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Andres wrote, about meridional parts-


"The model in Bowditch current edition for meridional parts is wrong.

See also the e-mail: [NavList 5837] Current Bowditch errors"


and pointed us, via this link, to a paper

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=297304

A Comment on Navigation Instruction. Michael A. Earle

The Journal of Navigation, Volume 58, Issue 02, May 2005, pp 337-340

=========================

Unfortunately, that link provides only an abstract of the paper, not the
whole thing, for which you need to pay a subscription.

However, I have that journal in print form, and would be happy to share the
article, if anyone asks. It's three A4 pages, and a bit.

Indeed, I hope someone does, because it contains an equation for meridional
parts which I have tried to implement but have been unable to obtain
sensible answers. So I would like someone else to check out what's going
wrong! It could well be a silly error on my part.

==========================

I suspect that the discrepancies between various calculations, that Andres
refers to, result from imprecise definitions of the nautical mile. If it's
assumed to be the length around theEarth's surface that subtends an angle of
1 minute between two local verticals, then when the Earth is taken as an
ellipsoid, that length varies (a bit) with latitude. It's even different, at
the same spot, when measured in different directions, because the Earth's
curvature is different measured North-South to what it is measured
East-West. For most purposes, we compromise on an average value of 1852
metres, which would be appropriate for a sphere. However, for geodetic
purposes of precise survey, a value for the "geodetic mile" is taken that
puts exactly 360 degrees, or 21600 miles, around the Earth's equatorial
circumference, making a geodetic mile 1855.3248 metres.

The divergence between different values for meridional parts seems to relate
to the difference between those different mile-units, of no more than 0.18%.
Hardly relevant to navigators, that, though it might matter to geodesists.
In this case, may we be preserved from the attentions of such nit-picking
purists! I am aware that this puts me in unfamiliar territory, as most
times, I'm in favour of nit-picking puristry.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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