The average height of the ocean's surface calculated over a specific time period, used as the standard reference datum for measuring elevation and depth.

Mean Sea Level

Definition

Mean sea level (MSL) represents the average height of the ocean's surface over an extended period, typically 19 years or more. This measurement serves as the primary vertical datum for surveying, mapping, and elevation measurements worldwide. MSL is calculated by averaging hourly tidal observations to eliminate the effects of daily and monthly tidal fluctuations.

Historical Significance

The establishment of mean sea level as a reference datum emerged during the 19th century as nations recognized the need for standardized elevation measurements. Before this standardization, different countries used various local reference points, creating inconsistencies in surveying data. The adoption of MSL enabled coordinated mapping efforts and facilitation of international surveying projects.

Determination and Measurement

Mean sea level is determined through continuous observation at tide gauge stations positioned along coastlines. These automated instruments record water surface elevations at regular intervals, typically hourly. Over an extended observation period, these measurements are averaged to account for seasonal variations, lunar cycles, and meteorological influences.

The National Oceanic and Atmospheric Administration (NOAA) in the United States maintains an extensive network of tide gauge stations that contribute to MSL determination. Similar agencies exist in other countries, creating a global framework for elevation reference data.

Variations and Adjustments

Mean sea level is not uniform across the globe. Gravitational anomalies, geoid variations, ocean circulation patterns, and tectonic activity cause MSL to vary by several meters between locations. These variations are accounted for through geoid models that describe the equipotential surface of Earth's gravity field.

Long-term changes in MSL also occur due to eustatic changes (global sea level rise or fall) and isostatic adjustment (local land subsidence or uplift). Climate change has accelerated sea level rise in recent decades, requiring periodic updates to MSL reference stations.

Surveying Applications

In surveying practice, all elevation measurements are referenced to a specific MSL datum. Benchmark monuments permanently mark established elevations relative to MSL, allowing surveyors to maintain consistency across projects and time periods. These benchmarks facilitate:

Topographic mapping - Establishing contour lines and terrain representation

Engineering projects - Setting grade elevations for construction

Hydrographic surveying - Charting underwater depths relative to water surface

Flood risk assessment - Determining hazard zones relative to established elevations

Datum Adjustment

North American Vertical Datum (NAVD 88) and older National Geodetic Vertical Datum (NGVD 29) represent different realizations of mean sea level in the United States. The shift between datums reflects improved understanding of MSL variations and geoid shape. Modern surveying increasingly employs ellipsoidal heights from GNSS receivers, which must be converted to MSL-based elevations using geoid models.

Modern Challenges

Climate-induced sea level changes present ongoing challenges to MSL-based reference systems. Rising global temperatures cause thermal expansion of seawater and glacier melting, increasing MSL at varying rates across regions. Satellite altimetry now provides unprecedented precision in tracking global sea level changes, revealing rates of approximately 3-4 millimeters per year.

Conclusion

Mean sea level remains the fundamental vertical reference datum in surveying despite technological advances. Its definition, maintenance, and periodic revision ensure that elevation measurements remain consistent, comparable, and relevant for mapping, engineering, and scientific applications worldwide.