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    Re: Poor St. Hilaire
    From: George Huxtable
    Date: 2007 Oct 17, 00:01 +0100

    John Karl wrote-
    
    | ...In my English translation of his
    | 1873 publication, he refers to the "estimated position" when
    | explaining his intercept method.  This unfortunate terminology has
    | guided many CN students to a Sea of Mystery engulfing the DR position
    | and the confusing concept of "assumed position".
    
    John makes a big thing of such confusion, but to me it seems pretty
    clear-cut and I really can't see where any such trouble arises.
    
    The "estimated position" is simply your best guess at where you are, taking
    everything possible into account, using dead reckoning and any other
    evidence..
    
    However, if you are going to use altitude azimuth tables, working from those
    tables is much simplified, and interpolation avoided, if you work from a
    whole number of degrees for your latitude, and a whole number of degrees for
    your local hour angle. So instead of using your estimated position, you
    choose as a starting point another position, nearby, which meets that
    requirement. That is the "assumed position". Always, it will be within 30',
    in lat and long, of your estimated position. With some tables, such as
    HO214, latitudes are tabulated in half-degree steps, and then the assumed
    latitude will be within 15' of the estimated value.
    
    Working from that assumed position, the tables will provide a calculate
    altitude, to 0.1' or 1', depending on which tables you choose, and you take
    the difference with observed latitude to give an intercept, towards or away
    from the assumed position. And the azimuth, to 0.1 deg or 1 deg, depending,
    tells you in which direction to displace that intercept from the assumed
    position.
    
    Working from a small boat, any of those tabulations is quite accurate enough
    to be better than the errors involved in estimating where the horizon is, to
    measure altitudes.
    
    If you are working with a calculator and not tables, there's no advantage in
    choosing whole-number values, and no point at all in bothering with an
    assumed position. Work directly from the estimated position.
    
    The procedure I've described above is well known to any navigator worth his
    salt. Indeed, there was hardly any point in writing it down, for most
    listmembers. The only reason for doing so was to explain the difference
    between estimated and assumed positions, and the simplicity of those
    concepts.
    
    So tell us, John, where this "Sea of Mystery" comes from. To most of us,
    it's no mystery.
    
    Perhaps, though, I have completely misunderstood what he is getting at.
    
    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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