Define mean sea level. How is this value determined? Is it constant or variable around the world? Explain.

Oceanography is the scientific study of the oceans, covering a wide range of topics including marine organisms, ecosystem dynamics, ocean currents, waves, and the geophysical properties of the seabed. It's essential for understanding mean sea level, as oceanographers analyze long-term data to establish this average reference point. By examining historical tide records and satellite measurements, these scientists can quantify variations in sea level over time and space, providing insights into ongoing changes and future trends caused by factors like climate change.

In the oceanographic context, mean sea level is not just a static number; it's a dynamic indicator sensitive to a host of environmental factors. These include thermal expansion of water due to warming, melting of glaciers and ice sheets, and various geological processes that can cause land to rise or fall over geological time scales.

Tide measurement is crucial for calculating mean sea level and involves long-term monitoring of the rising and falling of sea levels. Tides are caused by the gravitational pull of the moon and the sun and the rotation of the Earth. To determine mean sea level, tide gauges are used. These devices, situated at select coastal locations, record the height of the water at regular intervals over a long period of time, historically a full lunar cycle, to account for the complete range of tidal fluctuations.

Advanced technology in tide measurement includes floating buoys equipped with sensors, radar tide gauges, and even satellite altimetry, which measures the time it takes for a radar pulse to travel from a satellite to the ocean surface and back again. This data is invaluable not just for understanding sea level changes, but also for navigational purposes, coastal management, and predicting potential flooding from storm surges.

Gravitational differences are one of the factors that complicate the measurement of mean sea level around the globe. The Earth's mass is not distributed evenly—variations in the distribution of continents, mountains, and ocean trenches, for instance, can locally increase or decrease the gravitational force. These variations affect the way water is pulled towards different parts of the Earth, causing the sea level to fluctuate from one region to another.

The geoid, an imaginary shape of the Earth that represents global mean sea level and is perpendicular to the direction of gravity everywhere, illustrates these gravitational anomalies. Given that the shape of the geoid is irregular due to Earth's uneven mass distribution, mean sea level is correspondingly irregular and must be measured relative to the geoid, not a perfect sphere.

Mean sea level serves as the principal datum or reference point for measuring the Earth’s elevation. This elevation reference is used in cartography, navigation, and civil engineering to estimate the height of geographical features, ranging from mountains to man-made structures. It is derived from a calculated average of sea level measurements taken over time from various locations around the world.

However, it's important to understand that due to the gravitational differences and the other factors mentioned earlier, mean sea level varies at different locations across the globe. To account for this, local or regional datums are often used, which align more closely with the mean sea level at their specific locations. This local mean sea level is then adjusted to agree with the global mean sea level, providing a more accurate reference for elevation measurements.