NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: QMOW Days work in Navigation
From: Peter Hakel
Date: 2010 Jan 9, 11:20 -0800
From: Peter Hakel
Date: 2010 Jan 9, 11:20 -0800
Perhaps I should clarify why I found this detail so interesting. There are two reasons, in fact.
First, it is the extensive use of the running fix which has been both criticized and defended quite recently on this list.
Second, less recently some of us (Jeremy, Antoine, Andres,...) went through some real-life meridian transit data that resulted in longitudes that were better than mediocre, in my opinion. The construction of the noon or any other meridian transit curve indeed "require extended and precise observational data" as George said. However, according to the procedure you were supposed to do these noon-curve measurements anyway, so you're not doing any extra observational work beyond what is already mandated. Before computers the "noon curve longitude" determination would result in non-trivial amount of extra computational work, which I suspect as the main reason why this was not done traditionally. This led to my hypothesis that accuracy may not have been the issue; the extra labor required was, hence the running fix instead.
Now in 2010 we do have powerful computers and automatization of these calculational chores by software, essentially eliminating the extra labor problem caused by vessel's motion, for instance. This brings me back to the question of accuracy and also to my reason #1. If this "day's work procedures" were to be followed, wouldn't it be better to recommend "longitude by LAN" (which is a true fix) instead of doing the running fix using the DR advanced mid-morning Sun LOP and LAN latitude?
Peter Hakel
From: George Huxtable <george@hux.me.uk>
To: NavList@fer3.com
Sent: Sat, January 9, 2010 5:30:43 AM
Subject: [NavList] Re: QMOW Days work in Navigation
Peter Hakel wrote-
" ...experience has apparently shown that doing a running fix is preferable
to getting longitude from the time of LAN. This is very interesting. Does
anyone know how this rule was established? Has anyone tried both methods
and compared their accuracy? It is also conceivable that nobody has really
tested this and the reasons are historical: i.e. the LAN is used to give
latitude ONLY and thus the running fix is the next best thing to establish
position."
==================================
Peter has it right. Although much attention has been given on this list to
the determination of longitude from observations around Local Apparent Noon,
mentions of its use at sea in actual chronometer-navigation are rare. That
is because its defects were recognised, as a method which would require
extended and precise observational data, to achieve a mediocre result; there
were better ways of doing the job. The moment of Local Apparent Noon was
indeed calculated, but that was to provide the best monent to take an
altitude of the Sun to determine latitude, which would (unlike the maximum
observed altitude) be unaffected by the vessels course and speed.
[rest deleted by PH]
First, it is the extensive use of the running fix which has been both criticized and defended quite recently on this list.
Second, less recently some of us (Jeremy, Antoine, Andres,...) went through some real-life meridian transit data that resulted in longitudes that were better than mediocre, in my opinion. The construction of the noon or any other meridian transit curve indeed "require extended and precise observational data" as George said. However, according to the procedure you were supposed to do these noon-curve measurements anyway, so you're not doing any extra observational work beyond what is already mandated. Before computers the "noon curve longitude" determination would result in non-trivial amount of extra computational work, which I suspect as the main reason why this was not done traditionally. This led to my hypothesis that accuracy may not have been the issue; the extra labor required was, hence the running fix instead.
Now in 2010 we do have powerful computers and automatization of these calculational chores by software, essentially eliminating the extra labor problem caused by vessel's motion, for instance. This brings me back to the question of accuracy and also to my reason #1. If this "day's work procedures" were to be followed, wouldn't it be better to recommend "longitude by LAN" (which is a true fix) instead of doing the running fix using the DR advanced mid-morning Sun LOP and LAN latitude?
Peter Hakel
From: George Huxtable <george@hux.me.uk>
To: NavList@fer3.com
Sent: Sat, January 9, 2010 5:30:43 AM
Subject: [NavList] Re: QMOW Days work in Navigation
Peter Hakel wrote-
" ...experience has apparently shown that doing a running fix is preferable
to getting longitude from the time of LAN. This is very interesting. Does
anyone know how this rule was established? Has anyone tried both methods
and compared their accuracy? It is also conceivable that nobody has really
tested this and the reasons are historical: i.e. the LAN is used to give
latitude ONLY and thus the running fix is the next best thing to establish
position."
==================================
Peter has it right. Although much attention has been given on this list to
the determination of longitude from observations around Local Apparent Noon,
mentions of its use at sea in actual chronometer-navigation are rare. That
is because its defects were recognised, as a method which would require
extended and precise observational data, to achieve a mediocre result; there
were better ways of doing the job. The moment of Local Apparent Noon was
indeed calculated, but that was to provide the best monent to take an
altitude of the Sun to determine latitude, which would (unlike the maximum
observed altitude) be unaffected by the vessels course and speed.
[rest deleted by PH]
