NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fw: Re: Still on LOP's
From: George Huxtable
Date: 2002 Apr 27, 22:37 +0100
From: George Huxtable
Date: 2002 Apr 27, 22:37 +0100
Dan Hogan wrote- >How about comparing it with your Dead-Reckoning position? > > 25 Apr 2002, at 23:31, George Huxtable wrote: > >[Snip] >> This was the attitude taken by old-school navigators in the erroneous >> belief that the true position must always be embraced by the cocked hat. >> >> It interests me what degree of safety Barrie thinks he is achieving when he >> takes that part of the cocked hat that is closest to land as his position. >> >> It has, I hope, been generally accepted by most listmembers by now that >> there is only a one-in-four chance that the true position will be contained >> within the cocked hat at all. If a navigator insists on assuming, against the >> evidence, that he must be within the cocked hat, then he will get a marginal >> improvement in his safety by presuming that he as at that part of the >>triangle >> which is nearest the danger, rather than at its centre. But he is being >>unduly >> complacent, and there is a strong chance that he may be significantly nearer >> still to the danger. The only safe thing to do is to assess the likely errors >> involved, in the light of his previous experience, and allow an appropriate >> offset. This offset should always be greater, and might be many times >>greater, >> than the dimensions of a particular cocked hat. > >Cheers >-Dan- ================== Dan is quite right, of course. The position from a 3-bearing or 3-Sumner-line fix has to be given appropriate weight, and combined with other information, such as the DR position taken from an earlier fix, and perhaps soundings too, where these are relevant and weighted accordingly. Everything of any relevance should be taken into account. The master of a barque which had been beating against the Westerly winds and the current swirls off the Horn for a week without a view of the sky or the land (and that wasn't uncommon) would put little reliance on his DR. The true art of the traditional navigator is in estimating what weights to attach to the various bits of information, and thinking out how to combine them (in his head, most likely) to estimate a position and to estimate the possible errors in that position. That's an art which is becoming lost since satellites took over. Comparing a fix with your dead-reckoning position, as Dan Hogan suggests, can (if they agree) enhance confidence in the result. That confidence may be spurious if, as is possible, both are in error. Only if a previous fix was precise and recent, and the sailing in-between has been in an area where currents are small and well-known, is the dead-reckoning likely to add much to the information obtained in a 3-point fix. On the other hand, if the two positions being compared are discrepant, outside the limits of the expected errors, then that will at least inform the navigator that "something is up" and he will learn to distrust both the DR position and the fix until he has discovered what the problem is, or re-measured. It's all a matter of commonsense, really. I should add that others on this list (including Dan Hogan) have a much wider practical experience in such navigation than I do, so my pontifications should, accordingly, be given an appropriate weight. George Huxtable. ------------------------------ george@huxtable.u-net.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------