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    Re: Averaging sights
    From: Robert Gainer
    Date: 2004 Oct 29, 14:40 +0000

    The discussion about averaging data has been very interesting and for me the
    math has been quite a challenge to follow. From a practical point of view I
    have quite a different take on the whole process. I use the sextant as a
    tool to provide needed information. If the weather is good you don�t need
    the same degree of accuracy in navigation that you need in times of reduced
    visibility or extreme conditions and those are the times that celestial
    navigation is at its worst. During the good times you have many other tools
    that you can use to confirm your position and landfall. The simplest one is
    just looking up from the chart table and seeing land. Under those conditions
    even a poor sight will tell you if the harbor is above or below your
    landfall.
    
    A landfall under poor conditions is the worst situation that the navigator
    will ever find himself in. If you can only get a few quick shots though the
    overcast you need to get a feel for the validity of the sight without the
    luxury of time and averaging data that you just don�t have. With enough
    practice you can develop a feel for if the shot is worth anything and how
    far out it might be. You will of course have your taffrail log over the side
    to use as a check on the reasonableness of the position you get from the
    sight. The currents and leeway are very constant things and easier to allow
    for then the unknown of the sight. The angle that the taffrail log line
    makes with your course is the leeway that you are making though the water.
    The currents offshore are well known and covered in great detail by several
    publications. A DR that is maintained with care can be very accurate over a
    72-hour period. The last 48 to 72 hours is the most critical time period in
    a single-handed trip. If you are off in estimating the landfall and need to
    stay awake for more then 72 hours you are almost certainly going to get
    yourself into trouble.
    
    A more important point then the accuracy of the data itself is that I think
    you need to select the stars so that the LOP will give you the most useful
    information and if you select the right pair of stars the errors of the
    instrument and most other errors are canceled out. My plan has always been
    to take sights so that the LOP�s will give me the distance from the coast
    with the least amount of doubt. A LOP that is parallel to my course is not
    as useful as one that is parallel to the coastline. An early shot of the sun
    will give you a better approximation of your time of landfall then a noon
    sight regardless of the errors involved. Of course in better conditions you
    can get as many shots as you want, but you might see land before you have
    done all the calculations. Navigation to me is not the goal itself but is
    the means to an end.
    
    My expectation in using a sextant is to get a position at landfall within 3
    to 4 miles, which is close enough to make the transition from offshore
    navigation to coastwise navigation. When I am on soundings the sextant is
    occasionally used to get a LOP that will give me a position along the
    coastline where landmarks are scarce. There the accuracy is not very
    important to me because you are just getting a guideline as to the distance
    traveled between stops. The water depth will usually provide a better guide
    to the distance offshore then the sextant in those situations.
    All the best,
    Robert Gainer
    
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