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Subject: [Nml] Re: Call for Hands
HT.Feuerhelm@XXX.XXX
Date: Wed Mar 10 1999 - 17:13:36 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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