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Geopotential height differs from geometric height (as given by a tape measure) because Earth's gravity is not constant, varying markedly with altitude and latitude; thus, a 1-m geopotential height difference implies a different vertical distance in physical space: "the unit-mass must be lifted higher at the equator than at the pole, if the same ...
For example, at a radius of 6600 km (about 200 km above Earth's surface) J 3 /(J 2 r) is about 0.002; i.e., the correction to the "J 2 force" from the "J 3 term" is in the order of 2 permille. The negative value of J 3 implies that for a point mass in Earth's equatorial plane the gravitational force is tilted slightly towards the south due to ...
Geopotential is the potential of the Earth's gravity field. For convenience it is often defined as the negative of the potential energy per unit mass , so that the gravity vector is obtained as the gradient of the geopotential, without the negation.
The equation that relates the two altitudes are (where z is the geometric altitude, h is the geopotential altitude, and r 0 = 6,356,766 m in this model): = Note that the Lapse Rates cited in the table are given as °C per kilometer of geopotential altitude, not geometric altitude.
The term elevation is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and depth is used for points below the surface. Elevation histogram of the Earth's surface
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Travelling one metre in the direction of a gravity vector of strength 1 m·s −2 will increase your potential by 1 m 2 ·s −2. Again employing G as a multiplier, the units can be changed to joules per kilogram of attracted mass. A more convenient unit is the GPU, or geopotential unit: it equals 10 m 2 ·s −2.
It provides a raster of 2.5′×2.5′ and an accuracy approaching 10 cm. 1'×1' is also available [7] in non-float but lossless PGM, [5] [8] but original .gsb files are better. [9] Indeed, some libraries like GeographicLib use uncompressed PGM, but it is not original float data as was present in .gsb format.