# NavList:

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

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Re: Can I Navigate Without an Assumed Position?
From: Bill Lionheart
Date: 2017 Aug 26, 14:24 +0100

```Probably this is well known to navigators, it is just something I know
from geometry.

If you know the GP of a heavenly body and its altitude that means you
are on a  known "small" circle centred on the GP. On a map projection
that is conformal circles are taken to circles, so you have a position
circle on such a chart. If you have two such position circles you know
you are at one of the (typically two) places that they intersect.  If
you have three and they more or less agree at one point of
intersection you have your position. This does not use an assumed
position (other than you are on the surface of the earth).

The reason we need an assumed position is that we usually work on a
chart on the scale where we can approximate the small circles by
straight lines. This is easier and if you had a big enough chart to
draw the circles it would not be so accurate.  You would probably then
zoom in and do it with the conventional method using the circle method
as your GP. You can do it all computationally rather than graphically
of course.

[The more knowledgeable NavList members will no doubt reply with the
history and terminology of such methods in navigation]

Bill

--
Professor of Applied Mathematics
University of Manchester
http://www.maths.manchester.ac.uk/bl
```
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