Introduction to the geomagnetic survey of Switzerland

The last complete geomagnetic survey of Switzerland was carried out from 1974 to 1978 over a total of 448 sites. The country being rather small it is possible to represent the main field at a particular epoch in terms of a linear function of coordinates. Especially appropriate for that purpose are the kilometric coordinates of the topographic network. The remaining anomalies can then be mapped separately. Since the secular variation of the main field is known to have been particularly regular in Western Europe over the last two or three centuries, the assumption was made that, except for a position-dependent proportionality constant, this variation was the same in Switzerland as at Fürstenfeldbruck Observatory in southern Germany.

Over the last few years the Observatory data were modelled with a rational function, such as to yield an asymptotically linear extrapolation beyond the last value. The position-dependent proportionality constant was derived by modelling the data from more than 50 sites surveyed in the last few years. With this procedure a mathematical representation of the main field as a function of position over the entire country and the present period of time was obtained. Because of the quasi-linear extrapolation in time, and the regularity of secular variation mentioned above, our mathematical representation may constitute a reasonably credible forecast of the field variation over the next decade. Each component is thus described by:

with the time dependence

where t is the time (in years) from the time origin To, x and y are the distance (km) from the origin of the Swiss kilometric network (Xo=200, Yo=600). The following table gives the last update of the various coefficients:

Coefficient update: 2015/ Time origin To: 2003.5 (rev. 04/02/2016)

Coefficient
D (declination)
I (inclination)
F (total field)
A
0.2631560E+00
0.6284751E+02
 0.4729625E+05
B
-0.6025391E-03
0.8372933E-02
0.2901324E+01
C
0.3820359E-02
0.7483889E-03
 0.6070384E+00
b
0.9142639E-04 0.1076109E-04
0.4883035E-03
c
-0.2016985E-03
-0.3259985E-02
0.2626958E-03
alpha
0.9269611E-01
-0.2501858E-03
0.4517365E+02
beta
0.5498111E-01
0.8475191E+00
0.3137973E+01
gamma
0.1598345E-01
0.1033407E-06
0.4773487E+01


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