# 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 4, 12:06 -0800

```On 2019-02-04 9:01, Tony Oz wrote:
> One thing still is not yet clear - the scale to use as an intercept distance
can not be placed on the protractor directly - because that scale depends on
the latitude. Do you do an extra calculation - to convert the T|A miles into
the millimeters on the protractor's (DIY) scale?

If the plotting sheet scale is 1 mm per mile, or 2, 5, etc. it's easy to
mentally compute the distance to move the protractor toward or away.
However, the movement requires a straightedge graduated in millimeters
since my square protractor (purchased from Celestaire, I think) has its
edges graduated in degrees, not millimeters.

I have never tried it, but I think you could use dividers if you have
enough coordination to handle the protractor and straightedge with one hand.

I agree with Michael Bradley about the "bearing markings on its edge
which confuse matters." To move the protractor toward or away, one must
be careful to match the 90 degree mark (for example) on the square
protractor to the correct millimeter mark on the straightedge. There
would be less chance of a mistake if the protractor scale was a circle,
and only the 0, 90, 180, and 270 degree lines extended to the edges.

What you actually have in the interior of the square protractor is a
grid of 5 mm squares. I have never figured out a use for this grid. Anyone?
```
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