"JAXAshby" babble again:m25.aol.com...
jeffies, one at a time

That's one too many for you, jaxie.


Magnetic compasses can
certainly be accurate to 2 degrees,

we were discussing recreational sailboats here, so let's stay with the
magnetic
compasses one would find on such.

I am. I think you must get yours from a Cracker Jax Box.


but in the
old
days it was pretty common to have it professional done, and to check it with
known ranges at every opportunity.

and read by an amateur in a moving boat under at sea conditions. and how many
compasses on the boats under discussion could *you* read to 2* or even 5*?

Almost every approach I've made to Maine (of several dozen) has been in flat
conditions. The point is compasses are physically capable of being that
accurate.




All of them right, because you read it in a book.

until this very minute you didn't know that a compass rotates one way as th
boat goes up a wave and rotates the other way going down a wave. Same thing
as
the boat rolls one way and then the other.

Why must there always be large waves? You're the one who gets everything from a
book. A real scary one!



Holding a course to 2 degrees may be difficult for a long period, but for the
short time needed for a bearing is not too hard.

ever helmed a boat, jeffies?

Its looking like you never have, jaxie.



And errors multiply??? That's one of the stupidest things you ever come out
with jaxie! Are you claiming that a 5 degree compass error and a 5 degree
course error yield a net 25 degree error??? I think that one's another
keeper!

jeffies, you stupid cluck. YOU claim to have a degree in physics (okay, an
arts degree and from Potato State), and if you were even qualified to take
freshman courses in the subject you wouldn't make such a dumb statement.

So, let's start by admiting you lied about what you know and what you should
know.

Then, consider this, jeffies. why is it you think a 2* error here and a 5*
error that makes for a 3* to 7* total? It does not.

Now follow this. I'll use % of error rather than * because it makes it easier
for a rank beginner math person like you.

It may be "easy" for you, but that's why you always get the wrong answer!
Directional errors are always additive, jaxie. Especially when you're trying to
show the maximum possible error. Claiming you can multiply them is just plan
stupid. Insisting on it after your error marks you as someone who hasn't
achieve junior high level


a 2% error means the actual might be anywhere from 98% to 102%. Understand?

Now bring on the next error, of 5%. you do NOT take the 5% error against the
original 100% but rather against the 98% to 102%.

THIS MEANS you take 95% to 105% against the errored 98% to 102%. Giving you a
potential error of 0.95 times 0.98, or just over 93% on the lower side and
1.02
times 1.05 or just over 7% on the upper side.

The incremental error is small in this case because the original errors were
small AND there were just two errors to cascade. EACH succeding error is
MULTIPLIED by the total of the previous errors. A string of four or five
errors, each small, can make for a highly uncertain result.

Per Centages are not the same as degrees. The fact that your answer to this
little problem is not symetrical should be a dead giveaway. If there are 5
steps and each step has a possible error of up to 2 degrees, the total possible
error is 10 degrees. However, one could show that the typical error is actually
much smaller than that.



In the case of the radio compass, yo have the error of reading the mag compass
by the helmsman, the error of maybe forgeting variation, the error in the
compass, the error of maybe local deviation, the error of the compassman in
placing the compass, the error of the helmsman in holding course, the error in
the compassman's listening to and hearding the nullness of the signal, the
error of alignment of the compass dial with the antenna, and the error of
reading the dial after the reading was taken.

Yes, all those errors of forgetting things. You have a lot of experiance in
this area. All you proving is that an idiot like you is prone to stupid
mistakes.


Now, you HAVE AT LEAST TWO READINGS to take, *****each**** with potential
errors above.

Why is that? Is it because you have never navigated before? Is it necessary to
use two radio bearings?



In addition, you have the error of the boat movement, which can only be
estimated (no gps onboard, remember?)

Why is that relevant? Are you worried about Doppler effects?




In addition, you have more -- and potentially large -- errors if the two read
stations at not at right angles to the boat.

Why do you need two stations? You've never navigated or piloted, have you?




And why does RDF calibration depend on the ship's compass? There are others
ways to check for deviation that don't depend on the compass.

the discussion was a recreational sailboat.

What does that have to do with it? You can do the calibration at a known
location while at anchor. Your grasp on this is rather tenuous - is that
because you just read about it yesterday?

And, all of this is ignoring several fundamental facts. First, any information
you derive from RDF is better than not having it at all. You haven't given any
alternative other than to Turn Back! Next, the magnitude of the error is not
that important for an approach. Eventually, you will be guided into your
destination. If one bearing is off by 10 degrees it won't make that much
difference. Look at a chart of Matinicus and consider an approach from Cape
Ann. And finally, the bottom line is that RDF was used successfully by
thousands of mariners in vessels large and small. Arguing that it just doesn't
work is stupid!