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What is the difference between ellipsoid height and mean sea level?
The GPS uses height (h) above the reference ellipsoid that approximates the earth’s surface. The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth’s gravity and approximated by MSL. MSL is defined as the zero elevation for a local area.
What is ellipsoid and geoid?
Ellipsoid height (h) is the difference between the ellipsoid and a point on the Earth’s surface. Geoid height (N) is the offset value between the reference geoid and the ellipsoid models. Orthometric height (H)—AKA the one you really care about— is the distance between a point on the Earth’s surface and the geoid.
Why do we need to calculate Orthometric Heights?
Such heights are called orthometric heights (H), and are the most useful in practice because they give the direction of the flow of water. The simplest mathematical figure which describes the geoid is the ellipsoid, defined by its semi-major axis (a) and flattening values.
What’s the difference between AGL and MSL?
Above Ground Level, or AGL, describes the literal height above the ground over which you’re flying. Mean Sea Level, or MSL, is your true altitude or elevation. Interestingly enough, the MSL calculation is derived from observations of tides and seasonal variations over a 19-year period to arrive at the average MSL.
Does altimeter read AGL or MSL?
As Scott said, AGL means Above Ground Level, while MSL is Mean Sea Level, meaning the altitude above sea level. Altimeters are generally referenced to sea level because the ground level keeps changing as you move.
How tall is the geoid below the ellipsoid?
The equation states that ellipsoid height equals height above mean sea level plus the ellipsoid/geoid separation. A typical value for N in the Alberta area is -17 meters, meaning that the geoid is 17 meters below the ellipsoid.
How tall is the geoid in Alberta Canada?
A typical value for N in the Alberta area is -17 meters, meaning that the geoid is 17 meters below the ellipsoid. Heights as reported by GPS receivers operating autonomously are ellipsoid heights referenced to the World Geodetic System 1984 (WGS84) ellipsoid.
Is the MSL a close approximation of the geoid?
The MSL surface is in a state of gravitational equilibrium. It can be regarded as extending under the continents and is a close approximation of the geoid. By definition, the geoid describes the irregular shape of the earth and is the true zero surface for measuring elevations.
Which is the reciprocal of flattening in the ellipsoid?
The value resulting from the equation, a/a-b is called the reciprocal of flattening, and is often used with the semi-major axis to define the ellipsoid. The ellipsoid is a relatively simple mathematical surface on which distance and bearing can be computed and position referenced.