Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: The cocked hat
    From: Joel Jacobs
    Date: 2004 Apr 4, 09:41 -0400

    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
    > > >
    > > >
    > >
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site