Compass turning error

I got Pete's explanation, and it's close to my own mental way of remembering
this.

The metaphor that helps me to visualize the N/S compass turning error in the
northern hemisphere is a string with one end tied to the back side of the
compass ring, behind where "S" (South) is painted, and the other tied to the
north pole. When I'm near the world's equator, this string pulls the
compass in the direction it's supposed to go (it always keeps the side with
"S" painted on it as close to the north as possible since the string is
parallel to the ground at that point), and there's no turning error.
However, as I approach the north pole, the string begins to pull more and
more downward, toward the ground (the pole itself). Using this metaphor, I
can "see" the effect that the string has on the compass; as I bank, the
string will pull the side of the compass on which "S" is written downward,
toward the earth. The rotation of the compass on its axis (the turning
error) then becomes apparent as I follow this metaphor through by
visualization.


"Casey Wilson" wrote in message
...
Okay, I know the compass swings into a turning error when initiating
a
turn from any direction other than East or West and that the amount of
turning error is greatest when starting from either North or South. I
even
know a couple of the cute acronyms like UNOS (Undershoot North - Overshoot
South) to aid in rolling out on the correct heading.

My questions a

What is the mechanics for causing the turning error in the first place?

Why doesn't the effect happen when starting from East or West, yet
develops
as the turn progresses to the North or South?

Perhaps I should have mentioned my my post (above), that in the metaphor,
the "string" represents nothing more than the pull of the magnet (which is
physically located in the same place as the "string" is tied; on the back of
where "S" is painted) towards the north pole (the other side of the
"string").

"Marty Ross" wrote in message
et...
I got Pete's explanation, and it's close to my own mental way of
remembering
this.

The metaphor that helps me to visualize the N/S compass turning error in
the
northern hemisphere is a string with one end tied to the back side of the
compass ring, behind where "S" (South) is painted, and the other tied to
the
north pole. When I'm near the world's equator, this string pulls the
compass in the direction it's supposed to go (it always keeps the side
with
"S" painted on it as close to the north as possible since the string is
parallel to the ground at that point), and there's no turning error.
However, as I approach the north pole, the string begins to pull more and
more downward, toward the ground (the pole itself). Using this metaphor,
I
can "see" the effect that the string has on the compass; as I bank, the
string will pull the side of the compass on which "S" is written downward,
toward the earth. The rotation of the compass on its axis (the turning
error) then becomes apparent as I follow this metaphor through by
visualization.


"Casey Wilson" wrote in message
...
Okay, I know the compass swings into a turning error when
initiating
a
turn from any direction other than East or West and that the amount of
turning error is greatest when starting from either North or South. I
even
know a couple of the cute acronyms like UNOS (Undershoot North -
Overshoot
South) to aid in rolling out on the correct heading.

My questions a

What is the mechanics for causing the turning error in the first place?

Why doesn't the effect happen when starting from East or West, yet
develops
as the turn progresses to the North or South?

In article ,
says...
"Casey Wilson" wrote in message
...
What is the mechanics for causing the turning error in the first place?

In a magnetic compass, you've got a needle that is aligning itself with the
lines of Earth's magnetic field. In particular, the lines go between the
north and south poles, and the needle tries to line up parallel to them.
However, the magnetic field is not parallel to the surface of the Earth. It
curves down toward the poles, and the curvature is greater the farther north
you go.

If you're heading is due north or due south, the needle is aligned with the
airplane's longitudinal axis and you get an accurate reading. When you
bank, because the magnetic field is curved, the compass needle actually
deflects downward a bit to line up with the magnetic field, which causes it
to rotate on its pivot.

If you are headed due north, the needle's downward pointing results in a
heading indication opposite of the turn you're about to commence. Headed
due south, the same downward pointing is reversed relative to the airframe,
and results in a heading indication in the same direction of the turn you're
about to commence.

The effect becomes more pronounced the farther north you go, since the
magnetic field is more curved as you get closer to the pole.

Of course, everything is reversed in the southern hemisphere.

This effect is, of course, completely different from other types of compass
error, such as that caused by acceleration.

Bad example methinks - acceleration error is exactly the
same...
The effect is the same (the needle does not deflect
'normally'). The cause is the same (magnetic dip).

--
Duncan

Peter Duniho wrote:

Of course, everything is reversed in the southern hemisphere.

My understanding is that one end of the magnet inside the compass is attracted
to the magnetic north pole, and that causes the dip. If this is the case, the
effect would not be reversed in the southern hemisphere.

George Patterson
Brute force has an elegance all its own.