NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: George Huxtable
Date: 2006 Jul 23, 16:05 -0500
Robert Eno asked-
Anyone out there ever use their GPS to swing a steering compass.
I'd like to hear about your experiences.
=================
I am astounded at the complications people have got into in doing so,
in non-steel vessels..
First, I ask; is it really wise to use corrector magnets to zero out
the deviation? All that's needed is to chuck out any corrector
magnets, note what the deviation actually is, at each heading, make
out a deviation card to correspond, and allow for it by a bit of
mental arithmetic. Otherwise, you (or worse, someone else) must have
gone through the exercise of adjusting those correctors, and have Got
It Right, and presume that thereafter nothing ever changes. If you're
an ocean voyager, it's likely that big changes of latitude will render
those corrections invalid.
Robert Eno finds his deviations to reach 25 degrees, which ought to be
a cause for concern. If I were in his position I would go carefully
around the boat with a hand bearing compass to search out a better
position for the steering compass, if possible, and to understand why
it is so badly affected. We have to remember that he is in a region of
very high magnetic latitude, where the field is nearly straight up and
down, and the little horizontal component of field that remains is
likely to be badly affected by any steel around, such as his engine.
The rest of us, in lower latitudes, have an easier time.
But there are other possible culprits than the engine, and (for
example) any nearby loudspeaker, even the unit in a VHF radio, should
be treated with suspicion.
======================
Instead of using GPS to determine deviation, what's the matter with
simply using your chart, or else celestial bearings?
In harbour, look at your chart. Most places, there will be lots of
charted objects all around, and any pair of such objects, that you can
align, gives you a transit with a true bearing that you can measure
off the chart with a protractor. Get yourself in that line, put that
transit on the bow, then on the stern, then on each beam, read the
compass at each position, allow for variation, and already you have
four points on a deviation card. Find another transit, in a different
direction, and do it again. Be careful if sighting on floating marks,
unless they are very distant, because they can range about so much in
a tide.
If you can put the boat precisely at an identifiable spot on the
chart, transits are unnecessary; simple bearings to known landmarks
will suffice.
You can rig a simple device, using a protractor with sightvanes on the
coachroof, to set the bow at a known angle to an identifiable
direction.
But why not compare the compass reading with the bearing of a
celestial body, particularly one that's low, near the horizon? On
passage, point the bow, or the stern, at a low morning or evening Sun.
Head off course for a moment, if necessary, to do it. Note the time
and the compass reading. Later, work out the Sun azimuth at that
moment, allow for variation, and check the result against your
deviation card. You can keep on doing that job each day as a matter of
routine, as the sailing-ship navigators used to do. On a clear night,
there's a choice of low stars for doing the same thing.
So what's to be gained by invoking GPS, I ask? Certainly, you are
likely to get into trouble if you compare compass heading with the
course shown by a GPS receiver, under way. What you need, for the
deviation, is to know the heading of the bow. If there's any leeway,
your course through the water will differ from that. If there's any
current, your course over the ground will differ, again, from that,
and that's what the GPS will tell you.
On my boat, I do my very best to keep things simple.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 Jul 23, 16:05 -0500
Robert Eno asked-
Anyone out there ever use their GPS to swing a steering compass.
I'd like to hear about your experiences.
=================
I am astounded at the complications people have got into in doing so,
in non-steel vessels..
First, I ask; is it really wise to use corrector magnets to zero out
the deviation? All that's needed is to chuck out any corrector
magnets, note what the deviation actually is, at each heading, make
out a deviation card to correspond, and allow for it by a bit of
mental arithmetic. Otherwise, you (or worse, someone else) must have
gone through the exercise of adjusting those correctors, and have Got
It Right, and presume that thereafter nothing ever changes. If you're
an ocean voyager, it's likely that big changes of latitude will render
those corrections invalid.
Robert Eno finds his deviations to reach 25 degrees, which ought to be
a cause for concern. If I were in his position I would go carefully
around the boat with a hand bearing compass to search out a better
position for the steering compass, if possible, and to understand why
it is so badly affected. We have to remember that he is in a region of
very high magnetic latitude, where the field is nearly straight up and
down, and the little horizontal component of field that remains is
likely to be badly affected by any steel around, such as his engine.
The rest of us, in lower latitudes, have an easier time.
But there are other possible culprits than the engine, and (for
example) any nearby loudspeaker, even the unit in a VHF radio, should
be treated with suspicion.
======================
Instead of using GPS to determine deviation, what's the matter with
simply using your chart, or else celestial bearings?
In harbour, look at your chart. Most places, there will be lots of
charted objects all around, and any pair of such objects, that you can
align, gives you a transit with a true bearing that you can measure
off the chart with a protractor. Get yourself in that line, put that
transit on the bow, then on the stern, then on each beam, read the
compass at each position, allow for variation, and already you have
four points on a deviation card. Find another transit, in a different
direction, and do it again. Be careful if sighting on floating marks,
unless they are very distant, because they can range about so much in
a tide.
If you can put the boat precisely at an identifiable spot on the
chart, transits are unnecessary; simple bearings to known landmarks
will suffice.
You can rig a simple device, using a protractor with sightvanes on the
coachroof, to set the bow at a known angle to an identifiable
direction.
But why not compare the compass reading with the bearing of a
celestial body, particularly one that's low, near the horizon? On
passage, point the bow, or the stern, at a low morning or evening Sun.
Head off course for a moment, if necessary, to do it. Note the time
and the compass reading. Later, work out the Sun azimuth at that
moment, allow for variation, and check the result against your
deviation card. You can keep on doing that job each day as a matter of
routine, as the sailing-ship navigators used to do. On a clear night,
there's a choice of low stars for doing the same thing.
So what's to be gained by invoking GPS, I ask? Certainly, you are
likely to get into trouble if you compare compass heading with the
course shown by a GPS receiver, under way. What you need, for the
deviation, is to know the heading of the bow. If there's any leeway,
your course through the water will differ from that. If there's any
current, your course over the ground will differ, again, from that,
and that's what the GPS will tell you.
On my boat, I do my very best to keep things simple.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
