NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sight reduction tables
From: Peter Fogg
Date: 2007 Oct 3, 10:01 +1000
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From: Peter Fogg
Date: 2007 Oct 3, 10:01 +1000
On 10/2/07, Ronald van Riet <ronald@van-riet.nl> wrote:
I am a relative newcomer to the celestial navigation arena so forgive
me if what I say has been said a zillion times before or makes no
sense at all, but George points out (rightly it seems) that there can
be large errors in azimuth with an example where the azimuth is up to
15 degrees off.
Why is that so bad? By how much does that displace the resulting LOP
and the fix using one or two more LOPs?
The importance of accuracy with azimuths is related to the shape the (three or more) LOPs make. Most commonly (but not ideally, if maximum accuracy is the goal) this shape is a triangle, made up of 3 LOPs. If each LOP is derived from an azimuth that is about 120d apart, which can be considered a 'best case' scenario when it comes to azimuth choice, then some inaccuracy in azimuth calculation matters little. This may become clearer if you plot a few examples, then swivel the LOPs (alter the azimuths).
The other extreme could be where you have only 2 LOPs, and they either have similar azimuths, or azimuths very nearly separated by 180d. In this case the two LOPs tend towards being parallel lines, and small changes in azimuth result in great changes to their intersection; the fix.
Before you make up your mind about:
"George points out (rightly it seems) that there can be large errors in azimuth"
consider that the possibility of this happening is rather remote, to say the least. Take a look at:
about halfway through that missive, where you come across:
"probability of error using these azimuth tables"
and which concludes:
"The highly selective example showing a 15 degree error is too remote to be of practical concern.
It seems that altogether too much could be made of the supposed shortcomings of the tables, by selecting 3 variables carefully chosen to show the maximum amount of error possible in a juxtaposition of exactly halfway values.
"
Its a beat up. A furphy. A deliberate mischief. A rare enough example of something actually created by George (albeit a somewhat perverse obsession) who delights in nit-picking the work of others, while apparently incapable of creativity himself. Where is your book, George? Where are your publications on navigation?
The first half of that message may become clearer if you examine:
which investigates, as its title suggests: "potential error introduced by rounded values".
Incidentally, the starting point for searching the archive is
which has a search box at the top
And if you want to be more precise, why not use the new fix as the
assumed position for a next round of calculations? Since this AP would
be much nearer the actual position, an azimuth offset by 15 deg won't
make any significant difference.
Yep, this is sound practice. But again, while this technique can be very useful, especially when the intercepts are large, you'll be a very old navigator indeed, it seems, before you're likely to come across the problem while exclusively using the azimuth tables in the book '
The Complete On-Board Celestial Navigator', and you would only do so after having ignored the warning about when the tables should not be relied upon, and failed to use one of the many alternatives proposed.
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