altitude|altitude The altitude is the vertical rise in a place or an object compared to a basic level.
Altitude is also a size which expresses a difference between a given point and a mean level; on Ground this level is generally the Sea level (or “level zero”). The Sommet S are associated at an altitude, calculated by various indirect means (Géodésie, Triangulation). Altitude is also a data useful Exogène for numerical calculation in various fields: Physical Meteorology, , Biology.
Effects of altitude
Certain physical fields vary according to altitude: reduction in the Atmospheric pressure variation of the Temperature, the solar radiations and the Hygroscopy in particular. One notes also particular reactions of the organizations vis-a-vis these new conditions, particularly at the plants, but also in the animals.
Rather than of effects of altitude, it would be advisable to speak about variations related to rise, because altitude is a raw data which does not have any consequence by itself. It is necessary to distinguish two types of effects:
- brutal and specific changes due to fast rise;
- adaptations of means and long terms of the organizations in altitude.
The first effects are spectacular and well-known Alpiniste S; the seconds are percussion and assign the men as well as the ecosystem in which they live or travel.
Physical variations of fields
It decreases with the altitude in manner Exponentielle. With the sea level it is worth 1 atmosphere (either 760 MmHg or 1013,25 mb) whereas with 1 000 m it are worth nothing any more but 674 mmHg, with 4 800 m 416 mmHg and with 8 848 m 236 mmHg.
Adaptation of the body to altitude
Short-term answer (a few days)
At the man the effects of altitude are mainly due to the partial pressure decrease in Oxygène in the air inspired, and to the fall of Température. Indeed, at rest, these elements cause in the short run a Hyperventilation, increase in ventilation, a Tachycardie, increase in the Heart rate, as well as a Diurèse (elimination of part of plasmatic volume (Blood plasma) intended to increase the proportion of red Globule in the Sang).
Long-term answers (several weeks)
Later on (as from approximately 3 weeks), one observes a big raise of the number of red Globules (Hématocrite) allowing a transport of the oxygen increased in the Sang. This is the consequence of a peak of EPO in the first days of exposure to the Hypoxie of altitude. The Vo2 max (maximum oxygen consumption) drops according to altitude, thus, with 0 meter (with the sea level), the man is to 100% of these possibilities, whereas with 4 810 meters (top of the white MT) it can have about it only 70% and only 20% with 8 848 meters (top of the Everest).
The effect “increase of the quantity of red Globules” is particularly required by certain sportsmen, it is the imperative reason of the organization of training course in altitude, sometimes with more 3 000 meters; however this Polyglobulie can involve, in certain cases an excess of red Globules, the formation of blood clots can then block the veins and cause a major venous Thrombose (phlebitis) which can result in death. The concentration in red Globules (Hématocrite) of the blood of the people living with high-altitude (Bolivia, Peru, Tibet) is naturally higher.
Can one live with 4.000 meters of altitude?An example: the inhabitants of Potosi in Andean Bolivia are perched to 4.040 Mr. They are completely accustomed to these conditions: their blood is richer in red globules which convey oxygen to the bodies. On the other hand, for the visitors, it is a little more complicated. The air and rarefying oxygen, their physical capacity is reduced from 30 to 40% in spite of cardiorespiratory acceleration. It takes approximately two weeks of adaptation. Meanwhile, the visitor can suffer from the " badly acute of the montagnes" : headaches, nauseas, oedemas,…
Calculation of altitude
The Calcul of an altitude always amounts measuring a difference between a starting level and the point in which one wishes to find rise compared to this level (Dénivelé). This calculation is generally indirect, i.e. it is carried out without measurements on the ground. The Géomètre S which measure distances carry out contrary to direct calculations, but these methods are difficult to implement for variations of several hundred meters or several kilometers, which more is in the vertical direction and with a chaotic Relief.
Impossibility of defining altitude correctly
Contrary to the good sense, it is very difficult to define the concept of altitude.
First of all it would be necessary to define what is the basic level.
It was of use to regard as basic level the mean level of the seas, which is a surface difficult to put in equation; it is a surface which moves according to astronomical elements like the the Moon or the Planet S (phenomenon of Marée), which is not an equipotential surface (because inter alia currents and variation of salinity), therefore is not comparable to the terrestrial Geoid, and which in any event does not exist with the vertical of a given terrestrial place.
The old method, which consists in walking on between the mean level of the sea and a place given by measuring each time the difference in level dh, is mathematically problematic, because the result depends on the followed way, in other words ∫ dh is not a perfect integral. On the other hand, energy to spend to go from a point to another, which is ∫ G dh, G being gravity in each point, does not depend on the followed way. Altitude is then calculated by measuring G regularly, and by dividing the value obtained by an average g0, the choice of this g0 conditioning of course the result.
The arrival of GPS allowed another definition of altitude: the distance from the center point of gravity of the whole of the satellites, which is about the center of gravity of the ground. The GPS gives then a triplet XYZ in a orthonormé reference mark, and it is necessary to project this point in a cartographic system to obtain altitude, the result depending of course on selected projection.
the calculation height of the Grande pyramid of Gizeh by Ératosthène is already a kind of calculation of altitude, between the top of the monument and the ground. The method used, an application of the famous Theorem of Thalès, was taken again for calculation of altitude of released tops. The associated margin of error is rather important.
- a method used on the ground and which does not require a tool utilized the aiming by approximation. In mountainous region, one can estimate that an adult fir tree is high of thirty meter on average. By superposition, one can estimate an altitude or a variation with an average margin of error, often acceptable.
- By using a Geographical map and a Compass graduated, the method of the Triangulation makes it possible to know the altitude of a close point.
- the Altimètre is an instrument which measures altitude while being based on the relation between rise and the atmospheric pressure. This relation is not linear, and sudden considerable variations due to the evolution of the masses of air during the catch of altitude by the walker who uses the altimeter. It is thus a means of measurement less reliable than it appears to with it: it is necessary to take care as often as to calibrate possible the altimeter at the points whose altitude is known. The altimeters are used in the Ballon-sonde S.
Measurements of altitude by the modern instruments are of a precision quite higher than what it is possible to make with the eye or the compass. The satellite S are made profitable to calculate and update the “heights” of the points of planet, tops or not. To the difference in the terrestrial methods which use a dynamic reference frame taking account of the local variations of the field of gravity (geoid) and give by là-même true “altitudes”, the satellites provide a height starting from a Ellipsoïde of reference (IAG GRS80). The differences between Geoid and ellipsoid are variable according to the place and can reach the hundred meters. Models geoid can however be integrated in a calculation programme which then makes it possible to find altitudes starting from satellite measurements. The precision depends then mainly on the smoothness of the model.
Altitudes in the international System
In the international System, altitudes are expressed compared to the mean level of the sea in Mètre S, level zero being fixed by a laboratory.
In FranceFor the France (altitude of the charts IGN), it is located at Marseilles, and was given thanks to 12 years of recording of maregraphy between 1885 and 1897.
In SwitzerlandFor the Swiss , the reference mark of the Pierre of Niton (in the roads of Geneva) is used for all altitudes and was given to 373,6 m above the mean level of the sea (measurements of the marigraph of Marseilles).
In Germany (FRG)
In East Germany (GDR) since 1958 until the reunification
For East Germany its designation was HN (Höhennormal) and its dimension of reference was established on a river scale of Kronstädt which was lower of 14 cm. However to East Berlin like on the rail network of GDR and for interior navigable water the dimensions of reference of FRG were always applied.
As SloveniaFor Slovenia the point reference is also in Trieste, but it is defined in 1900.
In CanadaThe Canada uses the height of the mean level of the oceans bordering the country like altimetric datum-line, so that altitude is simply a height above this mean level of the sea.
On Mars, in the absence of ocean, the origin of altitudes was fixed arbitrarily: it is altitude having an average atmospheric pressure of 615 pascals. This pressure was selected because it corresponds to the pressure of the Point triples water (273,16 K and 615 Pa), and that the level thus defined is close to the mean level of Martian surface.
- Altitude of all the cities and villages of the world
- Calculation of altitude according to the latitude and of longitude
- didactic Altitude.org
- Card of Ign on the calculation of altitudes
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