NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How accurate are fixes in practice?
From: Peter Fogg
Date: 2008 Aug 4, 11:15 +1000
--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Peter Fogg
Date: 2008 Aug 4, 11:15 +1000
Bill wrote:
From an approx. 11 meter sailcraft under
good conditions I would expect individual intercepts to be +/- 1.6 nm or
better 66% of the time, with an average intercept (plot fitted to Hc line as
per David Burch) better than 01.5 nm.
(average them on the fly then do a plot against
the actual Hc slope later).
How is this averaging on the fly achieved, Bill?
From an approx. 11 meter sailcraft under
good conditions I would expect individual intercepts to be +/- 1.6 nm or
better 66% of the time, with an average intercept (plot fitted to Hc line as
per David Burch) better than 01.5 nm.
Sounds pretty good to me, given those somewhat trying conditions. Its interesting that, if I have understood correctly, your actual sights have been improved via some sort of comparison with slope technique. And yes, please send me that PDF of the Burch article.
My worst set (first on-the-water experience) was using a kit cardboard
sextant against sand dunes with a hazy horizon and confused 8-10ft seas on a
34' fin-keel sailboat. My intercepts were about 5-7 nm off, all in the same
direction.
Still rather good for cardboard. I'll have to lift my game.
My second worst (using Astra) was a day with 4 ft seas but with the
totally-obscured horizon mentioned above. I looked at distance boats'
waterlines, and adjusted with dip short to guess about where above the
bottom and below the top of the mess the horizon might be. The range of
intercepts was approx. plus/minus 6'. Using the Hc slope method the
"averaged" outcome was almost spot on. Better lucky than good. <g>
Lucky or an endorsement of the slope technique?
In my last on-the-water attempt (July 2008) clouds rolled in just as I was
ready to shoot. I got off 2 observations shooting through the cloud before
the sun was obscured. 3 ft seas and a usable although not crisp horizon. I
had to shoot through sails/rigging so I was not able to fully rock the
sextant to my satisfaction. My intercepts, 0!7 and 1!9 (average 1!3) were
both larger than the Hc. That suggests to me the sextant was not plumb.
As to fixes, I cheat and do not follow established procedure. I factor
leeway into DR and use my EP as AP. I would agree that any running fix (5
hours) greater than 3 nm off my GPS position under average conditions would
have me looking for a reason why.
I don't think this is cheating. Sounds more like common sense.
--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
