Geodetic astronomy

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Geodetic astronomy is the application of astronomical methods into networks and technical projects of geodesy.

The most important topics 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 and
modelling the rock densities of the topography and of geological layers in the subsurface
Satellite geodesy using the background of stars (see also astrometry and cosmic triangulation)
Monitoring of the Earth rotation and polar wandering
Contribution to the time system of physics and geosciences

Important measuring techniques are:

Latitude 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 measurements
- 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 measurements 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.: