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    Sextant accuracy
    From: Peter Fogg
    Date: 2003 Mar 18, 11:09 +1100

    On a small boat out at sea I think of a fix within 10 nm of the actual
    position as being good, within 5 nm as excellent, and as the manual
    system of sight reduction I use only inputs and calculates values to
    the nearest nm or minute of arc then that is as good as can be got. In
    practice the various roundings can tend to cancel themselves out (most
    often) or accumulate so the final result without mistakes can indicate
    2 or more rarely 3 nm of error. As others have pointed out; far from
    land in slow moving craft this is more than good enough.
    
    The big advantage of the system I use is its simplicity and
    compactness, everything needed being contained in the one slim book. I
    think the theoretical advantage of the systems that use and calculate
    values to the nearest tenth of a nm is misleading, all that extra bulk
    of books and work of calculation to achieve this result is worthless
    when the various practical difficulties of taking sights from small
    unstable platforms tend to lead to errors of many nms.
    
     From a known fixed position on land its a little different, and using
    my navigational calculator which does accept data to many decimal
    places and displays results to the nearest tenth of a minute of arc or
    nm the actual position can be located to that level of accuracy -
    within 185 metres.  I can clearly remember (it wasn't all that long
    ago!) our class taking its first sights from a suburban headland
    overlooking the sea, the instructor checking the intercepts with his
    calculator and announcing 'You're in the carpark' where we were
    standing. It seemed like magic then and to some extent it still does.
    One practical problem I face taking sights from headlands is I may have
    to guess at the height of eye. Once I have established the position
    (sometimes the DR may be a guess too) then I believe it may be possible
    to calculate the dip by re-iteration.
    
    The key to this accuracy, the essential technique if accuracy is at all
    important, is to plot as many sights of the one body within 5 minutes
    of time as you can make and compare them with the slope of the body's
    apparent rise or fall, as detailed on this list previously. Otherwise
    you are simply taking a punt with any individual sight and its myriad
    of possible errors.
    
    As to whether CN has a place in coastal nav: I think it does. Along a
    coast with distinct features over a wide arc then it is quicker and
    simpler to plot 3 LOPs directly from them. But if you're out on the
    edge of the continental shelf, about 10 nm from land, as you may well
    choose to be to take best advantage of a current, the land could be
    just a smudge on the horizon and CN is the way to go. Even closer, you
    may have only one shore feature to use, so it can be very useful to
    observe the sun or moon to give you another LOP to play with. An LOP is
    an LOP, it doesn't matter where it comes from. You could just as well
    use a bearing to a radio source as a useful LOP to plot with the
    others. Lacking a Radio Direction Finder, let me say that the cheapest
    and nastiest of portable transistor radios are the best to swivel
    around to locate a bearing. The problem may be to find where the
    transmission tower is located - charts only tend to locate them if
    close to the coast, unfortunately.
    
    
    

       
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