Geodesy - Geodetic Observational Concepts

Geodetic Observational Concepts

Here we define some basic observational concepts, like angles and coordinates, defined in geodesy (and astronomy as well), mostly from the viewpoint of the local observer.

• The plumbline or vertical is the direction of local gravity, or the line that results by following it. It is slightly curved.
• The zenith is the point on the celestial sphere where the direction of the gravity vector in a point, extended upwards, intersects it. More correct is to call it a rather than a point.
• The nadir is the opposite point (or rather, direction), where the direction of gravity extended downward intersects the (invisible) celestial sphere.
• The celestial horizon is a plane perpendicular to a point's gravity vector.
• Azimuth is the direction angle within the plane of the horizon, typically counted clockwise from the North (in geodesy and astronomy) or South (in France).
• Elevation is the angular height of an object above the horizon, Alternatively zenith distance, being equal to 90 degrees minus elevation.
• Local topocentric coordinates are azimuth (direction angle within the plane of the horizon) and elevation angle (or zenith angle) and distance.
• The North celestial pole is the extension of the Earth's (precessing and nutating) instantaneous spin axis extended Northward to intersect the celestial sphere. (Similarly for the South celestial pole.)
• The celestial equator is the intersection of the (instantaneous) Earth equatorial plane with the celestial sphere.
• A meridian plane is any plane perpendicular to the celestial equator and containing the celestial poles.
• The local meridian is the plane containing the direction to the zenith and the direction to the celestial pole.