# NavList:

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

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Re: Pointless? Optimisation - Intercept Plotting
From: Paul Hirose
Date: 2019 Feb 3, 11:27 -0800

```On 2019-02-02 5:01, Michael Bradley wrote:
> The Douglas type Protractor has been tried, but by its nature it has bearing
markings on its edge which confuse matters, and in needs a copious supply of
meridians which you don't get on a plotting sheet.

I have not found the single meridian of a plotting sheet a problem with
the Douglas (square) protractor. Say the azimuth of the body is 231°. If
the protractor is oriented to north, the top and bottom edges of the
protractor are oriented for a line of position at azimuth 0 or 180°. To
change that to the required 231°, rotate clockwise, counting by tens and
ignoring the numbers on the protractor: 190, 200, 210, 220, 230. Go one
more degree to obtain 231°. Now with the help of a straightedge the
protractor can be moved to the assumed position without changing its
orientation. Place one of the correctly oriented edges on the AP, then
use the millimeter scale on the straightedge to slide the protractor
toward or away as required. This lets you plot the LOP without first
drawing an azimuth line to the body.

The 5 inch square protractor works well with 8.5 by 11 inch plotting
sheets when there's not much space.

Another shortcut is to use azimuth angle Z directly. For example, in the
northern hemisphere a traditional sight reduction would give Z = 129 in
the previous example. When the protractor is oriented north, the left
and right edges are oriented for a LOP at Z = 90. But you need 129, and
since the body is west of the meridian, the protractor must be rotated
counterclockwise.  Count by tens: 100, 110, 120, 130. Then turn back one
degree to get 129. This simple trick eliminates the azimuth rules, and
it works with any protractor.
```
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