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Hi Lu,

Let's clarify one thing before going any further. This discussion has
related, I think, to the intersection of lines of position obtained by
celestial observation, not to the accuracy of the resultant position which
may not be reflected by the nicety of intersection. One criterion sometimes
applied is that of "repeatability" , i.e., the coincidence of multiple
positions obtained in a static location over a period of time and obtained
by the same means - some years ago, the USCG established experimental Loran
stations for just this purpose, and the results seemed rather amazing as
respects the divergence of the resultant positions. Although they have
almost certainly been done, I know of no similar experiments with GPS, or
for that matter any other form of position finding - potential accuracy
would certainly be better served by such work, rather than by repeated
quotations from texts, the authors of which may well have never have
navigated a ship at sea and are published with the dollar market uppermost
in mind. It should be remembered that, until relatively recent time, most
charts were constructed on the basis of celestial observation, usually but
not always utilizing the artificial horizon. Most of these charts, albeit
not all, have been proved to be amazingly accurate - as a matter of simple
fact, in the late 1940s, there being no other charts immediately available,
I was obliged to use a 1908 BA blueback chart to navigate portions of the
West African coast, and found no appreciable discrepancy of position
anywhere.

For years, my only gauges of position accuracy were that the ship remained
always afloat, and the accuracy of any particular landfall. I have had
perfectly wonderful looking fixes and, a few hours later made the intended
landfall 6-miles or so either side of the intended course line while at
other times it was dead on. On one occasion, it was predicted that we would
raise Green Point Light at about 0600 - I was called at 2400 with Green
Point and Slangkop clearly above the horizon and the 3rd Mate in a bloody
panic; cross bearings quickly established that we were some 52-miles off
Green Point and with plenty of time for a good snooze before arrival. It was
the most significant case of refraction that I recall witnessing at sea.

Any position established at sea by whatever means must be viewed with
suspicion and the navigator must use every trick in his bag, the price of
safety being eternal vigilance - and nothing could be truer as respects
navigation. That being said, I simply refuse any static textbook definition
of sight accuracy - such definitions, in my humble opinion, lead to sloppy
if not careless workmanship and demean the potential of celestial
navigation; generally they lead to excessive "rounding off" which in itself
is conducive to accumulated errors. I have had good, bad, indifferent and
wonderful results with celestial navigation and it was my job to be able to
differentiate, based on existent conditions whatever they might be.
Normally, I endeavored to work to the accuracy permitted by commercially
available plotting sheets, and am of the opinion that average results, under
good conditions of horizon and ship steadiness, were probably in the range
of 2.5-miles of the truth with more often than not good LOP intersections.
Let there, however, be no mistake, there were also bad results - just not
too many.

Regards,

Henry
----- Original Message -----
From: "Lu Abel" 
To: 
Sent: Monday, December 11, 2006 12:56 AM
Subject: [NavList 1881] Re: Position lines, crossing


>
> With deepest respect, Henry, a previous poster pointed out that most
> texts suggest that LOPs taken at sea are at best good to +/- 2 NM and
> that for small craft +/- 5 NM is more likely.   I agree.   So I'm
> wondering under what conditions your sights were taken and/or what scale
> you used in plotting them!
>
> I don't mean in any way to demean your skill, I just am reflecting the
> track of this discussion, which is that any navigator worth his salt
> will not only be able to plot a LOP on a chart, but also fully
> understand the uncertainties associated with it.
>
> Lu Abel
>
> halboth wrote:
> > Hi Peter,
> >
> > I most certainly am Peter, and have hundreds, if not thousands, of
> > positions in my navigation workbooks to demonstrate this fact.
> >
> > Regards,
> >
> > Henry
> >
> >     ----- Original Message -----
> >     *From:* Peter Fogg 
> >     *To:* NavList@fer3.com 
> >     *Sent:* Sunday, December 10, 2006 11:55 PM
> >     *Subject:* [NavList 1877] Re: Position lines, crossing
> >
> >     Henry wrote:
> >
> >         In my opinion, "cocked hats" are most often the result of
> >         altitude errors induced by varying horizon conditions due, in
> >         the case of stars, to failing light conditions.
> >
> >
> >     Henry, are you saying that given ideal conditions (as you have
> >     described) LOPs can in practice meet, or almost meet, at a common
point?
> >
> >
> >
> >
> >     >
>
> >
>


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