[Rhodes22-list] Hanks & Daves GPS Compass Analysis

Thu Apr 7 16:55:35 EDT 2005

OR NOT!

When I grounded the "CoraShen" a few weeks ago,
navigating by pilotage, I turned on the GPS so I could
report co-ordinates to any would be rescuers.  GPS's
work great to avoid dangerous shoals, but technique
only, they work better when turned on and available at
the helm.

All this talk about compasses brings back bad memories
of old test questions and memory aids;  east is least,
west is best - accelerate north, decelerate south -
variation +/- deviation = true, etc., way to much to
remember this day and age.  Every boat needs a compass
to give it that salty, seaworthy look.  Bill got it
about right.  I think the best compass would be one
with a flat top you could rest your beer (or rum) on. 
Better yet, mount your GPS on it and monitor the
compass swing when you turn it on.

Brad

--- Ronald Lipton <rlipton at earthlink.net> wrote:

> The GPS knows it's location based on the relative
> time of reception
> of signals from the various satellites.  It solves
> an equation like
> (r-distance to satellite i) = speed of light (t -
> time of emission from 
> satellite i)
> so it can only determine it's position with respect
> to the constellation
> of satellites.  Direction and speed are calculated
> from sucessive 
> measurements.
> 
> The accuracy need to achieve on meter resolution is
> about 3 billionths of a
> second.  The fact that all of this works depends on
> corrections due to
> both special (the speed of the satellites) and
> general relativity (the 
> graviational
> field at the satellite radius.  So you all should
> thank Einstein the next 
> time
> you avoid a dangerous shoal.
> 
> Ron
> ----- Original Message ----- 
> From: "Hank" <hnw555 at gmail.com>
> To: "The Rhodes 22 mail list"
> <rhodes22-list at rhodes22.org>
> Sent: Thursday, April 07, 2005 3:01 PM
> Subject: Re: [Rhodes22-list] Hanks & Daves GPS
> Compass Analysis
> 
> 
> > Ed,
> >
> > Try this and see if it makes a difference.  Walk
> down the street with
> > the compass pointed ahead of you and then walk the
> same path with the
> > compass pointed to the side and see if the compass
> heading reads any
> > different.
> >
> > Hank
> >
> > On Apr 7, 2005 3:55 PM, ed kroposki
> <ekroposki at charter.net> wrote:
> >> 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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