# 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
Date: 2019 Feb 3, 14:40 -0800

Paul, you wrote:

"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."```

That's a great scheme, thankyou. I was previously pre-occupied with using just one piece of plastic.

Using your scheme, I used the Douglas Protractor entirely conventionally, not counting degrees, just rotating clockwise from 000 to on the central meridian of the UPS until the inner scale mark for Zn is aligned on the meridian.  No brain work needed there. The north end or the south end of the protractor can be used for the LOP - no brain work there, and possibly reducing the number of sliding movements needed to get a protracter north or south edge at the EP.

And to add a tweak to your scheme: Frank's UPS, on my printer at least, is scaled 1/16" to 1 minute of Lat - using a ruler marked up in 1/16" as the straight edge, you don't have to measure the intercept separately, mm or otherwise. Just lay the ruler on either the east or west of the side of the protractor, then use the rulers 1/16" marks to move towards or away from the EP before drawing the line. That arrangement then, gets us the Douglas Protractor to define the Zn on the plotting sheet, the ruler enables sliding the protractor edge to the EP, and the 1/16" marks on the ruler calibrate the exact  the intercept movement of the protractor edge from the EP. And you can plot from an AP  using rapid tables with a larger Intercept, because you have a ruler full of 1/16".  Who needs to be limited to a dozen or so marker lines on any plastic?

Isn't life rich? Thanks again Paul.

Good navigating

Michael

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