# NavList:

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

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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
> -----------------------------------------
>
>

```
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