Physical geodesyPhysical geodesy is the study of the physical properties of Earth's gravity and its potential field (the geopotential), with a view to their application in geodesy. Traditional geodetic instruments such as theodolites rely on the gravity field for orienting their vertical axis along the local plumb line or local vertical direction with the aid of a spirit level. After that, vertical angles (zenith angles or, alternatively, elevation angles) are obtained with respect to this local vertical, and horizontal angles in the plane of the local horizon, perpendicular to the vertical.
Référent altimétriqueUn référent altimétrique est, en géodésie, un point dont l'altimétrie est fixée et qui sert à calculer celle des autres points. Dans un système géodésique, l'altimétrie d'un référent altimétrique est généralement un niveau moyen de la mer observé et dans le cas de certains pays, le niveau d'un grand lac. Le niveau moyen des mers et océans du globe n'étant pas le même en fonction des secteurs où il est mesuré et de la période, des différences sont observées entre les référents altimétriques employés par les différents pays.
Height above ground levelIn aviation, atmospheric sciences and broadcasting, a height above ground level (AGL or HAGL) is a height measured with respect to the underlying ground surface. This is as opposed to height above mean sea level (AMSL or HAMSL), height above ellipsoid (HAE, as reported by a GPS receiver), or height above average terrain (AAT or HAAT, in broadcast engineering). In other words, these expressions (AGL, AMSL, HAE, AAT) indicate where the "zero level" or "reference altitude" – the vertical datum – is located.
HauteurLa hauteur mesure l'étendue de quelque chose selon la verticale. La hauteur prend néanmoins diverses significations suivant le domaine où elle est utilisée. En métrologie (science des mesures), la hauteur est la distance verticale entre un point (ou un objet assimilé à un point) et un niveau de référence spécifié. Par extension, c'est aussi la dimension d'un objet, prise dans la direction verticale. Voir Système international d'unités.
Dynamic heightDynamic height is a way of specifying the vertical position of a point above a vertical datum; it is an alternative for orthometric height or normal height. It can be computed by dividing the location's geopotential number by the normal gravity at 45 degree latitude (a constant). Dynamic height is constant if one follows the same gravity potential as one moves from place to place. Because of variations in gravity, surfaces having a constant difference in dynamic height may be closer or further apart in various places.
Height above mean sea levelHeight above mean sea level is a measure of the vertical distance (height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum. In geodesy, it is formalized as orthometric heights. The quantity is called "metres above mean sea level" in the metric system, while in United States customary and imperial units it would be called "feet above mean sea level". Mean sea levels are affected by climate change and other factors and change over time.
Orthometric heightThe orthometric height is the vertical distance H along the plumb line from a point of interest to a reference surface known as the geoid, the vertical datum that approximates mean sea level. Orthometric height is one of the scientific formalizations of a laypersons' "height above sea level", along with other types of heights in Geodesy. In the US, the current NAVD88 datum is tied to a defined elevation at one point rather than to any location's exact mean sea level.
Nivellement (topographie)Le nivellement ou nivèlement en topographie est l'ensemble des opérations consistant à mesurer des différences de niveau (ou dénivelés ou dénivellation), généralement pour déterminer des altitudes. En d'autres termes, le nivellement permet de mesurer des dénivelés puis de déduire l'altitude de repères ou de points caractéristiques du sol ou d'ouvrages.
Geopotential heightGeopotential height or geopotential altitude is a vertical coordinate referenced to Earth's mean sea level (assumed zero potential) that represents the work done by lifting one unit mass one unit distance through a region in which the acceleration of gravity is uniformly 9.80665 m/s2. Geopotential height (altitude) differs from geometric (tapeline) height but remains a historical convention in aeronautics as the altitude used for calibration of aircraft barometric altimeters.
Normal heightNormal heights is a type of height above sea level introduced by Mikhail Molodenskii. The normal height (or ) of a point is computed as the ratio of a point's geopotential number (i.e. its geopotential difference with that of sea level), by the average, normal gravity computed along the plumb line of the point. (More precisely, along the ellipsoidal normal, averaging over the height range from 0 — on the reference ellipsoid — to ; the procedure is thus recursive.) Normal heights are thus dependent upon the reference ellipsoid chosen.
