time to the seabed, while tide and weather continually move that surface. Plotting raw line or sonar depths would give the same bottom different numbers at different times. Hydrographic reduction brings observations to a specified vertical reference. The IHO defines chart sounding datum as the tidal datum to which charted soundings and drying heights are referred, usually corresponding to a low-water stage.
Chart datum is an abstract reference, not a physical layer that water can never cross. Charted depth is measured downward from it and tide height upward from the same zero. If a charted depth is 5.4 metres and water level is 1.7 metres above datum, simplified water depth is 7.1 metres. If pressure and offshore wind produce an observed height of minus 0.2 metre, depth is about 5.2 metres. Negative tide height can be valid in some datum systems.
LAT, MLLW and mean sea level describe different surfaces
Lowest Astronomical Tide is the lowest tide level predictable under average meteorological conditions and any combination of astronomical conditions. It excludes exceptional pressure and wind effects, so observed water can still fall below LAT. Many hydrographic offices in the British tradition use a chart datum approximating LAT. Approximating matters: local realization, legacy datums and continuity between charts can leave a difference.
NOAA commonly uses MLLW for chart products and predicted tides. Mean Lower Low Water averages each tidal day's lower low water over a National Tidal Datum Epoch; the current official U.S. epoch is 1983–2001. Mean Sea Level averages the water-level series and generally lies above a low-water chart datum. LAT, MLLW, MSL, NAVD88 and ellipsoid height cannot be exchanged by name because their separation varies spatially.
Depth, drying height and tide height occupy opposite sides of the zero
A seabed that remains submerged is normally shown by charted depth below datum. A bank or rock exposed at low water can carry a drying height measuring its top above chart datum. If a rock dries 0.8 metre and tide height is 0.5 metre, about 0.3 metre remains exposed; at a tide height of 1.4 metres, nominal depth above the rock is about 0.6 metre. The particular chart legend controls sign and typography.
A tide table height also has a reference note. Direct addition works only when tide height and charted depth share chart datum. A harbor gauge station zero, an engineering drawing on an orthometric land datum, or a GNSS ellipsoid height must first be transformed. NOAA preserves a tidal datum through local land benchmarks, while UKHO tidal metadata record the connection between a gauge zero and Chart Datum or Ordnance Datum.
A vertical section checks the arithmetic but does not decide clearance
Take a channel with 5.40 metres charted depth and a predicted tide of plus 1.70 metres CD. Nominal depth is 7.10 metres. A vessel drawing 5.80 metres with estimated squat of 0.35 metre has nominal under-keel clearance of 7.10 minus 5.80 minus 0.35, or 0.95 metre. Every value uses metres and the water values share one vertical reference, so the arithmetic is reproducible.
That 0.95 metre is not an automatic safety conclusion. Observed water differs from prediction through pressure, wind, river discharge and model error. Heel, trim, wave motion, density and loading change the vessel envelope, and seabed data carry survey uncertainty and an update date. Overhead clearance may use a high-water or purpose-specific clearance datum, so the depth formula cannot simply be inverted beneath a bridge.
Vertical and horizontal datums answer different questions
WGS 84 and NAD 83 primarily locate a point horizontally or relative to an ellipsoid. Chart Datum, MLLW, LAT and NAVD88 participate in depth and elevation. A chart may validly state horizontal datum WGS 84 and sounding datum LAT in separate notes. Transforming latitude and longitude alone does not convert its soundings to another vertical reference.
NOAA VDatum models transformations among MLLW, MSL, NAVD88, ellipsoid height and other vertical frames, with coverage and uncertainty information. A datum offset is not one worldwide constant: a 0.6-metre correction at one harbor cannot be copied to another coast. Combining surfaces without a supported grid, gauge tie or official transformation can create a smooth-looking seabed with a systematic vertical step.
An old chart's datum belongs to its local tidal history
The IHO Manual on Hydrography notes that varied tides have prevented one universal scientific chart-datum definition. MLW, LLWLT, LLWST and other low-water surfaces remain in historical and regional use. Early U.S. charts likewise used forms of spring-tide low water before observation periods, policy and control networks changed. A sounding-datum note is part of the dataset, not expendable footer text.
Before attributing a depth difference between editions to erosion or deposition, align depth unit, vertical datum, horizontal datum, survey method and survey year. A change may instead come from fathom-to-metre conversion, movement from an older low-water plane to MLLW, a horizontal shift or denser sonar coverage. Archive the source value and source datum, place transformed values in separate fields, and record the transformation model and version so another researcher can reproduce the comparison.
Questions
Continue exploring this subject
Is chart datum the same as mean sea level?
Usually not. Chart soundings commonly use a lower tidal surface such as LAT or MLLW, while mean sea level averages water level. Their separation varies by place.
Does water never fall below Lowest Astronomical Tide?
It can. LAT covers astronomical tide under average meteorological conditions; high pressure, offshore wind and other non-astronomical effects can push observed water below it.
How do I turn charted depth into water depth at a given time?
When tide height and charted depth share the same datum, simplified water depth is charted depth plus tide height. Prediction error and meteorological effects still matter.
Why is a drying height above chart datum?
It measures how far an intertidal object rises above the datum. Once tide height exceeds drying height, their difference is nominal water depth above that object.
Does WGS 84 identify the chart's depth datum?
No. WGS 84 identifies a coordinate or ellipsoid reference. Soundings still require a vertical datum such as LAT or MLLW and a supported transformation.
Sources
Continue the research
- Chart sounding datumInternational Hydrographic Organization S-32
- Lowest Astronomical TideIHO Geospatial Information Registry
- Manual on Hydrography, Chapter 2International Hydrographic Organization
- Nautical Charts Tutorial: Tidal datumsNOAA National Ocean Service
- A tutorial on datumsNOAA VDatum
- Estimation of Vertical Uncertainties in VDatumNOAA VDatum
- Tidal datums and their applicationsNOAA Tides and Currents
- UKHO Tidal Harmonic Analysis and PredictionUK Hydrographic Office