Geodetic astronomy or astronomical geodesy (astro-geodesy) is the application of astronomical methods into geodetic networks and other technical projects of geodesy.
The most important applications are:
Establishment of geodetic datum systems (e.g. ED50) or at expeditions
apparent places of stars, and their proper motions
precise astronomical navigation
astro-geodetic geoid determination
modelling the rock densities of the topography and of geological layers in the subsurface
Monitoring of the Earth rotation and polar wandering
Contribution to the time system of physics and geosciences
Important measuring techniques are:
Latitude determination and longitude determination, by theodolites, tacheometers, astrolabes or zenith cameras
time and star positions by observation of star transits, e.g. by meridian circles (visual, photographic or CCD)
Azimuth determination
for the exact orientation of geodetic networks
for mutual transformations between terrestrial and space methods
for improved accuracy by means of "Laplace points" at special fixed points
Vertical deflection determination and their use
in geoid determination
in mathematical reduction of very precise networks
for geophysical and geological purposes (see above)
Modern spatial methods
VLBI with radio sources (quasars)
Astrometry of stars by scanning satellites like Hipparcos or the future Gaia.
The accuracy of these methods depends on the instrument and its spectral wavelength, the measuring or scanning method, the time amount (versus economy), the atmospheric situation, the stability of the surface resp. the satellite, on mechanical and temperature effects to the instrument, on the experience and skill of the observer, and on the accuracy of the physical-mathematical models.
Therefore, the accuracy reaches from 60" (navigation, ~1 mile) to 0,001" and better (a few cm; satellites, VLBI), e.g.:
angles (vertical deflections and azimuths) ±1" up to 0,1"
geoid determination & height systems ca.
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In geodesy and navigation, a meridian arc is the curve between two points on the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length. The purpose of measuring meridian arcs is to determine a figure of the Earth. One or more measurements of meridian arcs can be used to infer the shape of the reference ellipsoid that best approximates the geoid in the region of the measurements.
An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations. It is a spheroid (an ellipsoid of revolution) whose minor axis (shorter diameter), which connects the geographical North Pole and South Pole, is approximately aligned with the Earth's axis of rotation.
In geography and geodesy, a meridian is the locus connecting points of equal longitude, which is the angle (in degrees or other units) east or west of a given prime meridian (currently, the IERS Reference Meridian). In other words, it is a line of longitude. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. On a Mercator projection or on a Gall-Peters projection, each meridian is perpendicular to all circles of latitude.
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