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Henry,

Thank you very much.

You stated the complete argument much better than I just did, though I think
any more than three sights is an over kill.

Joel Jacobs


----- Original Message -----
From: "Henry C. Halboth" 
To: 
Sent: Sunday, April 04, 2004 1:15 AM
Subject: Re: The cocked hat


> In my experience at sea, before other means of position finding were
> available offshore, conventional wisdom suggested the EFFORT to observe
> at least four stars (including planets), one each bearing respectively
> south, north, east, and west, or as close thereto as possible, so as to
> average out opposing horizon conditions. The sea horizon is seldom
> consistent at all points of the compass and discrepancies readily show up
> in utilizing this method - judgement being of course necessary in not
> using a portion of the horizon which may be obviously unsuitable.
>
> Obviously conditions do not always permit of such niceties and one must
> do what one can. It was, however, quite customary to pre-compute and
> observe at least five bodies, even if only to have a reserve in the event
> that some proved obviously in error, i.e, wrong body, poor horizon, time
> error, sextant reading error, etc. By the way, pre-computation is
> essential to accurate star sights - if you wait until you can see them
> it's too late to expect a good horizon on average, telescope power
> notwithstanding.
>
> With a well calibrated sextant, a good horizon, the correct time, and
> careful calculation, "cocked hats" of any great significance or magnitude
> were not the usual thing - if they occasionally were, judgement as to the
> value of the individual sights and as to the reliability of the fix as a
> whole were seen as more of a consideration than efforts to calculate, by
> whatever means, an MPP. If two sights are correct and a third is in
> error, calculating an MPP by any method is not going to provide a more
> correct position, especially if one can be evaluated out. If it is a
> matter of serious concern, it seems the careful navigator will carry
> his/her (almost slipped on that one) reckoning forward from each
> intersection of the fix to insure s/he is not standing into danger -
> assuming of course a "cocked hat" of a magnitude requiring calculation of
> an MPP.
>
> Just thought I'd put my oar in.
>
>                                         Henry
> On Sat, 3 Apr 2004 22:07:05 -0500 Robert Eno  writes:
> > I'll throw my two bits' worth in. I agree with Joel and in fact what
> > he
> > describes is more or less the method that I have employed for quite
> > a few
> > years.
> >
> > I'll take it a step further: I generally examine my intercepts
> > before
> > plotting them out, discarding the obviously wonky ones. I do,
> > however,
> > average out sights taken on land with a bubble attachment because
> > bubble
> > attachments are inherently inaccurate and in my experience, it is
> > better to
> > take multiple observations of a few stars and average them out.
> >
> > I was persuaded, some years ago, that the 3 star fix is overrated.
> > Better to
> > take two observations of two stars.  This is not an an original idea
> > on my
> > part: my inspiration comes from a little-known book -- Arctic Air
> > Navigation -- by a former RCAF Air Navigator, Keith Greenaway:. In
> > his book
> > Greenaway writes:
> >
> > "Navigators who use asto continually prefer 2-star fixes with each
> > star
> > sighted twice, rather than 3 star fixes with each star sighted only
> > once.
> > Although an extra sight is required, time is actually saved, because
> > only
> > two stars have to be located, the course setting of the sextant
> > changed only
> > once, and the tables entered for two stars only. It is also easier
> > to detect
> > inaccurate computation or observation as shown in Fig. 27".
> >
> > Figure 27 shows two parallel LOP's spaced very closely and two
> > parallel LOPs
> > at right angles to the former, spaced far apart. The figure beside
> > it shows
> > a classic "cocked hat". The caption to the figure reads: "Comparison
> > of a
> > 2-star and a 3-star fix. In the case of the 3-star fix, it is not
> > obvious
> > which sight may be in error, while in the case of the 2-star fix it
> > is
> > immediately apparent which star should be re-sighted or sight
> > computations
> > checked".
> >
> > While it is true that I have quoted from a book pertaining to Air
> > Navigation, I have used this technique, successfully both on land
> > and at
> > sea.
> >
> > As for MPPs, I cannot argue with George and Herbert about the
> > efficacy of
> > performing the necessary statistical computations to arrive at this
> > figure.
> > I have read only a little on MPPs but I certainly agree that it is a
> > very
> > real and very valid concept. Nevertheless, for purposes of practical
> > navigation, who is really going to perform this exercise while at
> > sea? In
> > reality, navigators will simply take the centre of the cocked hat or
> > the
> > intersection of two lines of position.
> >
> > I believe that the two times two star technique described by
> > Greenaway is a
> > reasonable compromise.
> >
> > Robert
> >
> >
> >
> > ----- Original Message -----
> > From: "Joel Jacobs" 
> > To: 
> > Sent: Saturday, April 03, 2004 7:58 AM
> > Subject: Re: The cocked hat, again. Was: "100 Problems in Celestial
> > Navigation"
> >
> >
> > > In an off group exchange with Herbert Prinz, to which I have added
> > a few
> > > words to for clarity, I said this:
> > >
> > > "It is really a matter of personal judgment. The navigator's
> > selection of
> > an
> > > MPP will vary as to how reliable or accurate the navigator thinks
> > the
> > sights
> > > were based in the conditions he encountered when taking the
> > sights. A
> > > mathematical solution may not be appropriate."
> > >
> > > I submit that this is what George is correctly introducing.
> > >
> > > Let me add this thought. When I took the USCG Celestial Exam, year
> > ago,
> > one
> > > of the questions had a series of sights that when plotted, had one
> > sight's
> > > LOP distant from the others. If  you answered the question
> > including that
> > > sight in your calculations, even by averaging, you were wrong. If
> > you
> > > rejected it as a bad sight, you were right. As a matter of pride,
> > I got
> > > 100%. Of course I was a lot younger and sharper.
> > >
> > > Joel Jacobs
> > >
> > >
> >

   

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