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    Re: Position lines, crossing
    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 Fogg
    Sent: Sunday, December 10, 2006 4:00 PM
    Subject: [NavList 1861] Re: Position lines, crossing

    Thanks 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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