NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Position lines, crossing
From: Henry Halboth
Date: 2006 Dec 10, 23:45 -0500
From: Henry Halboth
Date: 2006 Dec 10, 23:45 -0500
Hi Peter,
Sorry, but I have repeatedly spoken of the
methodology of observing bodies in opposition in previous posts, a matter
which has always been studiously ignored. From the precision point of view, it
is thoroughly discussed in "Hydrographical Surveying" by Wharton & Field, a
standard work on precision position finding by astronomical means - I know
that Frank has a copy of this work, but seems much too wise to weigh in on this
highly theoretical discussion.
From a practical point of view, given varying
horizon conditions possibly existent, all sights taken in the establishment
of a position are not necessarily equal; if they were, and assuming no
instrumental or time errors, there should be no significant "cocked
hats".
In my personal experience there were very few
"cocked hats" when using precomputed altitudes and a clear cut horizon,
i.e., observing stars a few minutes before/after sunrise/sunset on a
clear horizon, before the bodies were visible to the naked eye. When
conditions allow, this methodology is the simple secret to accurate stellar
positions - far above the accuracy generally discussed on this List or in most
texts. The method, of course, requires preparation in advance; to wait until the
stars can be identified by naked eye is usually too late to obtain an accurate
fix.
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. Although George is obviously textbook
correct in his analysis, it is probably more correct to evaluate the individual
sights employed, based on the conditions particular to each, and to discard
any taken under less favorable circumstances of horizon or otherwise. Generally
speaking, to me a large "cocked hat" almost always spelled "mistake" in capital
letters, and was viewed with suspicion, not technically analyzed.
Regards,
Henry
----- Original Message -----From: Peter FoggTo: NavList@fer3.comSent: Sunday, December 10, 2006 4:00 PMSubject: [NavList 1861] Re: Position lines, crossingThanks to Geoffrey for his detailed and clear explanation of the use of bodies in opposition to reduce systematic error. I can't remember this having being discussed here in the past.
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