Mean sea level definition
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Mean Sea Level: Basic Definition and Concept
Mean sea level (MSL) is commonly defined as the average height of the sea's surface, measured over a long period, with short-term variations such as tides, waves, and weather effects averaged out. This concept is used as a universal reference point for measuring land elevations and sea depths 2358. In practice, MSL is often determined by taking the average of sea level measurements at a specific location over a standard period, such as 19 years, to account for tidal cycles and other fluctuations 110.
Physical and Geodetic Interpretations of Mean Sea Level
There are two main interpretations of mean sea level. The first is a practical, time-averaged value at a specific place, such as a monthly or yearly mean, which is used for chart datums and navigation. The second is a more theoretical, geodetic definition: the surface the ocean would assume if it were at rest, undisturbed by tides, winds, or atmospheric pressure changes. This geodetic mean sea level is a hypothetical surface, as the real ocean is always in motion due to various forces 1368.
Factors Affecting Mean Sea Level Measurements
Several factors can cause changes in mean sea level, including:
- Tides and Waves: Regular, predictable changes due to gravitational forces from the moon and sun 35.
- Atmospheric Pressure and Wind: Weather conditions can temporarily raise or lower sea level at a location 45.
- Water Density and Temperature: Changes in water temperature and salinity (steric effects) can alter sea level 47.
- Long-Term Climate Change: Melting glaciers, thermal expansion of seawater, and changes in terrestrial water storage contribute to global and regional sea level rise 5710.
Mean Sea Level as a Reference Datum
Mean sea level serves as the standard vertical datum for mapping, navigation, and geodetic measurements. It is used to determine land elevations and sea depths worldwide. However, the exact value of mean sea level can vary from place to place and over time due to the factors mentioned above, making its precise determination complex 389.
Historical and Practical Considerations
The concept of mean sea level as a stable reference point is relatively recent, emerging from debates and measurements in the eighteenth and nineteenth centuries. Its adoption as a global baseline was based on the assumption that coastal measurements could provide a reliable standard, despite the inherent variability of the sea .
Conclusion
Mean sea level is the average height of the ocean's surface, used as a reference for elevations and depths. It is determined by averaging sea level measurements over long periods to smooth out short-term fluctuations. While the concept is simple in theory, accurately defining and measuring mean sea level is complex due to natural variability and the influence of multiple environmental factors 1234+6 MORE.
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