# NavList:

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

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Re: Remember your first time?
From: Bill B
Date: 2005 Oct 31, 17:14 -0500

> Given the accuracy of my DR, I expect I can get closer, and maybe my
> IE was not too accurate, but heck for a first shot, it was close
> enough for me.

Paul

Welcome to the list.  Started my adventure in cel nav a bit more than a year
and a half ago.  My goal was to learn the HO229 tabular method.  I found the
group very supportive.  But be forewarned, if you have an addictive
personality, you may get hooked and learn much more than you ever
anticipated ;-)

I would highly recommend David Burch's article(s) on plastic sextants
(useful for all sextants, but address some of the techniques needed to
optimize observations with a plastic unit.)

http://www.starpath.com/catalog/accessories/psextants_midpage.htm

Follow the "How to take plastic sextant sights" link.

Links in that article can take you to methods (fit slope) for determining
which observation to use and which to discard.  Different methods such as a
simple average of observations altitudes vs. average of time, or some form
of linear regressions (Least Mean Square?) have been argued almost to death
on the list, and are available in the archives.

All said and done, IMHO the fit slope method (a known slope based on the
body's real motion) is the best.  If one draws in or calculates a slope to
fit the data points, the slope is driven by the data points. That slope may
not represent the actual slope of the body's movement, so determining
outliers becomes difficult.  Burch's method reduces the tendency to force a
line or average from the data points by comparing the data points to the
actual path (or approximation thereof).

In any case, the actual path is seldom a straight line (slope) but a curve,
so with any method try to keep the observation time span down to 4 or 5
minutes so the straight line (or average) approximates a small section the
curve (much as we use an line of position to plot in cel nav, which is
actually a small section of a large circle of position).

Hope that helps

Bill

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