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    Re: Call for Hands
    From: Dan Hogan
    Date: 1999 Mar 11, 08:06 EST

    > Maybe I should pose a few questions which have bothered me for quite
    > some time, which i somehow discovered in silicon sea more than a year ago,
    > but i somehow lost trace of it:
    > ----------------------------------------------------------------- -
    > Sailing on the Chart, we use Mercator Projection, right? Now I found
    >   out that for some countries, they use different formulae for the
    >   "Mercator Bit":
    > - US uses an ellipse (one or another "Correction" to the main part of
    >   the formula) as model of the earth.
    > - In Germany, at least the navigation "hobbyists" use the formulae
    >   which correspond to a sphere (no ... additions to the "main" formula). -
    > I dont know much about other countries, but at least there seem to be
    >   more and different "Mercator Projections" floating around, for example
    >   for geodetic (??) purposes like actual chart making.
    The best answers are in a good Navigation manual, Duttons, Bowditch(Pub
    Yes and you will get differences in position from using different
    formulas or other methods also.
    > Now comes the problem:
    > - In Silicon Sea i learned a few years ago, that the american use of an
    >   ellipse-model would be more precise (as you can also find out when you
    >   look at a GPS display, where "Chart Datum" is very improtant), which
    >   caused me some headaches to squeeze both versions into a spreadsheet for
    >   solving silicon sea problems.
    Silicon Sea was based on the U.S. methods presented in pre-1995
    editions of "Bowditch". You would only get EXACT answers to the
    problems if you use the same program and methods.
    >   BTW, I have found that working german navigation exam problems with
    >   those "ellipse type formulae" would produce errors which could result to
    >   a FAIL in an actual test, therefore, those differences are at least
    >   somewhere important...
    Yes they also use different formulae for HE.
    This also varies with different other countries.
    > - However, in astronavigation, we all use a spherical model !
    >   1) Does that make sense instead of being more simple for calculation ?
    The catch is which spherical model of the earth is used.
    >   2) Has anybody ever tried to evaluate the deviations / errors produced
    >      when using those different formulae (I would suspect those
    >      differences to be negligible, but one never knows....)
    Never tried it, too much work. Celestial Navigation is still an "art"
    the accuracy of electronic positioning will never be there. However ,
    if you follow the procedures; maintain a DR and the celestial sight
    reduction. You should be within a reasonable circle of error for
    *practiacal* navigation 4-6 Nautical miles.
    >   3) Are there any programs out there to make printouts for plotting
    >      sheets in which you could adjust for the different formulae, and
    >      would that make sense at all ?
    There are computer programs that adjust for different ellipsis of the
    earth. But why not pick a method that is comfortable for you and stick
    with that?
    >   4) Would we not, at least in principle, have to correct positions
    >      obtained by astronavigation to the correct chart datum to the chart
    >      which is used on the ship ?
    Yes, but the most accuracy that is needed is that provided by a
    Universal Plotting sheet.
    > Can anybode help me out of this mess? Maybe some of you will judge my
    > "problem" to be nonexistant, but I feel it to be quite annoying, when a
    > science and art like navigation would depend on "country" and "politics"
    > :-))
    The science is only in the mathematical foemulae involved. Believe me, the rest is an art.
    Dan Hogan WA6PBY
    Catalina 27 "GACHA"
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