.. _abs_mes_deffreq:

Definition and Frequency
========================

Absolute control at a magnetic observatory requires periodic
measurements of the absolute value of the geomagnetic field. At
present, this requires manually operated instruments; a single
observation takes several minutes.

Absolute control is required in order to correct for the
long-term instability in the variometer installation, and
thereby enabling the determination of very long-term changes in
the magnetic field. The frequency with which these measurements
should be made depends on several factors: the intrinsic and
thermal stability of the variometer electronics, sensor, and
piers; the method of temperature stabilization and monitoring;
the accessibility of the observatory; availability of staff;
and other considerations.

The choice of period between measurements should be influenced
by:

-  the change in baseline values from measurement to
   measurement,
-  the consistency between primary and alternative variometers
   (including intermediate Reference Measurements),
-  the consistency between total field derived from the vector
   variometer Fv, and those measured using a recording absolute
   scalar magnetometer Fs, and
-  known disruptive events.

INTERMAGNET recommends a maximum period of one week between
absolute measurements.

Observations should be obtained for different levels of
magnetic activity and for different magnetic field values.