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Subject: Re: [Nml] Re: Call for Hands
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Thu Mar 11 1999 - 08:06:10 EST
HT:
> 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
No.9)
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
dhhogan@XXX.XXX
Catalina 27 "GACHA"
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