# NavList:

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

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Re: Understanding a 1902 Sumner
From: David C
Date: 2017 Jan 23, 13:31 -0800

Here is the next instalment.........

The difference between the intercept method and CoP app positions is partly due to errors in the azimuth plotted on Google Earth. I decided to eliminate this error by using Sumner's chord (two point) method. It is easy to plot accurate positions on google earth and draw lines between them. I have attached a revised plot. The sumner method position includes rounding errors so is not a perfect match to the CoP position. As would be expected the sumner and intercept position lines are parallel  -  it is satisfying to demonstrate this.

The position lines are approximations of a circle of position. The error can be determined by stringlining a curve. See the attached 2009 photo which shows measurements being taken without the ubiquitous GPS receiver on a pole. For details of the method see

v = approx  L2/(8R)

where v =versine

L = chord length

All values in the same units. I will use nm.

T Alt approx = 44°

ZD = 46° so radius = 46 X 60 nm

Chord length = 1.91nm  (measured on Google Earth)

From which v = 0.000165 nm

=  .01"

Clearly in this case the assumption that the CoP can be approximated by a straight line is valid.

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