[Rhodes22-list] Hanks & Daves GPS Compass Analysis

[David Walker]

Ed,

I agree.  My GPS is a Garmin and they all have basically the same screens.
As long as its moving it works like a relatively perfect compass giving you
your direction of motion.  Note also that the GPS can be set to read in
either True or Magnetic.  Most sailors I know keep their set for magnetic.

Also note that when sailing (assuming you are set for Magnetic) there are
two reasons why your compass and GPS won't neccessarily agree.  One is
deviation, the other is leeway.  The ships compass (if perfectly calibrated
and with 0 Deviation) will tell you at what direction the bow is pointed.
Due to leeway (wind and current) you probably will not be going in that
direction.  Hence the COG or course over ground is what the GPS is actually
telling you.  The difference (in degrees) is very useful information to the
sailor for optimizing trim and course.

Dave

David Walker
David Walker Photography
davidwalkerphotography.com
----- Original Message -----
From: "ed kroposki" <ekroposki at charter.net>
To: "'The Rhodes 22 mail list'" <rhodes22-list at rhodes22.org>
Sent: Thursday, April 07, 2005 3:55 PM
Subject: RE: [Rhodes22-list] Hanks & Daves GPS Compass Analysis


Hank,
You made me go back and look at the directions.  Yes, movement is
involved, but the heading is based on satellite fixes.  The north stays put
at north, the heading varies.  So yes you can figure out true north and set
a compass by it.
The compass mode is not instantaneous but takes 15 to 20 feet to
tune in an accurate heading.  My driveway is about 10 degrees west of north.


Dave,
Note that my GPS is a Magellan Meridian with a compass mode, not
just a heading.  You can also use the heading mode to a fixed landmark.  If
you walk slowly the compass mode works like a hand held compass.  If you
stop it maintains the last direction.  I just went and walked up and down
the street in front of my house to check it out.  As long as you are moving,
it has an N that rotates like a compass.  As you change directions the big N
points north within the distance stated above.  Turn the GPS while walking
and N points north.
Ed K
Greenville, SC, USA



-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Hank
Sent: Thursday, April 07, 2005 9:11 AM
To: The Rhodes 22 mail list
Subject: Re: [Rhodes22-list] Slim's Compass

Ed,

It is true that the GPS tries to show a heading of where it is
pointed, but I suspect it is deriving that from forward movement.
Next time you go out, put the GPS perpendicular to your direction of
travel for a few minutes and see what it shows for a heading.  I
suspect it will be based upon your movement, not on where it is
pointed.

Hank

On Apr 7, 2005 9:00 AM, ed kroposki <ekroposki at charter.net> wrote:
>
>
> Slim:
>        If you are going to keep the compass on the boat, then do a basic
> calibration.  If your drive is paved, then get a cheap scout compass and a
> piece of chalk.  Plot N-S, E-W on the driveway, or street in front of your
> house.  Adjust the compass to N-S, then E-W, then S-N, then W-E and again
to
> fine tune.
>        The headings may not be perfect, but they will give you the general
> direction, so that you do not do Brad's trick of going in circles.  If you
> decide to do serious navigating then do more serious calibrations of the
> compass.  Keep in mind it will never be perfect to a degree.
>        As to which way do charts use, just look at a navigation chart in
> the light.  The answer is usually somewhere on the chart in degrees.
>        Bill said that a GPS is referenced from where you are coming from.
> On my Magellan Meridian, it tries to show a heading of where it is
pointed.
> If you use a Meridian GPS, you should be able to calibrate the compass to
> within a few degrees.
>        Are you planning a cruise across Lake Superior this summer?
> Are you planning a visit to your mother's anytime soon?
>
> Ed K
> Greenville, SC, USA
> Addendum:  Navigators answer:   A number of different approaches are being
> tried. (We are still guessing at this point.)
>
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