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: Plotting 3-Body fixes
    From: Peter Fogg
    Date: 2003 Sep 4, 10:10 +1000

    Normally you would only use one DR for a 3 body fix observed over 15
    minutes. If your craft is doing 4 knots you will only travel one nautical
    mile in this time.
    
    I don't understand 'L and Lo'. What you need to plot with is the azimuth
    (bearing to body observed) and the intercept, the distance towards or away
    from that azimuth. This gives you a position line which runs at right angles
    to the azimuth. The 3 position lines (or LOPs) can be extended to form a
    triangle. If all 3 sights were perfect then they would meet at a single
    point, your fix postion. So the centre of this triangle is your calculated
    fix position.
    
    The tools needed are plotting sheets (you can plot directly onto the chart
    but will end up with lines everywhere), a ruler, something to draw right
    angles with. Use a 2B pencil for azimuth lines and a 4B (soft and broad) for
    LOPs. The convention is to put single arrow-heads on the ends of your LOPs
    and double ones on LOPs used for running fixes.
    
    As an aside, my nav. calculator can have the speed and craft's bearing
    entered with the other data and will advance or retard the other sights
    accordingly, leading to a more accurate result for the time chosen.
    
    'Can anyone recommend a CN book with the best chart-plotting content?'
    
    Well funnily enough, the book  "The Complete On-Board Celestial Navigator"
    by George Bennett has worked out examples of, among other things, a 3 body
    fix then shows this plotted. It also has the necessary blank forms,
    including a plotting sheet, that you can photo-copy.
    
    ----- Original Message -----
    From: "Jim Thompson" 
    To: 
    Sent: Wednesday, September 03, 2003 7:45 PM
    Subject: Plotting 3-Body fixes
    
    
    > This is a "newbie" question.  I am learning celestial navigation.  What is
    > the convention for plotting 3-body fixes taken within a few minutes of
    each
    > other?  Say I take 3 sights on 3 stars, on a moving vessel, all within a
    > span of 15 minutes.
    >
    > Method A:
    > 1. Work out DR positions for each of the sights (L, Lo).
    > 2. Use the L and Lo for each sight's DR to work out the intercept, azimuth
    > and LOP for that sight.
    > 3. Advance the LOPs for the first two sights to the last DR to draw the
    > cocked hat fix estimate.
    >
    > Method B:
    > But the Canadian Power and Sail Squadrons' course manual implies a
    different
    > method, although I am reading between the lines to operationalize what
    they
    > wrote:
    > 1. Work out DR positions for each of the sights (L, Lo).
    > 2. Use the L and Lo for each sight's DR to work out the intercept, azimuth
    > and LOP for that sight.
    > 3. Simply plot the LOPs for for all 3 sights from the last DR to draw the
    > cocked hat fix estimate (without formally advancing the 2 earlier LOPs).
    >
    > Jim Thompson
    > jim2{at}jimthompson.net
    > Outgoing mail scanned by Norton Antivirus
    > -----------------------------------------
    >
    >
    
    
    

       
    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