Gravity ground network
A regional or national network of fixed points on which the strength of the earth's gravitational field was determined with high accuracy is referred to as a gravity network .
The measured values belong in any case
- the local acceleration due to gravity , which in Central Europe varies between about 980 and 981 Gal (cm / s²) and is determined to a hundredth to a thousandth of a milligal ( i.e. at least 10 −8 of its value),
- the sea level (to at least mm accuracy) and
- usually also the vertical gravity gradient .
- A reduction in the severity values are only for the earth tides of the (up to 0.3 milligal) and any instrumental error gravimeter (zero errors and drift).
The measuring points of the gravity network are about 30–50 km apart in industrialized countries ; often they are also more closely spaced along the lines of the state precision leveling system . Your markings must be extremely stable, i.e. H. either on solid rock or in the basement of very stable or very old buildings (e.g. historical monasteries ). Even small changes in height (e.g. due to subsidence of the structure) would already result in small systematic errors in the system. In order to exclude local geological effects, the gravity and height control points are "insured" by additional measurement marks or stabilization in the immediate vicinity.
The network of these focal points is part of the basic survey of a country and serves as a reference system for all measurement campaigns with which the detailed points of the gravimetric land survey (regional gravimetry ) or the geoid determination are later obtained.
The detailed measurements are usually carried out in loops lasting several hours, which before and afterwards connect to one or two points in the gravity network. By comparing measurement and reference data, the current instrumental equation of the field gravimeter and its current accuracy can be determined - a kind of local calibration . Since the field measurement takes place at the same point as in the gravity network, there is no reduction in the terrain or topography .