Topic summary
Geomagnetic
Extracted from the Wikipedia article Earth's magnetic field.
Measurement
Geomagnetic observatories provide continuous records of the magnetic field at particular points on land. The instruments are of two kinds. Variometers record the relative variations of the field in arbitrary units such as electrical voltage. These can operate with minimal human intervention for do not provide absolute magnitudes. The other kind, usually a combination of fluxgate theodolite and proton precession magnetometer, make absolute measurements and can be used to adjust the variometer data.