NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Deviation Card with GPS
From: Lu Abel
Date: 2006 Jul 23, 12:10 -0500
First and foremost, unless you're on a steel-hulled vessel, you should
be able to reduce deviation to nearly zero with your compass's
compensating magnets. That does, of course, still require swinging your
boat.
The way to do it with GPS is to find a good spot on the water with
navigation aids or other visible landmarks nicely around a 360 degree
circle. Using a chart, enter them as waypoints in your GPS, entering
their latitude and longitude as accurately as possible (for floating
aids, sailing past them and using the "Mark" button on your GPS is a
great way to get exact coordinates).
Now go to approximately your chosen spot (with GPS you don't have to be
exactly there). Bring up each of your landmarks on your GPS (there are
a variety of ways to do this and they differ somewhat on each GPS). Get
the bearing from your current location to the landmark. Point your boat
at the landmark and read its compass. Voila! Deviation.... Repeat
for other landmarks.
This is a heck of a lot easier than the traditional methods of either
anchoring at a precise spot and rigging a yoke so you can point your
boat at various landmarks, or running courses between landmarks (where
you have to worry about getting set sideways).
One hint: PLOT your deviation curve. X-axis is 0-360 degrees heading,
Y-axis is deviation. It should look like a sine wave (again, except
for steel boats). If the axis of the sine wave is not at zero
deviation, then your compass's lubber line is not aligned with your
keel! Many compass alignment books (or alignment chapters in more
comprehensive navigation texts) for some reason do not mention this.
When I followed this advice I discovered that my otherwise
well-constructed 36' sloop came from the factory with the compass
misaligned by two degrees. Once I correctly aligned the lubber line I
was able to get my deviation below 1 degree all the way around using my
compass's compensating magnets.
In addition to the usual chapters on compass adjusting in all the
standard navigation textbooks, it is my understanding the the
International Marine division of McGraw Hill will be publishing a
"Captain's Quick Guide to Compass Adjusting" soon, but I don't have a
date. It will be a waterproof fold-out guide, similar to their current
Captain's Quick Guides" on other topics (
http://books.mcgraw-hill.com/getdiscount.php?q=quickguides&template=im )
Lu Abel
Robert Eno wrote:
> Anyone out there ever use their GPS to swing a steering compass.
>
> I'd like to hear about your experiences.
>
> Robert
>
> >
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Lu Abel
Date: 2006 Jul 23, 12:10 -0500
First and foremost, unless you're on a steel-hulled vessel, you should
be able to reduce deviation to nearly zero with your compass's
compensating magnets. That does, of course, still require swinging your
boat.
The way to do it with GPS is to find a good spot on the water with
navigation aids or other visible landmarks nicely around a 360 degree
circle. Using a chart, enter them as waypoints in your GPS, entering
their latitude and longitude as accurately as possible (for floating
aids, sailing past them and using the "Mark" button on your GPS is a
great way to get exact coordinates).
Now go to approximately your chosen spot (with GPS you don't have to be
exactly there). Bring up each of your landmarks on your GPS (there are
a variety of ways to do this and they differ somewhat on each GPS). Get
the bearing from your current location to the landmark. Point your boat
at the landmark and read its compass. Voila! Deviation.... Repeat
for other landmarks.
This is a heck of a lot easier than the traditional methods of either
anchoring at a precise spot and rigging a yoke so you can point your
boat at various landmarks, or running courses between landmarks (where
you have to worry about getting set sideways).
One hint: PLOT your deviation curve. X-axis is 0-360 degrees heading,
Y-axis is deviation. It should look like a sine wave (again, except
for steel boats). If the axis of the sine wave is not at zero
deviation, then your compass's lubber line is not aligned with your
keel! Many compass alignment books (or alignment chapters in more
comprehensive navigation texts) for some reason do not mention this.
When I followed this advice I discovered that my otherwise
well-constructed 36' sloop came from the factory with the compass
misaligned by two degrees. Once I correctly aligned the lubber line I
was able to get my deviation below 1 degree all the way around using my
compass's compensating magnets.
In addition to the usual chapters on compass adjusting in all the
standard navigation textbooks, it is my understanding the the
International Marine division of McGraw Hill will be publishing a
"Captain's Quick Guide to Compass Adjusting" soon, but I don't have a
date. It will be a waterproof fold-out guide, similar to their current
Captain's Quick Guides" on other topics (
http://books.mcgraw-hill.com/getdiscount.php?q=quickguides&template=im )
Lu Abel
Robert Eno wrote:
> Anyone out there ever use their GPS to swing a steering compass.
>
> I'd like to hear about your experiences.
>
> Robert
>
> >
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
