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    Re: Coastal Plotting Sheets
    From: George Huxtable
    Date: 2007 Apr 1, 23:26 +0100

    I suggest that the point of horizontal sextant angles, compared with compass
    bearings, is being forgotten.
    
    The difference lies in the precision, which is why the technique is more
    important to hydrographic surveyors, to whom precision really matters, than
    to navigators, to whom it usually doesn't.
    
    If you try to determine an offshore position in relation to coastal features
    or landmarks, magnetic compass bearings don't establish a bearing to better
    than a degree or two, even in calmish conditions. If you are plotting 2 or 3
    bearings, that error applies to each. If you want to find the angle between
    two landmarks, the error in the difference is somewhat greater.
    
    And that's just the magnetic bearing. To make it true you have to correct
    for variation, and also for deviation, which depends on the vessel's
    heading, and those add additional error (though difference angles are
    unaffected).
    
    Compare that with what an old hand with a sextant can manage, even in rough
    conditions. He can measure the horizontal angle between two landmarks to
    less than an arc-minute, without problems. Much more precisely that he can
    plot, indeed. For distant objects, over the horizon, he can measure from
    high up, in the main top (when ships had them).
    
    Yes, to fix by horizontal sextant angles you need to have three marks on
    shore, and in the right sort of geometrical alignment to give a good cut
    between the circles, as against only two marks for compass bearings. But in
    terms of the precision of the result, there's really no contest.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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