From the lessons of geography, everyone is familiar with the phrase “sea level”. Which sea serves as a reference point and what is the absolute height?
What is “sea level”?
This is the position that the free surface of the oceans occupies. Measure it using a plumb line in relation to a specific conditional reference point. Sea level depends on various factors:
- gravity force;
- temperature;
- tides;
- fluctuations in atmospheric pressure;
- Earth's rotation moment, etc.
Given all the above factors, sea level is constantly changing. Specialists distinguish several levels, for example, tidal, average annual, daily average, etc.
Thanks to constant observations and research, scientists calculate the mean annual sea level and use it as that conditional reference point. The heights on land are counted from this level, and the note “above sea level” or “below sea level” appears on the maps.
In this way, height above sea level - this is the difference between the origin and a certain point on the earth's surface. The initial level is taken as zero. There are several height systems, for example, dynamic, orthometric, normal.
Why is the sea level different?
Planet Earth has a not perfectly even shape and surface - there are several kilometer highs, and similar troughs. Scientists call this shape a flattened ellipsoid.Therefore, it is not surprising that different levels of the ocean are observed in different parts of our planet.
Interesting fact: the highest point on the planet is Mount Jomolungma (Himalayas, 8848 m). The lowest point on land is the coast of the Dead Sea (the border between Israel and Jordan, 430 m below sea level). The deepest point on the planet's surface is the Challenger Abyss (Mariana Trench, Pacific Ocean, 10,994 m below sea level).
Despite the plasticity of water, the ability to fill all kinds of recesses, in different parts of the globe it still does not fill them completely. Because of this, the level of the oceans is different everywhere, as well as the level of individual seas. For example, the Mediterranean Sea is 15 cm lower than ocean level.
How to determine the absolute height in different countries?
Russia and many other countries that used to be part of the USSR use the average long-term level of the Baltic Sea as a reference point. It is determined using a special device - a foot rod located in the city of Kronstadt. Measurements are carried out within the framework of the Baltic system of normal heights.
The United States and Canada are guided by the water surface in St. Lawrence Bay near the town of Rimuski. In the UK, sea level is Newlin Harbor in the southwestern part of England. In Northern Ireland, there is a water surface near Belfast. In Sweden, Denmark, the starting point is the sea near Amsterdam. China counts from the water surface near the city of Qingdao.
France uses, as a zero point, the averages of the Marseille tide gauge, taken from 1885 to 1897. Since then, the water level has risen 16 cm, but the data has not been updated.
Interesting fact: since each country has its own idea of sea level, different data on the height of certain objects are indicated on the maps.
This feature of using different systems of altitudes and sea levels should cause many difficulties. For example, how is the design and construction of large facilities carried out if countries focus on different indicators?
In such cases, before starting work, they come to some kind of agreement. All parties choose a specific altitude system and sea level, in accordance with which the calculations are carried out.
There is an idea that it would be easier for the whole world to choose a single system for measuring absolute height. Then there would be no discrepancies in the maps and other areas where this information is needed.
It is not so difficult to find a compromise, how to remake a huge number of cards, as well as change information in documents. Yes, and the surface of the planet does not make it possible to calculate the average absolute height - the differences are too great.
Many natural factors cause uneven water levels in the oceans. There is no single sea in the world for counting absolute heights. Each country adheres to a specific system and takes measurements, focusing on the most convenient body of water. Because of this, the designation of heights and troughs on maps may differ. In Russia and the countries of the former USSR, the long-term average level of the Baltic Sea is taken as a zero indicator.
