Vertical fluctuation

The vertical direction of a place is not fixed relative to the earth's body , but fluctuates slightly due to some effects:

1. Tides through the sun and moon
2. Mass shifts near the earth's surface ( snow , groundwater , earthquakes )
3. Ground movements ( warming , air pressure , etc.)
4. Geodynamics ( plate tectonics , mass displacements in the earth's mantle ...)

The largest are the tidal effects, which are around 0.02 ″. They allow conclusions to be drawn about the elasticity of the earth and, like the influences (2-3.), Must be taken into account before small inclinations due to (4.) emerge from the measurement data. The latter, however, are more likely to be determined with other, indirect means ( GPS , satellite geodesy , VLBI ).

Since the direct measurement of the vertical direction - with methods of astrogeodesy - is limited to an accuracy of approximately ± 0.1 ″, various special instruments have been developed to determine the fluctuations:

• Horizontal pendulum - precision instruments of various types, which are mostly set up in abandoned mines and swing around an almost vertical axis of rotation
• Vertical pendulum , borehole tidal pendulum - pendulums a few meters long whose fluctuations are measured to a hundredth of a micrometer and more precisely
• Partly also measurement of severity gradients .