What is Chart Datum (CD) and how does it relate to LAT?

Chart Datum (CD) is the reference level below which depths are shown on nautical charts and above which tide heights are measured. In most IHO member countries, CD is defined as Lowest Astronomical Tide (LAT) — the lowest predicted tide under average meteorological conditions. In the United States, CD is defined as Mean Lower Low Water (MLLW).

How do I convert a depth sounding from MLLW to MSL?

To convert a depth sounding from MLLW to MSL, you subtract the MSL-to-MLLW offset for your tidal station. For example, if MSL is 1.05 m above MLLW at your station, a depth of 12.0 m below MLLW equals 12.0 + 1.05 = 13.05 m below MSL. This tool automates this calculation for common tidal stations worldwide.

Why is datum conversion critical for marine construction?

Marine construction projects (piers, bridges, dredging, offshore platforms) require accurate vertical positioning. Confusion between chart datum, tidal datums, and geodetic datums can cause under-dredging (navigation hazard), over-dredging (cost overrun), incorrect structural clearances, or non-compliance with port authority requirements. A 0.5 m datum error on a large dredging project can cost millions in rework.

Marine Bathymetry & Hydrographic Chart Datum Offset Calculator

⚓ Chart Datum Offset Calculator

⚠️ Important: Tidal datum offsets are location-specific. This calculator uses published reference offsets for major tidal stations. For precise work, always verify against your local tidal datum relationships from NOAA (US), UKHO (UK), AHO (Australia), or your national hydrographic office.

Region / Tidal Station

Source Datum

Target Datum

Depth / Elevation Value (m)

Sign Convention

📊 Conversion Results

📋 Full Tidal Datum Reference Table

Below is the complete vertical datum relationship table for the selected station. All values are in meters relative to Mean Sea Level (MSL = 0.000 m).

Understanding Vertical Datum Relationships

Typical vertical datum stack for a tidal station. Values vary by location — always reference your local tidal datum epoch.

Chart Datums Explained for Marine Surveyors

What is Chart Datum (CD)?

Chart Datum is the plane of reference to which all charted depths and drying heights on nautical charts are reduced. It is chosen so that the actual water level rarely falls below it, ensuring that depths shown on charts represent near-worst-case conditions for safe navigation.

Regional Chart Datum Definitions

Region	Chart Datum Definition	Authority
IHO Member States (most of world)	Lowest Astronomical Tide (LAT)	International Hydrographic Organization
United States	Mean Lower Low Water (MLLW)	NOAA / NOS
United Kingdom	Lowest Astronomical Tide (LAT)	UK Hydrographic Office (UKHO)
Australia	Lowest Astronomical Tide (LAT)	Australian Hydrographic Office (AHO)
Japan	Lowest Astronomical Tide (LAT) — since 2002	Japan Coast Guard (JCG)
Canada	Lower Low Water Large Tide (LLWLT) — Pacific coast	Canadian Hydrographic Service

Why Datum Conversion Matters in Marine Construction

⚠️ Real-World Cost of Datum Errors: A 0.5 m vertical datum confusion on a 500,000 m³ dredging contract can cause $2–5 million in rework or result in under-dredging that creates navigation hazards. Always verify datum references with the port authority before commencing hydrographic survey or marine construction.

Common Conversion Use Cases

Dredging depth specification: Contract may specify design depth relative to MLLW, but GNSS survey measures ellipsoidal height — requires datum chain conversion
Bridge/pier clearance: Minimum air draft above MHW or MHHW for vessel passage
Offshore platform installation: Deck elevation referenced to LAT or MSL
Environmental monitoring: Sea-level rise studies require consistent MSL epoch reference
Bathymetric chart production: IHO S-44 standard requires soundings reduced to chart datum

Related Geodetic & Vertical Datum Tools

🏔️ Vertical Datum Benchmark Comparator

Compare geoid undulation (N) across EGM96, EGM2008, NAVD88, AHD, and EVRF2007 vertical datums at any position.

🌍 Geoid Height Calculator

Convert between GPS ellipsoidal height and orthometric (sea-level) elevation using EGM96 geoid model.

📐 Geoid vs Ellipsoid Explained

Understand the difference between ellipsoidal height (h), orthometric height (H), and geoid undulation (N).

🇪🇺 EVRF2007 European Vertical Datum

Understanding the European Vertical Reference Frame 2007 and its relationship to national height systems.

🇺🇸 NGVD 29 vs NAVD 88

Critical differences between the two US national vertical datums and how to convert between them.

🌏 Tectonic Drift Epoch Converter

Calculate continental plate motion and cumulative displacement between dynamic GNSS and plate-fixed datums.

Frequently Asked Questions

How accurate are the tidal datum offsets in this calculator?

The offsets used in this calculator are based on published values from NOAA, UKHO, AHO, and JCG for their primary reference stations. For project-grade accuracy, always verify against the current National Tidal Datum Epoch (NTDE) values from your national hydrographic office. In the US, the current NTDE is 1983–2001; the updated NTDE 2002–2020 is being implemented.

Can I use this for GNSS-based hydrographic surveying?

Yes. When using RTK GNSS for hydrographic surveys, the transducer depth is measured relative to the ellipsoid (WGS84). To reduce soundings to chart datum, you need the full datum chain: Ellipsoid → Geoid (N) → MSL → CD. Use the Geoid Height Calculator for the ellipsoid-to-MSL step, then this tool for MSL-to-CD conversion.

What is the difference between LAT and MLLW?

LAT (Lowest Astronomical Tide) is the lowest predicted tide level under average meteorological conditions over a full 18.6-year tidal cycle. MLLW (Mean Lower Low Water) is the arithmetic mean of the lower of the two daily low tides averaged over the NTDE. In regions with significant diurnal inequality, LAT is typically 0.1–0.5 m below MLLW.