French Repeat Station Survey
What is it?
Geomagnetic repeat stations are defined locations within a certain region (usually a country) where absolute measurements are carried out in regular time intervals, usually annually to once every 5 years. Repeat station data are used to map the main field and study secular variation in detail. The French National Observatory (CLF) is responsible for the French repeat station measurements.
How is it made?
Until 2012, the French geomagnetic repeat station network consisted of 32 points out of a denser network from former ground vector surveys. Regular repeat station surveys started in 1948 and were carried out every 5 years. The measurements are taken by a fluxgate theodolite (D, I) and a proton magnetometer (F). Here is the 2007 map:
The relevance of traditional magnetic repeat station networks to geomagnetic modeling has been increasingly challenged during the last decade, as the Ørsted and CHAMP satellites provided data of unprecedented precision and spatial resolution. Yet magnetic repeat networks organized on a national or regional basis can still be viewed as relatively inexpensive safety nets in case satellite data are not available. A new method for magnetic repeat measurements has been developped, where repeat stations are located on airports, azimuth sightings are determined using GNSS geodetic receivers and magnetic measurements are performed at night (around 0200 local time). It is believe this method improves the measurement precision while keeping the cost low, by facilitating the measurement execution. It was first implemented in metropolitan France during the summer 2012. A description can be found in this publication.
In order to calculate the declination where you are, download this programme.