Some clarifications.

"K. Smith" wrote in message
...

Not really, most ports have a compass swinging place set aside have a
look at the charts or ask. It usually has a set of posts you can tie to
& hold the boat on the various headings & "usually" they're sited so
there are at the very least a set of transits (2 or more landbased
objects that are on the chart & when in line mean your boat is pointing
to a known true heading as close to nth as reasonable given the
available transits)

You will have to keep the boat aligned and that's probably by keeping it on
the move along the line of sight. Tying it to posts that probably contain
steel reinforcements isn't a good idea. If you can keep it in a straight
line against a landmark, a GPS will work. Yes, you are correct about being
mindful of cross-track movements.


A GPS has no clue which way the bow of the boat is pointing & this is
what you're trying to determine. All a gps can do is tell you your
history track across the seabed & clearly you need to be moving for it
to even be able to tell you that.. You might be moving sideways,
backwards or whatever the GPS will not know so can't tell you.

The suggestion provided assumes a knowledge of GPS usage and remains a valid
suggestion.

I also note point 15 below??? DON'T ever do this, the compass must be
mounted so it's aligned with the boat's fore/aft centre line. It doesn't
need to be "on" or even near the centre line but must be completely
parallel with it.

The steps preceding 15 are used to determine the index error, that is how
much the compass is not aligned to the fore-aft axis of the boat. Hence it
is necessary adjustment by rotating the compass to bring it into alignment.

Tan PS

Subject: compass deviation
From: " Tuuk"

Another typical useless post from Harry,,

If you want to read another one, just reread your reply to this question.

Capt. Bill

Tan PS wrote:
Some clarifications.

"K. Smith" wrote in message
...


Not really, most ports have a compass swinging place set aside have a
look at the charts or ask. It usually has a set of posts you can tie to
& hold the boat on the various headings & "usually" they're sited so
there are at the very least a set of transits (2 or more landbased
objects that are on the chart & when in line mean your boat is pointing
to a known true heading as close to nth as reasonable given the
available transits)


You will have to keep the boat aligned and that's probably by keeping it on
the move along the line of sight. Tying it to posts that probably contain
steel reinforcements isn't a good idea. If you can keep it in a straight
line against a landmark, a GPS will work. Yes, you are correct about being
mindful of cross-track movements.


Tan,

I think we're at crossed purposes here, I think GPS is the single
biggest contributor to boat safety ever no exceptions. However it's
totally different method of navigation if indeed it can even still be
called navigation.

If the magnetic compass of any type is to be employed for navigations
then it needs to have a starting point & that's usually knowing which
way the bow of the boat is pointed & after that the navigator can make
decisions, mostly by calculation but eventually with ingredient "X",
which comes with experience & in small craft particularly knowing the
boat itself.

We've all come across the nuff nuff who's done all the nav courses etc
& will mindlessly hold a compass course straight up onto the bricks,
such is the belief that boats actually go where they're pointed over any
sort of distance.


A GPS has no clue which way the bow of the boat is pointing & this is
what you're trying to determine. All a gps can do is tell you your
history track across the seabed & clearly you need to be moving for it
to even be able to tell you that.. You might be moving sideways,
backwards or whatever the GPS will not know so can't tell you.


The suggestion provided assumes a knowledge of GPS usage and remains a valid
suggestion.

I explained my position so I guess you'll explain why you hold yours, I
can assure you there is no manner the gps knows which way the bow is
pointing.


I also note point 15 below??? DON'T ever do this, the compass must be
mounted so it's aligned with the boat's fore/aft centre line. It doesn't
need to be "on" or even near the centre line but must be completely
parallel with it.


The steps preceding 15 are used to determine the index error, that is how
much the compass is not aligned to the fore-aft axis of the boat. Hence it
is necessary adjustment by rotating the compass to bring it into alignment.

This is patently incorrect for any sort of proper compass Tan for the
reason already given. Again you need to know which way the bow is
pointing so you can apply various & varying adjustments, a skew wiff
mounted compass is an ornament.

K

Tan PS