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en:help:1.4:geolocation [2014/11/05 23:22]
alpinequest
en:help:1.4:geolocation [2017/09/12 11:21] (current)
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 ===== Elevations management ===== ===== Elevations management =====
  
-The elevation of a point is the difference between the altitude of this point and a reference surface of altitude 0 (also called "sea level").+The elevation of a point is the difference between the altitude of this point and a reference surface of altitude 0 (usually the sea level).
  
 However, depending on what is considered to be the surface of reference, elevations can be quite different. This surface depends on the representation of the Earth, which has been shaped in many ways over the last centuries. However, depending on what is considered to be the surface of reference, elevations can be quite different. This surface depends on the representation of the Earth, which has been shaped in many ways over the last centuries.
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 The GPS, working worldwide, had to choose a system defining a shape that match the entire Earth. A particular oblate spheroid, called [[http://​en.wikipedia.org/​wiki/​World_Geodetic_System|WGS84]] has been chosen as shape, because it provides a good overall approximation while remaining simple. The GPS, working worldwide, had to choose a system defining a shape that match the entire Earth. A particular oblate spheroid, called [[http://​en.wikipedia.org/​wiki/​World_Geodetic_System|WGS84]] has been chosen as shape, because it provides a good overall approximation while remaining simple.
  
-Based on this choice, GPS's "​WGS84"​ elevations are a good approximation at the Earth scale, but compared to per-country systems elevations, we can have discrepancies up to 50m.+Based on this choice, GPS's "​WGS84"​ elevations are a good approximation at the Earth scale, but compared to per-country systems elevations, we can have discrepancies up to 100m.
  
-**To reduce these discrepancies and provide more precise elevations, the application convert all GPS elevations towards another system using a more precise shape of the Earth, ​called ​[[http://​en.wikipedia.org/​wiki/​Geoid|Geoid]]. However, some slight discrepancies can still be found.**+**To reduce these discrepancies and provide more precise elevations, the application convert all GPS elevations towards another system using a more precise shape of the Earth, ​the [[http://​en.wikipedia.org/​wiki/​Geoid|Geoid]], which is called EGM96. However, some slight discrepancies can still be found.**
  
-{{:​icon_help.png?​nolink}} You can find detailed ​article of the shape of the Earth on [[http://​en.wikipedia.org/​wiki/​Figure_of_the_Earth|Wikipedia]].+{{:​icon_help.png?​nolink}} You can find detailed ​articles on [[http://​en.wikipedia.org/​wiki/​Geoid|the Geoid]] and on [[http://​en.wikipedia.org/​wiki/​Figure_of_the_Earth|the shape of the Earth]] on Wikipedia. 
 + 
 +Here is the difference between GPS and Geoid altitudes all around the world (from Wikipedia):​ 
 +{{ :​en:​help:​1.4:​geolocation:​elevations-management.jpg?nolink }}

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