NavList:
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Re: Interpolation of Meridional Part Table
From: George Huxtable
Date: 2009 Mar 26, 00:48 -0000
From: George Huxtable
Date: 2009 Mar 26, 00:48 -0000
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. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.navlist.net To post, email NavList@navlist.net To , email NavList-@navlist.net -~----------~----~----~----~------~----~------~--~---
