Sea level, also referred to as mean sea level, is the level at which the oceans exist when averaged between high and low tides. It is calculated by measuring the level of the ocean over extended periods of time and under all types of weather conditions. Mean sea level is used by geographers to calculate the height or depth of any point of the earth’s land surface, for instance the height of Mount Everest is considered to be 29,028 feet (8,848 meters) above sea level.

Sea level is constantly changing as tides rise and fall, and as surrounding winds and currents change. Other factors that cause changes in sea level include water temperature, air pressure, season of the year, the presence of ice and snow, and the amount of salt in the water. All of these widely varying factors can be completely different from place to place, therefore sea level can vary depending on where it is being measured in the world, and it is not a constant.

Mechanical tide gauges are just a small part of the complicated process that is used to determine the measurement of mean sea level. These devices measure the average height of the sea with respect to the land over a period of time, which means a change in the height of the land can also affect the measurement of the sea level. They can be used independently to calculate the mean sea level of a specific location, or many tide gauges can be used simultaneously to determine the sea level of a large body of water.

The measurement of mean sea level has many uses. One of these is for aviation, where it is often crucial for a pilot to be able to determine his or her height above sea level at any given time. Another important function of the mean sea level is to determine how much the sea is rising due to factors such as the melting of the polar ice caps. Mostly, sea level is calculated in order to determine heights of geological formations and land.

While most of the land in the world lies above sea level, there are still several places that actually exist below sea level. Death Valley, California, for instance, is 282 feet (85.5 meters) below sea level, and there are many other similar locations throughout the world. The Dead Sea is the lowest of these, measuring an amazing 1371 feet (418 meters) below sea level.