# NavList:

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

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Re: Circle charts
From: Richard M Pisko
Date: 2009 Jan 30, 23:52 -0700

```On Wed, 28 Jan 2009 12:34:27 -0700,  wrote:

> It occurred to me that there is intermediate variant between printing
> circle of position on a chart and constructing them manually every time
> it is needed. What can be done instead, is for certain pair of bases
> every point of its middle perpendicular can be graduated with the angles
> (in degrees) at which this pair of bases is visible from any point of
> the circle with the center on this point and radius equal to the
> distance between this point and one of the bases.

I like that idea for my intended trials down at the river bottoms, with a
number of unknown locations smaller than that required for dredging.  I
even have a beam compass that will let me work to a larger scale than the
available topographic quadrangles, by plotting on a large sheet of butcher
paper with an A2 sheet in my expected location.  It seems to me that I
could plot intersecting circles fairly quickly that way; just on that
particular A2 sheet.

In my opinion, the advantage of using your lines directly is being able to
pick out the observed angles quite accurately.  The advantage of
pre-plotting circles to a larger scale onto an A2 sheet is being able to
locate your (reasonably close) position more quickly; and without needing
a large plotting table.

> Attached PDF shows what I mean by that. Hopefully.

Is your formula for the circle centers on the perpendicular bisectors
something like: <(distance between station points)/(2 x tan of the
observation angle)>?  Which drawing program did you use?

Looks good at first study; but I think it is preferable to have the middle
shore station closest to your expected position to avoid the indeterminate
case of the circle of position.  The scales should then be diverging.

We could continue the lines in the other direction for observation angles
of greater than 90 degrees, I would think; even though that case may not
be as common.  My preference would be to choose shore stations giving
angles not too much closer than 20 degrees; even better around 60, so as
to get good intersections of position circles.  Of course, it is not
necessary to use a common point, four shore stations could be used as two
pairs instead, but there would still be a minimum of two intersecting
circles.

Thank you.

--
Richard . . .

Using Opera 9.2.4 after the "Dog" died

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