NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Coastal Plotting Sheets
From: George Huxtable
Date: 2007 Apr 1, 23:26 +0100
From: George Huxtable
Date: 2007 Apr 1, 23:26 +0100
I suggest that the point of horizontal sextant angles, compared with compass bearings, is being forgotten. The difference lies in the precision, which is why the technique is more important to hydrographic surveyors, to whom precision really matters, than to navigators, to whom it usually doesn't. If you try to determine an offshore position in relation to coastal features or landmarks, magnetic compass bearings don't establish a bearing to better than a degree or two, even in calmish conditions. If you are plotting 2 or 3 bearings, that error applies to each. If you want to find the angle between two landmarks, the error in the difference is somewhat greater. And that's just the magnetic bearing. To make it true you have to correct for variation, and also for deviation, which depends on the vessel's heading, and those add additional error (though difference angles are unaffected). Compare that with what an old hand with a sextant can manage, even in rough conditions. He can measure the horizontal angle between two landmarks to less than an arc-minute, without problems. Much more precisely that he can plot, indeed. For distant objects, over the horizon, he can measure from high up, in the main top (when ships had them). Yes, to fix by horizontal sextant angles you need to have three marks on shore, and in the right sort of geometrical alignment to give a good cut between the circles, as against only two marks for compass bearings. But in terms of the precision of the result, there's really no contest. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---