On Sat, 09 Sep 2006 20:31:27 +0200, Mxsmanic
wrote:

RK Henry writes:

Except that apparently true north DOES move. The tsunami/earthquake of
a year and a half ago reportedly caused the north pole to shift about
an inch as well as decreasing the length of a day. Just goes to show,
you can't depend on anything.

All earthquakes cause such effects. However, if they produce changes
that are below the resolution threshold of avionics, they aren't a
problem. The magnetic pole, on the other hand, moves perceptibly over
time--enough to require new charts and new regulation of stations and
instruments.

So does GPS. The system changes over intervals of hours, with such
factors as satellite outages or signal propagation, rather than the
months or years that it takes for magnetic variations. Generally, the
GPS computer applies corrections automatically. That makes it
transparent to the user, but it doesn't mean that the system is
inherently perfect. If GPS were inherently perfect, WAAS would be
unnecessary.

All instruments are inaccurate. In order to use them you have to
calibrate them.

RK Henry