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    Re: Call for Hands
    From: Tom Feuerhelm
    Date: 1999 Mar 10, 17:13 EST

    Dear ALL,
    When I read that Call for hands, I could no longer resist from
    answering!! YES, THERE ARE INDEED more people watching closely all of
    your discussions and deliberations! Some of them only have little
    time....
    When you call for HANDS, i myself could, because of lots of other things
    to be done, at this moment only offer my FEET, and it is very difficult
    to do the necessary trig an HO and DR and SOG and so on with feet only
    ;-))
    Seriously, I do enjoy and learn a lot from all those discussions,
    therfore, please do not despair and keep the thing going!!!
    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.
    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.
      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...
    - However, in astronavigation, we all use a spherical model !
      1) Does that make sense instead of being more simple for calculation ?
      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....)
      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 ?
      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 ?
    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"
    :-))
    My very best regards to all,
    AND keep up the good work!
    Tom Feuerhelm
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