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Trevor,you and Bill are not missing anything reguarding this matter.I fully
understand what you both are saying as to "confirming" the longitude of the
DR or EP.I also wished to show that if it is the only thing one has to get
an approximation of longitude one at least has an idea of where it is.
Now,say the vessel is on a coarse of 103*T.The auto-pilot is set and one
calculates the DR accordingly.The vessel travels through an unknown set of
170*T with a drift of 2kt. for 5 hrs with no calculations for leeway.The
last sight taken was 40 hrs before.The position of the vessel is now nowhere
near the DR.If the vessel has no electronic nav. equipment one won't be
aware that the vessel has been pushed off the trackline by the amount it is.
Conditions are such that 1 sight can be taken around the time of LAN.One
gets the estimate of latitude at LAN from the reduced noon sight.One now has
an idea of ones' latitude.Are you going to trust a position line 40 hrs old
to get an idea of the longitude?Yes,one can advance the line.But,in
reality,how accurate will the results be from this r-fix?I was taught and
practiced that the optimal time to advance or retard any line was 3-6 hrs.
So,one is in the position in this scenario not having the "warm,fuzzy"
feeling of confidence reguarding the ships'position.
At the least one can use the r-fix and the noon GHA of the DR or EP to get a
better idea of where  the position is or if some gross error exists in one
of the parts.


-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Trevor J.
Kenchington
Sent: Wednesday, January 21, 2004 14:06
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Timed Noon sights for position


Doug,

I dare say that I am missing something but I have read through your
original message twice and I am in the same position as Bill Noyce. As
best as I can see from your logic, you first determine the time (GMT) of
LAN for your EP, then you observe the Sun's altitude at that time, then
you assume that your observation was taken at LAN and use its time to
determine your longitude. That, however, is circular reasoning and can
never do anything but "confirm" the longitude of your EP, no matter how
erroneous it really is.

If there is more to your method, please explain it.

If there isn't anything more, I would say that the correspondence
between your GPS readings and your "observed" longitudes only
demonstrates that your EPs are very accurate.


Trevor Kenchington


Doug Royer wrote:

> There are many procedures and many ways to accomplish the same goal.In my
> origonal post on this matter I put forward this method as one of the
methods
> used.Some list members were horrified.Most times one doesn't use the Noon
> sight to obtain longitude.I wished to show that when needed(deteriorating
> conditions or circumstances)it can be used to give an approximation of
> longitude where only an estimated latitude was calculated.One more tool.
> Sometimes one doesn't have the luxury of turning the ships head from the
> trackline to obtain celestial course lines(as Joel noted)or the time to
> shoot double altitudes or multiple averaging sights.Are there pitfalls
> useing this method?You bet.


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

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