"Why is Mean Sea Level used as a reference?"

"Mean Sea Level provides a globally consistent and stable reference for measuring land elevation and water depth. It averages short-term fluctuations due to tides, weather, and other factors, enabling accurate comparisons across regions and supporting navigation, engineering, and aviation safety."

"How is Mean Sea Level determined?"

"MSL is determined by averaging hourly sea level measurements from tide gauges over a standard period (usually 19 years, a Metonic cycle). This process accounts for tidal, seasonal, and meteorological variations to provide a reliable reference."

"What is the difference between Global and Local Mean Sea Level?"

"Global Mean Sea Level (GMSL) is the average sea surface height across all the world’s oceans, typically measured by satellites. Local Mean Sea Level (LMSL) is the average at a specific location, measured by tide gauges, and can vary due to regional factors like currents, wind, and land movement."

"How does land movement affect sea level measurements?"

"Land movements such as subsidence (sinking) or uplift (rising) can cause local sea level changes independent of actual ocean volume changes. This is why both relative and absolute sea level measurements are important for accurate assessments."

"How do satellites measure sea level?"

"Satellite altimeters use radar to measure the distance to the sea surface from orbit, combining this with precise satellite positioning data to compute sea surface height globally. This method provides near-global coverage and high accuracy."

Mean Sea Level (MSL)

Mean Sea Level (MSL) is the average sea height, serving as the global reference for elevation and depth in mapping, surveying, and aviation.

Geodesy Earth Science Surveying Aviation

Mean Sea Level (MSL) is the arithmetic mean of hourly sea level heights observed over a specific, standardized period—typically 19 years, known as a Metonic cycle—at a particular location. MSL serves as the foundational reference for elevation and depth measurements in surveying, mapping, aviation, and geodesy. The use of MSL provides a common vertical datum for comparing heights across regions, enabling consistency in navigation, engineering, and scientific research.

MSL is a dynamic quantity, influenced by a variety of physical phenomena such as tides, atmospheric pressure, oceanic currents, temperature, and salinity. To mitigate these short-term fluctuations, MSL is not an instantaneous measurement but an average, smoothing out periodic and episodic changes. In geodetic terms, MSL approximates the geoid—Earth’s equipotential gravitational surface—though, in reality, local mean sea level may deviate from the geoid due to regional oceanographic and atmospheric processes.

Use in Aviation and Surveying

In aviation, MSL is the reference baseline for all altitude readings. For example, when an aircraft’s altimeter reads 3,000 feet, it is 3,000 feet above MSL. In surveying and civil engineering, land elevations and water depths are measured relative to MSL, ensuring a standardized basis for maps, charts, and infrastructure design.

Global Versus Local MSL

Global Mean Sea Level (GMSL): The spatially averaged height of the oceans, monitored by satellites.
Local Mean Sea Level (LMSL): Determined at specific tide gauge locations, can differ significantly from GMSL due to regional influences such as currents, temperature, atmospheric conditions, and land movement.

ICAO Reference

According to ICAO Annex 14, all aerodrome elevations and obstacle heights are referenced to MSL, and accurate determination of MSL is essential for the safety and standardization of international aviation operations.

Vertical Datum

A Vertical Datum is a fixed reference surface from which vertical measurements, such as elevations and depths, are made. Vertical datums provide the “zero” elevation level, enabling consistent measurement across different locations and over time. Examples include the Ordnance Datum Newlyn (UK), NGVD 29, and NAVD 88 (US). Modern approaches use geoid-based global vertical datums, enabled by satellite geodesy.

Tide Gauge

A Tide Gauge is a precision instrument installed along coastlines to measure the sea level relative to a fixed benchmark on land. Tide gauges are fundamental for defining local mean sea level, tracking sea level change, and supporting navigation, engineering, and climate science. They can use floats, pressure sensors, or radar technologies.

Satellite Altimeter

A Satellite Altimeter is a radar instrument on satellites that measures the precise distance from the satellite to the sea surface, enabling calculation of Sea Surface Height (SSH) globally. Since the 1990s, satellites like TOPEX/Poseidon and the Jason series have revolutionized global sea level monitoring.

Geoid

The Geoid is a theoretical surface representing the shape Earth’s oceans would take under the influence of gravity and rotation alone—an equipotential gravitational surface. The geoid is the zero-elevation surface for most vertical datums, closely approximated (but not matched exactly) by MSL.

Ellipsoid

An Ellipsoid is a mathematically defined, smooth, regular surface that approximates the Earth’s shape (slightly flattened at the poles). GPS and GNSS systems use ellipsoidal models (like WGS84) as their geometric reference, but these differ from the geoid and MSL due to gravity variations.

Global Mean Sea Level (GMSL)

Global Mean Sea Level (GMSL) is the spatially averaged height of the world’s oceans. It is a key indicator of climate change, integrating the impacts of ocean warming, ice melt, and land water storage. GMSL is measured by satellite altimetry and tide gauges.

Local Mean Sea Level (LMSL)

Local Mean Sea Level (LMSL) is the average sea level at a specific location over a 19-year period. LMSL can differ from GMSL due to regional factors (currents, tides, wind, and land movement).

Metonic Cycle

The Metonic Cycle is a 19-year period used for averaging tide gauge data, encompassing all major tidal and astronomical cycles that affect sea level variation.

Sea Surface Height (SSH)

Sea Surface Height (SSH) is the height of the ocean surface above a reference surface (typically the geoid or ellipsoid), measured globally by satellite altimetry.

Bathymetry

Bathymetry is the measurement of water body depths relative to MSL, essential for navigation, geology, and marine ecology.

Orthometric Height

Orthometric Height is the elevation of a point above the geoid (mean sea level) measured along the direction of gravity. It is the standard elevation used in most practical applications.

Ellipsoidal Height

Ellipsoidal Height is the geometric height above the ellipsoid (GNSS reference surface), converted to orthometric height using geoid models.

Geodetic Leveling

Geodetic Leveling determines differences in elevation between points, referencing a vertical datum like MSL. Techniques include spirit leveling, trigonometric leveling, and GNSS-based leveling.

Post-Glacial Rebound (Glacial Isostatic Adjustment)

Post-Glacial Rebound or Glacial Isostatic Adjustment (GIA) is the gradual uplift of land previously compressed by ice sheets, affecting local sea level measurements.

Subsidence

Subsidence is the sinking of land due to natural or human-induced causes, influencing local relative sea level rise.

Thermal Expansion

Thermal Expansion is the increase in ocean volume due to warming seawater, the dominant contributor to 20th-21st century sea level rise.

Glacial Melting

Glacial Melting adds freshwater to the oceans from land-based ice, accelerating global sea level rise.

Water Storage Change

Water Storage Change involves the transfer of water between land and ocean reservoirs (e.g., groundwater depletion, reservoir filling), affecting global sea level.

Floodplain

A Floodplain is a flat area near water bodies subject to flooding, with mapping dependent on accurate MSL and elevation data.

Tidal Datum

A Tidal Datum is a standard elevation defined by a phase of the tide (e.g., Mean High Water, Mean Lower Low Water, MSL) and is essential for hydrography and navigation.

Equipotential Surface

An Equipotential Surface is a surface of constant gravitational potential; the geoid is such a surface, approximating global MSL.

Gravity Anomaly

A Gravity Anomaly is a deviation of measured gravity from a reference model, revealing Earth’s mass distribution and informing geoid models.

Eustatic Sea Level

Eustatic Sea Level describes global ocean volume changes from ice melt or thermal expansion, as opposed to local relative changes.

Relative Sea Level

Relative Sea Level is the ocean’s height relative to the land at a given location, integrating both ocean volume changes and land movement.

Geodetic Reference System

A Geodetic Reference System is a standardized coordinate and reference surface for mapping, navigation, and geodesy (e.g., WGS84).

Summary

Mean Sea Level (MSL) is a fundamental geodetic concept underpinning mapping, navigation, engineering, and climate science. As both a local and global reference, it enables the consistent measurement of elevation and depth, supporting safe aviation, robust infrastructure, and reliable risk assessment for natural hazards and climate change.

For organizations that depend on precise elevation data, understanding MSL and related terms—vertical datums, geoid, ellipsoid, and more—is essential for accuracy, safety, and regulatory compliance.

Explore our resources or contact us to learn how accurate MSL data and geodetic solutions can enhance your operations.

Frequently Asked Questions

Why is Mean Sea Level used as a reference?: Mean Sea Level provides a globally consistent and stable reference for measuring land elevation and water depth. It averages short-term fluctuations due to tides, weather, and other factors, enabling accurate comparisons across regions and supporting navigation, engineering, and aviation safety.
How is Mean Sea Level determined?: MSL is determined by averaging hourly sea level measurements from tide gauges over a standard period (usually 19 years, a Metonic cycle). This process accounts for tidal, seasonal, and meteorological variations to provide a reliable reference.
What is the difference between Global and Local Mean Sea Level?: Global Mean Sea Level (GMSL) is the average sea surface height across all the world’s oceans, typically measured by satellites. Local Mean Sea Level (LMSL) is the average at a specific location, measured by tide gauges, and can vary due to regional factors like currents, wind, and land movement.
How does land movement affect sea level measurements?: Land movements such as subsidence (sinking) or uplift (rising) can cause local sea level changes independent of actual ocean volume changes. This is why both relative and absolute sea level measurements are important for accurate assessments.
How do satellites measure sea level?: Satellite altimeters use radar to measure the distance to the sea surface from orbit, combining this with precise satellite positioning data to compute sea surface height globally. This method provides near-global coverage and high accuracy.

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