What Is Sea Level?
We say that Mount McKinley in Alaska is 20,320 feet above sea level, or that Death Valley, in California, is 282 feet below sea level. And we say that the surface of the ocean is at sea level. But where exactly is the level of the sea? The surface level of the ocean is measured with special instruments called tide gauges.
Scientists measure the highest high tide and the lowest high tide, and then they figure out what the average is. They do the same for low tide. The point midway between average high tide and average low tide is called sea level.
In the United States and its territories, local mean sea level is determined by taking hourly measurements of sea levels over a period of 19 years at various locations, and then averaging all of the measurements.
The 19-year period is called a Metonic cycle. It enables scientists to account for the long-term variations in the moon’s orbit. It also averages out the effects of local weather and oceanographic conditions. Sea level is measured in relation to the adjacent land. Just like the ocean, the elevation of land may rise and fall over time.
For example, the tremendous weight of a glacier on land pushes the land down, closer to sea level, that same land bounces back (a process called post-glacial rebound) if the ice retreats, or melts, and its weight is removed. Local mean sea level measurements are a combination of sea level variations and movement of the land.