[Rhodes22-list] Sextants

Tue Oct 12 15:59:03 EDT 2004

Bill,

You mean you don't triangulate compass bearings to determine a position fix
independently of GPS?  Why do you think they make hand bearing compasses?  I
have a bearing compass built-in to my Bushnell 7X50 marine binoculars.  This
type of position fix is one of the 1st things they teach in any navigational
course.  It's been my experience that I can reliably shoot a compass bearing
to better than +/-5 degrees accuracy, even from the deck of a small boat.  I
only apply the local magnetic declination for a correction factor, which is
printed on any navigational chart.  I stand away from magnetic metals when I
take the compass bearing & assume the deviation correction is zero.  Since
we're talking about coastal navigation here, the distances involved are not
global in scope.  Many landmarks are difficult to recognize over a mile
away.  Over a mile, a 5 deg error in compass bearing causes a position error
of:

[Error] = 5280 ft * tan(5) = 462 ft.

With two or three overlapping compass bearings to triangulate on known
landmarks, the errors in the individual lines of position tend to cancel out
& the final position accuracy is usually good to +/- a couple hundred feet
as checked against GPS.  OK, it's not GPS accuracy.  But, the individual
compass bearings are quick to plot on the chart - much quicker than a
sextant sighting can be worked up.  No batteries, no calculator to fail at
the wrong moment, no books of tables, & it works on a cloudy or overcast day
or night.  Just a bearing compass, a chart, a parallel ruler, & a pencil.

As far as the laser rangefinder not being able to lock onto a target quickly
enough from the deck of a small boat, that doesn't seem to be a problem.
You put the crosshairs on the target which is magnified 7X, push a button, &
the distance measurement is made essentially instantly.  You can repeat the
measurement & recall the last 10 readings to check reproducibility.  I
suppose you could average those 10 readings if you wanted even better
accuracy.  The instrument also makes a measurement of the reflected laser
light off the target & reports a "target quality" index to alert you if you
might be getting a bad reading due to insufficient signal.

I believe it's prudent to have as many options as possible to determine
one's position.  A compass bearing & laser range to a known landmark would
certainly be better than a dead reckoning plot.  I'm not claiming my laser
rangefinder is the ultimate aid to navigation or even the 1st instrument I'd
purchase.  But, it adds coastal navigational capability &, for many
purposes, it would be good enough.

Roger Pihlaja
S/V Dynamic Equilibrium

----- Original Message -----
From: "Bill Effros" <bill at effros.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Tuesday, October 12, 2004 1:32 PM
Subject: [Rhodes22-list] Sextants

> Bob,
>
> Compasses are wildly inaccurate, including fluxgates.  Nobody corrects
them anymore.  Most of them can't even be corrected.  Roger's toy may be
interesting for other reasons, but not as a navigational tool.  The boat is
moving, you are moving, and the compass bearing is a guess at best.
>
> A sextant is a much better tool.  Start on land.  I can put myself within
6 feet of my actual location on land.  Sextant bearings on a boat are
harder, but the people who know how to do it, and practice all the time will
give you better bearings than laser range finder binoculars every time.
Sextant bearings are more reliable than GPS bearings.  Last time I checked
their was still a guy on all our big military ships taking sextant bearings.
>
> You can use a sextant horizontally to get compass bearings far more
accurately than fooling around with those oil filled things with a magnet
inside.
>
> You can get a high quality plastic sextant for $250 that will test the
accuracy of a traditional fix far more accurately than a laser range finder
binocular.  There are numerous programs available so you just plug your
readings into the program to calculate the full sight reduction.
>
> Compass bearings used as Roger proposes also require computers to
calculate the compass deviation for the exact spot you think you're at,
unless you know how to use a pelorus--another dying art--another forgotten
tool.
>
> Bill Effros
>
>
>
> To download a free copy of the electronic book "Quote Without Comment"
>
> Click on or copy this address and load it into your web browser:
> http://www.quotewithoutcomment.com/qwc.cgim?template=FreeBook
>
> Want to see more quotes?
> http://www.QuoteWithoutComment.com
>
>
> ----- Original Message -----
> From: Bob Weber
> To: rhodes22-list at rhodes22.org
> Sent: Tuesday, October 12, 2004 11:53 AM
> Subject: RE: [Rhodes22-list] Radio - FM Recption - GPS Map
>
>
> Brad, right now I am pushing for Rogers laser range finder, binoculars,
> compass, thing.  I would propably have a heart attack at the price.  But
if
> it had a bottle opener and tweezers that might sway the deal.  Seriously I
> am looking toward getting certified to teach navigation and that could
> really help with fixes or at least test the accuracy of a traditional fix.
> Bob
>
> >From: brad haslett <flybrad at yahoo.com>
> >Reply-To: The Rhodes 22 mail list <rhodes22-list at rhodes22.org>
> >To: The Rhodes 22 mail list <rhodes22-list at rhodes22.org>
> >Subject: RE: [Rhodes22-list] Radio - FM Recption - GPS Map
> >Date: Tue, 12 Oct 2004 08:18:34 -0700 (PDT)
> >
> >Bob,
> >
> >About half the DC-10 fleet has Inertial Nav and half
> >has two VOR's, same as a Cessna 150.  Often times
> >center will give us direct to somewhere and I have to
> >reply, "we're navigationally challenged" or "we're
> >heading equipped". Some guys were carrying handhelds
> >but the FAA and the company put the squash on it.  The
> >non jurrasic jet equipment, Airbus and MD-11's have
> >GPS moving map displays.
> >
> >Frankly, my handheld is more than enough for the boat.
> >  Christmas is coming though and I don't know what to
> >buy for myself.
> >
> >Brad
> >
> >
> >--- Bob Weber <ruba1811 at hotmail.com> wrote:
> >
> > > Brad, I have found the GPS to be great on the boat.
> > > I have brought it in
> > > the car before but never found a use for it.  If I
> > > were to be travelling to
> > > parts unknowen it would be helpful.   It doesnt have
> > > verbal comands "turn
> > > right in 50 feet" but you should be able to get that
> > > from the display.  I
> > > cant believe you dont already have a handheld GPS
> > > for backup while flying.
> > > I guess it is more helpful in the egg beaters than
> > > in commercial jets.  As
> > > far as XM.  I am not in a rush to pay for radio.  We
> > > all were in a hurry to
> > > pay for tv.  Remember the promise.  Pay monthly and
> > > get commercial free tv.
> > > That didn't last too long.  I am due for a new car I
> > > will keep that in mind
> > > when ordering the stereo equipment on the car.  If
> > > it is removable, it will
> > > be a no brainer.  Bob
> > >
> > > >From: brad haslett <flybrad at yahoo.com>
> > > >Reply-To: The Rhodes 22 mail list
> > > <rhodes22-list at rhodes22.org>
> > > >To: rhodes22-list at rhodes22.org
> > > >Subject: [Rhodes22-list] Radio - FM Recption - GPS
> > > Map
> > > >Date: Tue, 12 Oct 2004 05:27:36 -0700 (PDT)
> > > >
> > > >Am I the only one on a lake with almost no FM radio
> > > >available? Last Christmas I purchased an XM
> > > satellite
> > > >radio for CoraShen and it has turned out to be a
> > > >wonderfull investment.  Now I don't have to drag
> > > CD's
> > > >to the boat or listen to "swap shop" and the
> > > >obituaries on the local stations.  Both Sirrius and
> > > XM
> > > >get about 160+ stations and the reception is
> > > >outstanding.  The equipment is about $125 and the
> > > >service $10 per month.  I move the equipment to my
> > > car
> > > >when not on the boat.
> > > >
> > > >This experiment with dual use equipment has gotten
> > > me
> > > >started looking at portable GPS moving map
> > > displays.
> > > >The Garmin 176 looks interesting.  You can use it
> > > on
> > > >the boat and then in the car as well.  Anyone on
> > > the
> > > >list have experience with one?
> > > >
> > > >Brad Haslett
> > > >"CoraShen"
> > > >
> > > >
> > > >
> > > >_______________________________
> > > >Do you Yahoo!?
> > > >Declare Yourself - Register online to vote today!
> > > >http://vote.yahoo.com
> > > >__________________________________________________
> > > >Use Rhodes22-list at rhodes22.org, Help?
> > > www.rhodes22.org/list
> > >
> > >
> >_________________________________________________________________
> > > Get ready for school! Find articles, homework help
> > > and more in the Back to
> > > School Guide!
> > > http://special.msn.com/network/04backtoschool.armx
> > >
> > > __________________________________________________
> > > Use Rhodes22-list at rhodes22.org, Help?
> > > www.rhodes22.org/list
> > >
> >
> >
> >
> >
> >__________________________________
> >Do you Yahoo!?
> >Yahoo! Mail Address AutoComplete - You start. We finish.
> >http://promotions.yahoo.com/new_mail
> >__________________________________________________
> >Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>
> _________________________________________________________________
> Don’t just search. Find. Check out the new MSN Search!
> http://search.msn.click-url.com/go/onm00200636ave/direct/01/
>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>
>
>