Southern Hemisphere Compass - Worth it?

Hello All. I'm new to the group and to SCUBA in general. Was certified
in Bora Bora earlier this year (ANMP-CEDIP 1*) and and just recently
picked up a PADI "Peak Performance Boyancy" Specialty rating. Will now
proceed to get PADI Advanced Open Water in early January, 2006.

I just purchased most of my major hardware (Suits, Regulator, Octo, BCD
etc.), all pretty much Oceanic products (no, I don't have stock in the
company). For the computer, I selected the Datamax Pro Plus 2 with
attached compass.

Which leads me to my question. Oceanic also makes (as an option) a
"Southern Hemisphere" version of the compass head for that computer.
Since I do intend to return to Bora Bora perhaps as often as once a year,
(I'm New York based), is it worth buying the Southern Hemisphere head?

I'm not sure it's worth the money for using it just once a year at most, and
it's not like it's easy to switch the two heads - it does require a little surgery.
I could of course, just use it stand alone, but it gets awkward to use it that
way when you're trying to hold onto the computer itself. So if the "northern
hemisphere" compass is totally useless down there, I might buy it for there
and for the GBR in Australia.

Worth the expense/trouble?

Thanks very much for your help.



____________________________________________
Regards,

Arnold

Dan Bracuk wrote:
> mag3 <zmpmag3-plongee@yahoo.com> pounded away at his keyboard

> Welcome to rec.scuba.equipment and to diving, Arnold.
>
> I don't think it's worth the expense and trouble to get the Southern
> Hemisphere head for your compass.

northern compass? Australian compass?

the only difference seems to be the claim that the needle dips
and is not dead level in different areas of the world.
just try dipping the compass so the needle floats freely,
and it should work.

do you think that the navy carefully changes compasses
every time they cross the equator?

use the same compass anywhere you go.

mine works perfectly anywhere in the world I dive.
east west north south, never a problem.

someone is trying to sell you two compasses
that do the same thing.

In article <hlr5p1t6lfaf1b273i6heq540gu2q7ocpq@4ax.com>, mag3
<zmpmag3-plongee@yahoo.com> wrote:

€ Hello All. I'm new to the group and to SCUBA in general. Was certified
€ in Bora Bora earlier this year (ANMP-CEDIP 1*) and and just recently
€ picked up a PADI "Peak Performance Boyancy" Specialty rating. Will now
€ proceed to get PADI Advanced Open Water in early January, 2006.
€
€ I just purchased most of my major hardware (Suits, Regulator, Octo, BCD
€ etc.), all pretty much Oceanic products (no, I don't have stock in the
€ company). For the computer, I selected the Datamax Pro Plus 2 with
€ attached compass.
€
€ Which leads me to my question. Oceanic also makes (as an option) a
€ "Southern Hemisphere" version of the compass head for that computer.
€ Since I do intend to return to Bora Bora perhaps as often as once a year,
€ (I'm New York based), is it worth buying the Southern Hemisphere head?
€
€ I'm not sure it's worth the money for using it just once a year at most, and
€ it's not like it's easy to switch the two heads - it does require a little
€ surgery.
€ I could of course, just use it stand alone, but it gets awkward to use it
€ that
€ way when you're trying to hold onto the computer itself. So if the "northern
€ hemisphere" compass is totally useless down there, I might buy it for there
€ and for the GBR in Australia.
€
€ Worth the expense/trouble?
€

Probably not for a once a year trip, but there are differences in
compasses intended for different parts of the globe. Here's a link that
covers the topic pretty well:

http://www.geocities.com/magnetic_declination/

Most dive compasses are very sensitivie to declination, and one
intended for North America will be severely tilted if you use it in
Australia. I have a compass I bought in Thailand (Suunto) that is
almost unusable in San Diego because the card tilts and rubs against
the case.

Having said that, I'd buy a compass for your console that suits the
majority of your diving, and just rent/borrow a wrist mounted compass
for those times you travel to Bora Bora.

And to answer Crownfield's uninformed (gee, what a suprise) comment,
large ships used to deal with declination by having compasses that
could accomodate card tilt. Today, they're more likely to use fluxgate
(electronic) compasses that aren't affected by declination.





€ Thanks very much for your help.
€
€
€
€ ____________________________________________
€ Regards,
€
€ Arnold

I think you are confusing two different issues.

1) the dipole field of the earth is not coaxial with the earth's axis of
revolution. The difference between magnetic north and true (celestial)
north is known as declination. You can correct this anywhere in the
world by placing a mark on the compass to indicate the difference.

2) the earth's magnetic field is best approxmiated as a dipole,
especially close to the surface. The orientation of the field lines at
any given point on the surface of the earth is not necessarily parallel
to the surface. At the magnetic north pole, they will be almost
vertical whereas at the equator, they will be roughly parallel. The
compass needle will tend to align with the fieldlines, even if
constrained to a particular plane. The compass would be most effective
if it is allowed to align three dimensionally to the dipole field. I
suspect based on other posts that the "southern hemisphere" compasses
are designed with this in mind by being magnetized such that the part
you read is flat relative to the surface while magnetically pointing
slightly downward.

Alan Street wrote:
> In article <hlr5p1t6lfaf1b273i6heq540gu2q7ocpq@4ax.com>, mag3
> <zmpmag3-plongee@yahoo.com> wrote:
>
> € Hello All. I'm new to the group and to SCUBA in general. Was certified
> € in Bora Bora earlier this year (ANMP-CEDIP 1*) and and just recently
> € picked up a PADI "Peak Performance Boyancy" Specialty rating. Will now
> € proceed to get PADI Advanced Open Water in early January, 2006.
> €
> € I just purchased most of my major hardware (Suits, Regulator, Octo, BCD
> € etc.), all pretty much Oceanic products (no, I don't have stock in the
> € company). For the computer, I selected the Datamax Pro Plus 2 with
> € attached compass.
> €
> € Which leads me to my question. Oceanic also makes (as an option) a
> € "Southern Hemisphere" version of the compass head for that computer.
> € Since I do intend to return to Bora Bora perhaps as often as once a year,
> € (I'm New York based), is it worth buying the Southern Hemisphere head?
> €
> € I'm not sure it's worth the money for using it just once a year at most, and
> € it's not like it's easy to switch the two heads - it does require a little
> € surgery.
> € I could of course, just use it stand alone, but it gets awkward to use it
> € that
> € way when you're trying to hold onto the computer itself. So if the "northern
> € hemisphere" compass is totally useless down there, I might buy it for there
> € and for the GBR in Australia.
> €
> € Worth the expense/trouble?
> €
>
> Probably not for a once a year trip, but there are differences in
> compasses intended for different parts of the globe. Here's a link that
> covers the topic pretty well:
>
> http://www.geocities.com/magnetic_declination/
>
> Most dive compasses are very sensitivie to declination, and one
> intended for North America will be severely tilted if you use it in
> Australia. I have a compass I bought in Thailand (Suunto) that is
> almost unusable in San Diego because the card tilts and rubs against
> the case.
>
> Having said that, I'd buy a compass for your console that suits the
> majority of your diving, and just rent/borrow a wrist mounted compass
> for those times you travel to Bora Bora.
>
> And to answer Crownfield's uninformed (gee, what a suprise) comment,
> large ships used to deal with declination by having compasses that
> could accomodate card tilt.

alan may not yet have noticed, as most divers have,
but a hand held compass 'could accomodate card tilt' easily.

> Today, they're more likely to use fluxgate
> (electronic) compasses that aren't affected by declination.

(I have done flight simulation for aircraft, and they have been using
fluxgates for more than half a century now. big news.)

both of which use gymboled mounts.
obviously the needle points to the magnetic pole, which is not always
tangent to the earths surface.

maybe your compass has a problem, but mine works from Boston harbor to
the barrier reef with no big problems. any one with any diving ability
just adjusts the compass so the needle moves freely. east, west, north,
south, deep, shallow, cold, warm, clear, low vis: not a problem.

are you a barking moonbat alan?
or just a mentally challenged diver?

this I would have expected from JOF.

>
>
>
>
>
> € Thanks very much for your help.
> €
> €
> €
> € ____________________________________________
> € Regards,
> €
> € Arnold

zippthorne wrote:
> I think you are confusing two different issues.

Please! don't interupt the two fools. They haven't even figured out the
difference between DECLINATION and INCLINATION yet.

In article <11p773e4vl44014@corp.supernews.com>, James Connell
<jconnell@gci.net> wrote:

€ zippthorne wrote:
€ > I think you are confusing two different issues.
€
€ Please! don't interupt the two fools. They haven't even figured out the
€ difference between DECLINATION and INCLINATION yet.

Oops

Why does it matter where the compass is pointing? It would seem that unless
you were trying to get to a certain place on a certain heading, it wouldn't
matter if the compass was off at all.

If you are using it to figure out how to get back to where you started from,
you'd simply reverse course (or adjust for that). It seems to me that if you
start out going North (according to your compass) and it really isn't North,
reversing course and going (according to the compass) South would work
regardless of what "North" or "South" really were.

Or am I missing something?

In article <BqSdnUTcgfwfuQveRVn-vA@adelphia.com>, David Gintz
<dgintz@gmail.com> wrote:

€ Why does it matter where the compass is pointing? It would seem that unless
€ you were trying to get to a certain place on a certain heading, it wouldn't
€ matter if the compass was off at all.
€
€ If you are using it to figure out how to get back to where you started from,
€ you'd simply reverse course (or adjust for that). It seems to me that if you
€ start out going North (according to your compass) and it really isn't North,
€ reversing course and going (according to the compass) South would work
€ regardless of what "North" or "South" really were.
€
€ Or am I missing something?
€
€

Somewhat. I was using the wrong term. Declination is the difference
between true north and magnetic north, and you're right, it doesn't
generally matter for diving if you just use relative out and back
headings.

Inclination, or dip, is what causes the compass card to tilt as you
move from high latitudes to the equator. This doesn't cause an error in
heading, but it can prevent the compass card from spinning freely.

"Alan Street" <agstreet@nonono_san.rr.com> wrote in message
news:061220051602078125%agstreet@nonono_san.rr.com ...

> Inclination, or dip, is what causes the compass card to tilt as you
> move from high latitudes to the equator. This doesn't cause an error in
> heading, but it can prevent the compass card from spinning freely.

Once again, I may be showing my ignorance but how is that dip any different
from what I could cause by holding the compass in a non-level orientation?
In that case, I just level it myself. Isn't that the solution in this case?
I could see a problem if the compass was mounted in a fixed position on a
ship but it ain't too stable on my console anyways!