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Herbert

> Paul Hirose has surely found the most sophisticated way of landing in
> the reefs with the combined assistance of GPS _and_ a sextant! A GPS
> receiver normally shows distance between two points along the geodesic.
> For the points in the given example the answer using WGS84 is 2229 nm,
> being 3 miles off the GCD. In a worst case scenario, (e.g. establishing
> latitude at N45 from the sun at winter solstice) the discrepancy is 20
> miles!

My Garmin 76 agrees with your 2229 figure--2229 nm using WGS84 datum is the
exact figure my Garmin 76 "LDPW" spit out (approx 3.5 years old, original
shipping 2.06 firmware)
>
> In order to use your GPS receiver as a makeshift sightreduction
> computer, you have to cajole it into using a spherical earth model. On
> some models this is not a trivial task - if possible at all. (To say
> nothing of the practical aspect of making sure that a GPS receiver that
> has been upset in such a fashion does not end up in the nav station by
> mistake, quasi as a time bomb ready to explode at the moment of
> landfall. But practical aspects hardly play any role in this discussion.)

As I have posted before, the 76 gives great-circle distance (documented) and
initial great-circle course; as well as continuos course updates
(undocumented) as standard.  It does not have to be cajoled into doing this.
I know of no standard menu items to stop it from doing this.

As to your comment, "Paul Hirose has surely found the most sophisticated way
of landing in the reefs with the combined assistance of GPS _and_ a
sextant!" I whole heartedly disagree.

There are many more efficient ways:

Don't cross check your navigation tools/position with every means possible.

Trust your sextant, reductions, timepiece, auto-helm and GPS without
question.

Don't cross check your GPS, radar, RDF etc. against each other.

Don't do the reverse time shot posted earlier to check your timepiece.

Don't use coastal piloting techniques.

Trust charts to be 100% accurate.

Don't keep an eye on your depth meter.  Don't set its alarm.

Don't watch the waves for indications of a shoal or reef.

Don't bother with checking the compass with cel nav, boxing the compass, or
updating charts with recent magnetic variation.

Don't bother with determining your crafts leeway under various points of
sail, wind, and sea conditions.

Don't keep a regular log.

I had long ago recounted tales of one leg of the (Chicago to...) tri-state
race where several boats with the latest and greatest racing software and
instrumentation (pods all over the mast with readouts of VMG, target speed,
etc.) ran aground in southern Lake Michigan in heavy fog.  (And you just
can't get closer to point-and-shoot than southern Lake Michigan.) I'm sure
they knew their lat and long to 100 meters or less.  They just didn't check
out just where that lat and long actually was on a chart.

Paul's method requires active participation in the navigating process (and
cross checking instruments).  After all, removing the merchant marines,
military, and navigation buffs on the list, it is pleasure boating.  Drink
heavily, relax, and enjoy ;-)

I do share your implied concern regarding the his nm figure/HC, and
resultant intercept/correction to EP.  In the real world, I would steer well
clear of a reef.  If it came down to picking a passage through a reef, I
would put my trust in local knowledge and every scrap of information I could
glean from  multiple sources.

Having been deprived of making a pleasure-sailing passage at 45N near the
winter solstice--and for future reference--could anyone on the list please
provide me approximate locations of the reefs I need to avoid?

Bill

   

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