# NavList:

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

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Re: Averaging
From: Gary LaPook
Date: 2004 Oct 20, 15:16 -0700

```Not to be too "nit picky" but it should be a plot of a "small circle,"
not a "great circle." A straight line is a good approximation of the
small circle at lower latitudes and short intercepts. At high latitudes
you should use the "Ellsworth Tables" to calculate how much the small
circle diverges from the simple straight line and plot a modified LOP
which is then more accurate.

As long as we are talking about the St. Hilaire method (computing an
azimuth and intercept from an assumed position) we should remember that
it was developed as an easy method of laying down the "Sumner Line," now
called an  LOP,  requiring only one computation. The original Sumner
method required computing two time sights, twice as much work. With
programmable calculators it is now just as easy to do the two
computations and lay down the LOP without measuring an azimuth or
intercept  or using an assumed position. You  simply choose two
longitudes, one east and one west of your DR, and the calculator
calculates the latitude where the LOP crosses those longitudes. You
prick those latitudes on the chart and draw a straight line between them.

Gary LaPook

Chuck Taylor wrote:

>Standard procedure for plotting a line of position
>using the St. Hilaire method calls for plotting a
>straight line, when we know that what we "should" be
>plotting is the arc of a great circle.  Still, the
>straight line is useful.
>
>--
>Chuck Taylor
> 47d 55' N
>122d 11' W
>
>
>
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>
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```
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