On Mar 12, 4:13 pm, "Jozef" wrote:
...
A quick series of measurements (beginning north and with increments of 30°)
reveal a positive deviation with a maximum of +6.5° (from 0° to 180°) and
maximum deviation of -5° (from 180° to 360°). Drawing a graph with de
deviation values as a function of the bearing angle gives a sort of
sinusoïdal curve. ...

You probably know this but magnetic "variation" or "declination" is
the difference between magnetic north and true north. Ideally a
magnetic compass will always point to magnetic north. However, even a
well adjusted compass will tend to have different errors on different
headings. These errors are called "deviation" and are typically
tabulated and corrected for. Because these errors are result of local
magnetic fields a compass, like yours, that doesn't have a fixed
location may have different deviation functions depending on its
surroundings. The errors you are seeing may be a result of where you
are using the binos. You could try testing them in another location
and see if they are still the same. However, my suspicion, given that
these are quality glasses and the errors are large, is that the
problem is more local yet. Magnetic objects on your body like wire
frame glasses, watches or jewelry may be contributing to the errors.
If that is the case and you can't or don't want to remove the
offending metal you could make up a deviation table and use it to
correct you readings. However, be aware that as you move about you
may well find yourself in a place where the deviation will deviate
from what are correcting for. With portable compasses there isn't
much to be done about this other than being very aware of your
surroundings.

Of course, it could be that there is a design or manufacturing error
on the glasses themselves. This seems unlikely with quality equipment
like Steiner but if different people in different places tend to see
the same deviation errors then there is a defect with the product.

-- Tom.