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. Changing weather or atmospheric conditions near the observation site can negatively affect
atmospheric refraction in the
zenithal direction, referred to as
anomalous or
zenithal refraction; anomalous refraction is considered to be the primary source of error in geodetic astronomy deflection data. 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 cm •
astronomical 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)
Astrogeodetic leveling is a local
geoid determination method based on
vertical deflection measurements. Given a starting value at one point, determining the
geoid undulations for an area becomes a matter for simple
integration of vertical deflection, as it represents the horizontal
spatial gradient of the geoid undulation. == See also ==