Editing Vertical deflection (section)

==Application==
Vertical deflections are principally used in four matters:
# For precise calculation of '''survey networks'''. The geodetic [[theodolite]]s and levelling [[measuring instrument|instrument]]s are oriented with respect to the true [[vertical direction|vertical]], but its [[Deflection (physics)|deflection]] exceeds the geodetic measuring accuracy by a factor of 5 to 50. Therefore, the data would have to be corrected exactly with respect to the global ellipsoid. Without these reductions, the surveys may be [[distortion|distorted]] by some centimeters or even decimeters per km.
# For the '''geoid determination''' (mean sea level) and for exact transformation of [[elevation]]s. The global geoidal [[Undulation of the geoid|undulation]]s amount to 50–100&nbsp;m, and their [[region]]al values to 10–50&nbsp;m. They are adequate to the [[integral]]s of VD components ξ,η and therefore can be calculated with cm accuracy over distances of many kilometers.
# For '''[[GPS]] surveys'''. The [[satellite]]s measurements refer to a pure [[geometry|geometrical]] system (usually the [[WGS84]] ellipsoid), whereas the terrestrial heights refer to the geoid. We need accurate geoid data to combine the different types of measurements.
# For '''[[geophysics]]'''. Because VD data are affected by the physical structure of the Earth's [[Crust (geology)|crust]] and mantle, [[geodesist]]s are engaged in [[model (abstract)|models]] to improve our knowledge of the Earth's interior. Additionally and similar to ''applied geophysics'', the VD data can support the future [[exploration]] of raw materials, [[Petroleum|oil]], gas or [[ore]]s.