Where is sea level measured?

The heights of different places are measured relative to a zero-elevation surface called a vertical datum, just like the NAVD 88 of the United States based on a point in Canada.

The summit of Mont Blanc is 15,777 feet (4,809 meters) above mean sea level. The height of the sea changes by a few feet due to the tides, and this is something that can be seen by everyone who travels along the Atlantic shore. Despite this, the height of Mont Blanc remains constant throughout time. What could possibly account for such a contradiction? The height of Mont Blanc, for instance, along with all of the other altitudes in metropolitan France, is measured in relation to a reference point located on the Mediterranean, at the Marseille tide gauge, which defines the conventional zero altitude. This is done so that the height of Mont Blanc can be compared to other altitudes.

This instrument was created in the 19th century and consists of a float that is positioned in a well that looks out over the ocean. The float is placed on the surface of the water where it is shielded from the surge and waves and is attached to a device that keeps track of each of its vertical movements caused by changes in the level of the sea. The measurement of the average sea level that was taken in Marseille between the years 1885 and 1897 provides the basis for the zero height.

A lengthy time span provides the opportunity to break away from the influence of tidal and swell phenomena, the impacts of which, over the long term, cancel one another out and have essentially little effect on the mean sea level. A marking that has been hermetically sealed into the floor of the structure that houses the tidal gauge serves as the physical representation of the zero height. For all intents and purposes, it is situated precisely 5,450 feet (1.661 meters) above the zero point in historical measurement that was established in 1885. However, this standard is not applicable in every situation.

Recent studies of sea level made by the Hydrographic and Oceanographic Service of the French Navy reveal that the zero height designated by the Ajaccio tidal gauge is about 10 cm higher than that of Marseille. These observations were made just on the island of Corsica. In addition, every nation has its own datum, which may be a source of serious issues in situations when many nations are working together on international development initiatives, for instance.

The North American Vertical Datum of 1988 (NAVD 88) is a vertical reference system for North America that uses leveling points spread out throughout the continent from Alaska to Canada and the rest of the United States to provide a common vertical reference. It is referenced to a point in Quebec, Canada.


On a global scale, and particularly at sea, allusions to individual nations are meaningless because of the interconnectedness of the world’s oceans. The position that the sea would hold if it were standing still is the location that is used to define the zero point. This zero is the basis for a fictitious horizontal surface that is referred to as the “geoid.” Each point on this surface is perpendicular to the vertical direction, which corresponds to the direction of the gravitational field.

The gravitational field is the force that keeps us rooted to the surface of the earth and gives objects their weight. The gravitational field is not uniform over the surface of the Earth, which results in its surface having an uneven, potato-like shape. Using gravimeters located either on the ground or on satellites, scientists may calculate the contours of the geoid by determining how the Earth’s gravity field varies over a large number of locations.

However, the behavior of the seas is not nearly the same as that of a glass of water sitting on a table. As a result, they have a substantial angle of departure from the geoid. Because of shifts in water density, winds, and dominant currents like the Gulf Stream, as well as shifts in air pressure, the average level of the sea can shift by several feet relative to this point of reference. Therefore, the water level in the Mediterranean Sea is 6 inches (15 centimeters) lower than the water level in the Atlantic Ocean.

The geoid is modeled on a worldwide scale nowadays, and the precision of the model may vary from as little as 4 inches to as much as a few feet, depending on the location. But our GPS can’t handle the complexity of this modeling. Because of this, they employ a streamlined version that provides approximations of elevations determined in reference to a global zero. Carry out the examination in the mountainous region. On a peak, the height that is indicated by the GPS and the official altitude are not always the same. In many cases, there is a difference of several feet.

By Bertie Atkinson

Bertie Atkinson is a history writer at Malevus. He writes about diverse subjects in history, from ancient civilizations to world wars. In his free time, he enjoys reading, watching Netflix, and playing chess.