The background for this question is a user that expects something around the 6000 nT mark, and the reason is that the user is looking to use an AquaPro instrument in a polar region.

They are also asking whether there are any special procedures they need to follow, because they will be almost on top of magnetic North then it might be hard to get correct bearings. They also suggest using XYZ and know which way the instrument is facing at the point of measurement...

What is measured by the compass is the horizontal component of the magnetuc field. You can find the strength of the horizontal field in your specific location on a map online (see example of such a map in the link below, provided by NOAA). We also have the following general guidelines:

• We do not correct for declination so that has to be corrected in post processing
• Make sure the compass is not exposed to soft iron effects
• Take care to do an accurate hard iron calibration, we recommend doing that in the horizontal plane
• If possible, the compass should be checked at the deployment location against some reference in order to have an estimate of any heading error, for example in steps of 45 degrees

It is not impossible that the magnetometer is usable with 6000 nT horizontal magnetic field strength, but note that we have had cases with measurement errors from instruments working in the far north. Using XYZ might be a solution, as suggested, if it is possible to know the exact mounting direction.

How to adjust for magnetic declination, you may ask? You adjust for declination directly in ENU coordinates, either by rotating your components by multiplying by a heading matrix, or if you calculate speed and direction you can adjust the direction directly.