NavList:

Robert Eno wrote:

> My compass does not have corrector magnets. It is one of those (new) old
> fashioned flat top steering compasses. Dirigo, made in Seattle.

Given Red's subsequent posting, would it be possible to send a pointer
to an illustration of your compass.  The only Dirigos I can find are
manufactured in British Columbia (close to Seattle :-) ) by CommNav.
Looks like they build a variety of professional-looking compasses (and
I'm therefore surprised yours came without compensators).
>
> Nav aids are few and far between in my part of the world but I imagine I
> could find a spot where there are some identifiable landmarks that I could
> use.  I will have to get my chart out tonight and start plotting some
> positions.
>
> Your idea sounds a lot slicker than the way I have been doing it.
> Regardless, I am getting deviation errors as much as 25 degrees. How this
> can happen on a fibreglass boat is a mystery to me.  Perhaps the diesel
> engine in the back is throwing out some fearsome magnetic fields.

The steering pedestal on my 36' sloop is directly above the rear of my
diesel (and thus my compass is about 4' above a 4-cylinder diesel).
Much to my surprise, it seems to have little effect on the compass.  So
I'm having a hard time imagining your diesel throwing you 25 degrees of
deviation (are your compass and diesel both on the keel-line?).  Any
other hidden iron?   My previous boat, which had its compass mounted on
the cabin bulkhead, an eager crew stuffed the interior locker protecting
the rear of the compass with a half-dozen steel food cans at the start
of a cruise, playing hob with my deviation before I discovered the
problem!!!

> I have
> also been told that GPS heading can sometime be off.

Something I should have put in the previous note:  it's important to
distinguish between the a course-made-good reading from a GPS and a
bearing.  GPS's work entirely on determining latitude and longitude.
Bearing to a landmark can be determined very accurately by comparing its
L/Lo to one's current L/Lo (exactly the same calculations that are used
to determine course to a distant waypoint).  But CMG is calculated from
closely spaced L/Lo readings of the position of your boat.  A bit of
jitter in those readings and a GPS's CMG can bounce around a lot (I
think certain GPS models actually incorporate a smoothing function to
cut down on the jitter).   Other than that, the biggest problem with GPS
headings or bearings is the accuracy of the charted location of objects.
    Problem #1 is paying attention to the chart datum, since charted
items' L/Lo can change 1/4 mile or more between WGS-84 and earlier
datums.  Problem #2 is paying attention to the likely accuracy of the
chart.  There are a hell of a lot of charts (eg, of the Pacific islands)
that are based on British Admiralty surveys of the 19th century.  Those
were remarkable surveys, but plugging in thousandth-of-a-minute accuracy
coordinates of an object charted in the 19th century is just asking for
trouble.

Aha, maybe that's an argument in favor of celestial.  A good celestial
navigator knows his/her fixes are likely to be off by a mile or more, so
leaves a wide margin around hazards, etc.  Hand a bad navigator a modern
GPS and they'll easily enter highly precise coordinates for hazards and
never give a thought to the accuracy (vs precision) of those coordinates.

Lu Abel

PS - before anyone takes exception, my comment above about GPS's working
solely based on latitude and longitude measurements is for basic
hand-held or chartplotter marine GPS's.   Not included are Furuno's "GPS
compass" (which calculates north by comparing the phases of the signals
received by an array of GPS antennas) or aviation or military GPS's that
may use other techniques to derive VMG.

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