"Jim Macklin" wrote in message
news:_lOIg.6238$SZ3.83@dukeread04...
The flux valve is usually located well away from steel and
electrical components, so it avoids many deviation errors.
It also is designed to be more accurate and reduce turning
errors sent to the indicator.

But there's no way, absent some sort of absolute position information (eg
GPS) and a detailed database, to ascertain deviation due to local geology or
other external magnetic interference, nor due to magnetic variation (which
was Bob's point).

Of course, my point is that correcting those errors isn't relevant to ATC.
They don't care if your compass is actually right or not. They just care
that you can hold it to a specific heading. But I think it's fair to
comment on the statement that "the slaved compass is corrected so
as...variation is also corrected". That's clearly just not true, at least
in most installations (I suppose there's at least one GPS-based slaved
compass setup in which it is corrected...never say never...but generally,
this isn't used)

Pete