On Thu, 13 Mar 2008 21:09:24 -0700 (PDT), "
wrote:

On Mar 13, 5:39 pm, Bruce in Bangkok wrote:
... Perhaps you need to go to:http://www.steiner-binoculars.com/bi...arine/395.html
and read up on these glasses. They have a world wide chart showing
deviation in various areas. The maximum I noticed was 12.0 degrees.

The chart seems to indicate that the compass is "balanced" for the
specific area where the glasses are sold.

Good link. I don't believe that they are really talking about
deviation errors -- it's been a long hot day and beer has happened so
I could be wrong... I think they are talking just about tilt.
However, if that's true then the design is disappointing. It isn't
hard to make compasses that are much, much less affected by tilt.
I've got a couple of Plastimo Iris 50's that are Australian and I've
used them all over the Pacific with no serious problems. Tilt is
generally less than 10 degrees anywhere where the water is likely to
be liquid so, any sail boat compass will need to be designed to work
at angles much greater than those induced by magnetic tilt. I've even
seen flat compasses with an extra movement that lets the compass
magnet align itself with the Earths field in 3D. If the problem
really is tilt, and it is easy to tell by tilting the glasses up and
down, then the design is seriously flawed.

-- Tom.


I'm sure that you are right as the only thing that varies N or S would
be tilt. But they do show an illustration of how the needle is mounted
at angle depending where the glasses are used. In any event I think
the OP is looking for a needle in a hay stack if he wants 100%
accuracy from a compass. I used to swing aircraft compasses as part
of my job and never saw one that was 100% accurate on all headings.

I also think in real life that it doesn't make any difference. There
you are standing in a moving boat and taking a sight on two fixed
objects. You are just looking for the angle between the two objects.


Bruce-in-Bangkok
(correct email address for reply)

On Fri, 14 Mar 2008 06:08:16 -0400, "Roger Long"
wrote:

"Bruce in Bangkok" wrote
I used to swing aircraft compasses as part
of my job and never saw one that was 100% accurate on all headings.

You'll like this story then. Back when I was Maintenance Officer of the
flying club we put in a new windshield and some equipment that required
reswinging the compass. The shop said I couldn't do it because I wasn't an
AP but I was welcome to take the plane out and adjust it before they did the
official checks to save time and money.

I built a sight of a board with two nails in it and taped it to the
stabilizer carefully aligned with the aircraft center line. I then spent a
couple hours dragging the plane all over the ramp turning it 180 degrees and
tweaking out half the differences just as I used to do on boats. I did this
on 20 degree headings and got it damn near perfect, way within the limits
you can read while it's jiggling around in flight. I then put the plane on
the cardinal headings and ran the engine up to full power and turned all the
electronic equipment on and off and made radio checks to be sure nothing was
upsetting it. It was perfect.

The AP then came out with a little 5 dollar compass on a stick and had me
run the engine up to full power on six headings while he sighted down the
fuselage. The compass was shaking about 5 degrees each way from the
vibration of full power with locked brakes. I took his results and did a
polar graph as you are supposed to do with the data and said, "Look there is
no way you can have a 10 degree bulge on one heading sticking out like this.
You've got to recheck or do some smoothing of the data. Compass's don't act
like this. "

The shop said, "That's the FAA way. Here's your compass card." I had to
tell the club members to just ignore the compass care which would have lead
them 10 off in some directions but I could only do it verbally because, if
someone got lost and hit a mountain of course, it would have been my fault.

That's the way so much of the world works.


I worked for a company named Southern Airways at the time and we had a
"compass Rose". Put one wheel on a mark in the middle of the circle
and drag the airplane around until the tail wheel sat on another mark
and read the compass. At the time I was a lowly AP (actually A & E)
working on the flight line so just filled out a form with the compass
readings and turned it into the office. In due time an official
Compass Deviation Card was issued that we stuck in a little clip on
the instrument panel.

I used to listen to the instructors briefing the cadets for their
cross countries and I honestly think they could have flown them
without a magnetic compass as I used to hear the Instructors talk a
lot about the various railroad tracks in the area.


Bruce-in-Bangkok
(correct email address for reply)

On Fri, 14 Mar 2008 15:58:49 +0700, Bruce in Bangkok
wrote:

snip

In any event I think
the OP is looking for a needle in a hay stack if he wants 100%
accuracy from a compass

snip


It doesn't seem he's looking for 100% accuracy at all. The issue here
is whether we should be surprised at the Steiner's demonstrated
errors. There is a tendency to be appalled at the performance of such
a high quality instrument. But do our prior expectations come from
some source other than the high price?

Chuck

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On Mar 13, 10:58 pm, Bruce in Bangkok wrote:
I also think in real life that it doesn't make any difference. There
you are standing in a moving boat and taking a sight on two fixed
objects. You are just looking for the angle between the two objects.

I don't know how the OP intends to use the compass but 7 degrees is a
lot of error. More than twice what I'd expect from a well corrected
compass. Keep in mind, with deviation, the error will be between
headings so the angle between your two objects could be 7 degrees off
in this case... Good thing he's got a GPS.

-- Tom.

On Mar 14, 8:16 am, " wrote:
... Keep in mind, with deviation, the error will be between
headings so the angle between your two objects could be 7 degrees off
in this case... Good thing he's got a GPS. ...

And, while I'm in the bad habit of replying to myself and just for the
record: you can't get a fix with just the angle between two objects
and no azimuth. The angle only gives you a circle of position (or to
be fully correct two possible circular arcs of position except where
you have a transit and it becomes a line of position). With just
angles between objects you need three to get a 2d fix (unless you have
transits where you only need two). But you knew that

-- Tom.

On Thu, 13 Mar 2008 08:53:53 -0400, jeff wrote:

I had a learning
experience years ago where a small portable radio sitting next to the
compass shifted our course 15 degrees.

Could I point out the that speakers always have a magnet. About the
worst thing I can think of to put near a compass

Casady

On Fri, 14 Mar 2008 11:16:58 -0700 (PDT), "
wrote:

On Mar 13, 10:58 pm, Bruce in Bangkok wrote:
I also think in real life that it doesn't make any difference. There
you are standing in a moving boat and taking a sight on two fixed
objects. You are just looking for the angle between the two objects.

I don't know how the OP intends to use the compass but 7 degrees is a
lot of error. More than twice what I'd expect from a well corrected
compass. Keep in mind, with deviation, the error will be between
headings so the angle between your two objects could be 7 degrees off
in this case... Good thing he's got a GPS.

-- Tom.

I had assumed that he was using the glasses to measure the angle
between two known objects as is normal with a hand bearing compass. In
which case whether or not the glasses were reading the correct
magnetic heading is irrelevant.

Bruce-in-Bangkok
(correct email address for reply)

Richard Casady wrote:
On Thu, 13 Mar 2008 08:53:53 -0400, jeff wrote:

I had a learning
experience years ago where a small portable radio sitting next to the
compass shifted our course 15 degrees.

Could I point out the that speakers always have a magnet. About the
worst thing I can think of to put near a compass

Casady

No

On Fri, 14 Mar 2008 11:42:16 -0700 (PDT), "
wrote:

On Mar 14, 8:16 am, " wrote:
... Keep in mind, with deviation, the error will be between
headings so the angle between your two objects could be 7 degrees off
in this case... Good thing he's got a GPS. ...

And, while I'm in the bad habit of replying to myself and just for the
record: you can't get a fix with just the angle between two objects
and no azimuth. The angle only gives you a circle of position (or to
be fully correct two possible circular arcs of position except where
you have a transit and it becomes a line of position). With just
angles between objects you need three to get a 2d fix (unless you have
transits where you only need two). But you knew that

-- Tom.
Well, normally when you are doing it you take the angle between two
points a known distance apart...

Bruce-in-Bangkok
(correct email address for reply)

On Mar 14, 4:41 pm, Bruce in Bangkok wrote:
... I had assumed that he was using the glasses to measure the angle
between two known objects as is normal with a hand bearing compass. In
which case whether or not the glasses were reading the correct
magnetic heading is irrelevant.

I don't understand what you are saying. Bearing compasses are almost
always used to get azimuths which are then often plotted on a chart.
They have to read properly to work. If you just want to know the
angle between two objects you don't need a compass at all. For small
angles you can get within a few degrees just siting over you
outstretched hand (each knuckle is about 3 degrees). A cross staff,
peloris, kamal, sextant etc can be used for larger angles. If the
magnetic heading is irrelevant why have a magnet compass at all?

-- Tom.