Geodetic astronomy or astro-geodesy 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 expeditionsapparent places of stars, and their proper motionsprecise astronomical navigationastro-geodetic geoid determinationmodelling the rock densities of the topography and of geological layers in the subsurfaceSatellite geodesy using the background of starsMonitoring of the Earth rotation and polar wanderingContribution to the time system of physics and geosciencesImportant measuring techniques are:
Latitude and longitude determination by theodolites, tacheometers, astrolabes or zenith camerastime and star positions by observation of star transits, e.g. by meridian circles (visual, photographic or CCD)Azimuth measurementsfor the exact orientation of geodetic networksfor mutual transformations between terrestrial and space methodsfor improved accuracy by means of "Laplace points" at special fixed pointsVertical deflection measurements and their usein geoid determinationin mathematical reduction of very precise networksfor geophysical and geological purposes (see above)Modern spatial methodsVLBI 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. 5 cm up to 0,2 cmastronomical lat/long and star positions ±1" up to 0,01"HIPPARCOS star positions ±0,001"VLBI quasar positions and Earth's rotation poles 0,001 to 0,0001" (cm...mm)