What does "sea level" mean? Why countries have different “height from sea level”

sea ​​level

the position of the free surface of the World Approx., tending to be located perpendicular to the resultant of all forces applied to the mass of water. Changes in the position of surface levels are manifested in fluctuations in sea level.

Sea level

Sea level, the position of the free surface of the water of the seas and oceans, measured along a plumb line relative to the conventional reference point. There are “instantaneous”, tidal, average daily, average monthly, average annual, and average long-term sea levels. Under the influence of wind waves, tides, heating and cooling of the sea surface, fluctuations in atmospheric pressure, precipitation and evaporation, and river and glacial runoff, sea levels change continuously. The long-term mean sea level does not depend on these oscillations of the sea surface. The position of the long-term mean sea level is determined by the distribution of gravity and the spatial unevenness of hydrometeorological characteristics (water density, atmospheric pressure, etc.). The average long-term sea level, constant at each point, is taken as the initial level, from which the heights on land are measured. For reading the depths of seas with low tides, this level is taken as the zero depth - the water level mark, from which the depths are measured in accordance with the requirements of navigation. In the USSR, the absolute heights of points earth's surface are counted from the average long-term level of the Baltic Sea, determined from zero footstock in Kronstadt.

Lit .: Duvanin A.I., Sea level, L., 1956; Duvanin A. I., Kalinin G. P., Klige R. K., On long-term fluctuations in the level of the oceans, some seas and lakes, “Bulletin of Moscow State University. Series 5. Geography, 1975, ╧ 6.

Wikipedia

Sea level

Sea level- the position of the free surface of the World Ocean, measured along a plumb line relative to some conventional reference point. This position is determined by the law of gravity, the moment of rotation of the Earth, temperature, tides and other factors. There are "instantaneous", tidal, average daily, average monthly, average annual and average long-term sea levels.

Under the influence of wind waves, tides, heating and cooling of the sea surface, fluctuations in atmospheric pressure, precipitation and evaporation, river and glacial runoff, the sea level is constantly changing. The mean long-term sea level does not depend on these fluctuations of the sea surface. The position of the mean long-term sea level is determined by the distribution of gravity and the spatial unevenness of hydrometeorological characteristics.

The average long-term sea level constant at each point is taken as the initial level, from which heights on land are measured. To measure the depths of seas with low tides, this level is taken as the zero depth - the water level mark, from which the depths are measured in accordance with the requirements of navigation. In Russia and most other countries former USSR, as well as in Poland, the absolute heights of points on the earth's surface are counted from the average annual level of the Baltic Sea, determined from the zero footstock in Kronstadt. Depths and heights in Western European countries are calculated using the Amsterdam footstock (the level of the Mediterranean Sea is measured using the Marseille footstock).

A tide gauge is used to measure and record sea level fluctuations.

Since there are many factors that affect global weather changes (for example, Global warming), predictions and estimates of ocean level changes in the near future are not particularly accurate.

Sea level, the position of the free surface of the water of the seas and oceans, measured along a plumb line relative to the conventional reference point. There are “instantaneous”, tidal, average daily, average monthly, average annual, and average long-term sea levels. Under the influence of wind waves, tides, heating and cooling of the sea surface, fluctuations in atmospheric pressure, precipitation and evaporation, and river and glacial runoff, sea levels change continuously. The long-term mean sea level does not depend on these oscillations of the sea surface. The position of the long-term mean sea level is determined by the distribution of gravity and the spatial unevenness of hydrometeorological characteristics (water density, atmospheric pressure, etc.). The average long-term sea level, constant at each point, is taken as the initial level, from which the heights on land are measured. To measure the depths of seas with low tides, this level is taken as the zero depth - the water level mark, from which the depths are measured in accordance with the requirements of navigation. In the USSR, the absolute heights of points on the earth's surface are counted from the long-term average level of the Baltic Sea, determined from the zero of the footstock in Kronstadt.

Lit.: Duvanin A.I., Sea level, L., 1956; Duvanin A. I., Kalinin G. P., Klige R. K., On long-term fluctuations in the level of the oceans, some seas and lakes, “Bulletin of Moscow State University. Series 5. Geography, 1975, No. 6.

It was believed that the rise in sea level in the last 20 years has slowed down. However, a new study by surveyors from the University of Tasmania has shown that these calculations are incorrect. A group of scientists led by Christopher Watson published figures in the journal Nature Climate Change, according to which there is really no slowdown - sea levels are rising much faster than previously thought. Previous studies have shown that sea levels have risen by about 3.2 mm per year since 1993, but this figure may actually be an underestimate due to imperfect estimation methods, scientists say.

This means that many geographic locations will be at risk of flooding. "Russian Planet" ranked the most significant places, which can go under water already in this century. Among them are the most beautiful islands, as well as big cities: alas, our civilization has developed largely thanks to navigation, and many luxurious megacities are located precisely on the shores of the oceans and seas, at a low altitude above sea level.

1. Maldives

Located in the warm waters of the Indian Ocean, the islands, which many travelers perceive as an earthly paradise, have an average height above sea level of 1.5 meters. This means that by about 2100 their population, who lives mainly on the coast, will be forced to leave their homes. The islands are of coral origin, and the depth of the ocean around them is not high: when the atolls disappear under water, they will be visible from the air as vast shoals. Together with the archipelago, its exceptionally rich fauna will also perish.

2. Bangladesh

In this country, floods serve as a political factor: they are usually followed by famine (since water destroys not only people, but also crops), and this leads to crises of confidence in government. This was the case in 1974, when a catastrophic flood that killed 2,000 people and injured 1 million and made several million more homeless forced the government to impose martial law. The largest flood happened relatively recently - in September 1998, due to the flood of the Ganges, Brahmaputra and Meghna, about 300 thousand houses were flooded. Due to rising sea levels, researchers predict the growth of rivers and flooding of the coasts of Bangladesh. By the end of the century, half of the country will be at the bottom of a "sea" only one meter deep. Of course, it will be more likely not the sea, but a swamp covered with thickets of trees and shrubs.

Flooding in Bangladesh. Photo: Abir Abdullah / EPA / TASS

3. New Orleans

The birthplace of jazz is below sea level: in different quarters of the city, the figure varies from 1.5 to 3 meters. The city is saved by dams erected after the Flood Control Act was passed in 1965. However, the dams were not as reliable protection as the engineers who built them thought. On August 29, 2005, Hurricane Katrina caused water to breach one of them while waves were able to move over a number of others. As a result, more than 80% of the city area was under water. If the sea level rises, in a few decades the city will become an easy victim for hurricanes, which can be even 1.5-2 times weaker than Katrina.

4. New York

Like New Orleans, this American metropolis has suffered from hurricanes more than once in its history: in 2012, Hurricane Sandy claimed the lives of 185 local residents. Some parts of the city went under water. Eight months after the disaster, Mayor Michael Bloomberg unveiled a $19.5 billion plan to protect New York from natural disasters, one of the main elements of which would be to strengthen the coastlines. If a large-scale project is not implemented, by 2080 Long Island and the lower part of the Hudson River Valley may be under water.

5. Bangkok

Bangkok is half empty or half full? According to the most pessimistic expert estimates, part of the Thai capital will go under water already in the 2030s: in addition to rising ocean levels, coastal erosion is working on the death of the city. A giant dam along the Gulf of Thailand could save the city. But if in the case of New York or New Orleans there is hope for the construction of such giant dams, then the specifics of the political situation in the country will most likely not allow a national project to save the city to be carried out. Now more than a million houses, 90% of which are residential, are at risk, most likely, over time, their first floors will periodically flood.

Effects of heavy rains in Thailand. Photo: Narong Sangnak / EPA / TASS

Another beautiful American city, famous for its beaches, may become a victim of the waves of the Atlantic. The foundation of the city is formed by limestone rocks that pass water well, which means that the construction of dams, which could help New York, will be completely useless here. It is possible that by the end of the century the city will have to move the inhabitants of some neighborhoods to more high places: the metropolis, as it were, will “creep away” farther and higher from the ocean line.

7. Venice

One of the most romantic cities in the world will disappear into the waves if the level of the world's oceans rises "even" by one meter. In 2008, the city has already suffered from powerful floods: damage to buildings amounted to tens of millions of dollars. As noted earlier by the experts of the World Fund wildlife Italy as a whole could lose many of its beaches, resorts and unique coastal flora and fauna. About 60% of Italians live near the sea coasts, and coastal erosion over the past quarter century has led to the disappearance of 42% of the country's beaches.

8. Amsterdam

This city has already been conquered from nature: a third of it is an artificial embankment, which was built over several centuries. The very name of the city, by the way, reflects this fact - it is translated as "Amstel dam", "dam on the river Amstel". Perhaps it is time to pay back to nature: if the water rises by two meters, the city will almost completely go under water. This may happen by the middle of the next century, but even in this century a large part of the city may be at the mercy of the waves.

9. Shanghai

The name of this city literally means "on the sea", and such a location is fraught with a certain danger. The average height of the city above sea level is quite high - 6.5 meters, but there are "lowlands" on its territory, and the number of citizens living in them would be an honor for a large European city - about 5.5 million people. Complicating the situation is the fact that there is almost no flood protection system in the city: the authorities have only recently begun to think about the need for it. In general, Shanghai is far from the only city in China that is threatened by rising sea levels: vast areas of cities such as Guangzhou or Hong Kong are located below sea level.

10. Hamburg

Due to climate change, the city, which is the second largest in Germany, has had to deal with storm surges, rising sea levels and extreme heat in the past few years. Although flood protection systems have a long tradition here - the first dams were built in the Middle Ages, their development may not keep pace with rising sea levels. Now the low places of the city, adjacent to the Elbe, are protected by a continuous dam 14.5 m high on both sides of the river. If the climate on the planet continues to change at the same rate as now, this powerful dam may no longer serve as a reliable cover for the city.

Flooding in Hamburg. Photo: Kay Nietfeld / DPA / TASS

From Wikipedia, the free encyclopedia

Height above sea level, absolute height , absolute mark or altitude(lat. Altitudo- height (points of the earth's surface above ocean level)) - one coordinate in three-dimensional geospace (the other two are latitude and longitude), showing at what level, relative to the sea level taken as zero, this or that object is located.

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Notes

Links

• // Encyclopedic Dictionary of Brockhaus and Efron: in 86 volumes (82 volumes and 4 additional). - St. Petersburg. , 1890-1907.
• - article from the Great Soviet Encyclopedia (3rd edition)

An excerpt characterizing Altitude

ocean surface level is the free water surface of the oceans and seas, close to the geoid shape (Fig. 1).

Rice. 1. Ocean surface level

The initial level of the ocean is the standard from which the absolute height of the land surface and the depth of the seas is measured. In our country, this is the average long-term level of the Baltic Sea near the city of Kronstadt (Baltic height system).

Level fluctuations can be periodic - These are daily fluctuations due to tides and non-periodic- arising from tropical cyclones, tsunamis, etc.

Periods of fluctuations in the level of the World Ocean are short(high-low tide after 6 hours 12.5 minutes) and lengthy or century old(hundreds of years) (Fig. 2).

Rice. 2. Ocean level fluctuations over the past 200 thousand years

Secular changes occur for a number of reasons, such as changes in the volume of water in the ocean or changes in ocean capacity. The first of them occurred during glaciations, when a huge mass of water in the form of ice was conserved on land, and the ocean level dropped by 100-200 m. During the interglacial period, when water entered the ocean as a result of ice melting, the ocean level rose by 20-30 m. According to calculations, as a result of climate warming on Earth, a further increase in the level of the World Ocean by about 30 cm is possible by the middle of the 21st century. The second type of secular fluctuations in the level of the World Ocean is caused by tectonic disturbances of the ocean floor, which entails a change in the volume of the ocean's capacity.