NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: What precision is required in cel nav?
From: Stephen N.G. Davies
Date: 2015 Jul 17, 11:08 +0800
From: Stephen N.G. Davies
Date: 2015 Jul 17, 11:08 +0800
Well, I was an RN navigator, and they seemed to think that + or - one mile was quite good for everyday work (a bit different for the surveyors!). As an ocean racing navigator (back in the days before GPS) landfall accuracy to + or - a mile was well better than most could manage (yachts lurch about a bit), though if one could get that close, it could be a race winner (once was for me). I suspect that key variables are going to prove to be visibility and the nature of the waters near landfall. In excellent visibility super-accuracy matters less because one get the necessary visual clues to start coastal navigation well outside a + or - 1 mile CPE (I’ve taken a dawn bearing on the high mountains of northern Luzon at something like 100 miles out (very useful)). Equally, if a coast is steep to and without a litter of offshore hazards in the approaches, being accurate to + or - 10 miles is OK because, again, coastal marks will be visible in good time - inbound to HK from the Philippines I’ve picked up the chain of the Lema Islands and the high land of HK and the China coast when 40 miles out (c.25 miles from the Lemas) - a raft of bearings on all the visible peaks and then some tracing paper and getting all the bearings to line up on identifiable peaks had a visual fix (that actually confirmed what had felt like a rather iffy sun-run-sun snatched during some momentary clearances during the forenoon watch).
Like most navigation, a bit suck it and see.
It’s obviously how the old hands did it too. I re-ran the passage of the East Indiaman Albion on a voyage to China and back in 1787-89 from a 4th mate’s logbook not long back. It was very interesting how primarily dependent the passage was on position check landfalls were. Daily navigation (this was pre- normally embarked chronometers (or the managing owners were skinflints)) was traverse table stuff plus a daily latitude sight at mer. pass. Other than landfall checks (Madeira was the only landmark seen between The Lizard and the Cape of Good Hope) the only cross-checks were provided by TWO lunars. One came 9 hours after having taken a position from Madeira, perhaps because of doubts over estimations of distance off (the log doesn’t say), the other just north of the Equator, the longitude from which differed by 26’E from the DR position. There were NO lunars observed between the Cape, Johanna in the Comoros and landfall at Ceylon (Sri Lanka) - the whole was done by latitude observations and DR!
Best,
Stephen D
Dr Stephen Davies
Department of Real Estate and Construction
EH103, Eliot Hall
University of Hong Kong
Office: (852) 2219 4089
Mobile: (852) 6683 3754
stephen.davies79@gmail.com
daiwaisi@hku.hk
Department of Real Estate and Construction
EH103, Eliot Hall
University of Hong Kong
Office: (852) 2219 4089
Mobile: (852) 6683 3754
stephen.davies79@gmail.com
daiwaisi@hku.hk
On 17 Jul, 2015, at 10:41 am, John D. Howard <NoReply_Howard@navlist.net> wrote:A question for all:
What precision is required when doing cel nav ?
The reason I ask is I am a pilot and when we had to do cel nav in the C-141 trying to find Canton I. or Wake we were happy to get a fix + or - 10 miles. ( I was happy - maby my navigator was not. )
It seems to me a ship in the middle of the ocean does not NEED one mile accuracy. Close to port or hazards yes but in the Pacific ?
I just revieved the book " Hawaii by Sextant" from Starpath and it goes to great lengths doing statics to get a set of sights to yeild a fix 1/2 mile better. Is that extra work nessasry or pride ina good job?
Don't want to step on any toes, just want to know. Flying is different the sailing.
Thanks -- John H.
