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Dear Jim,
Here are my comments on your summary:

On Fri, 22 Oct 2004, Jim Thompson wrote:

> So, is this a fair summary?
>
> Sextant sights are subject to a variety of errors,
> leading to imprecision
> and inaccuracy. One way to deal with random
> observational error is to
> average a set of several sights, and then
> reduce and plot the average time
> and altitude.
>
> Always apply basic principles to a run of sights:
>
> 1. Use only sights taken within a few minutes,
> a minute or less between
> sights.

> 2. Use either the raw sextant observations,
> or reduce each observation and
> use the reduced set.

If you reduce each observation, there is no point
in averaging them.
And there is no point in taking them in a short time
interval.

> observations. This elimates variation owing to the way
> the corrections
> themselves might vary, but it still does not
> make the run of sights linear.

The non-linearity question is relevant ONLY if you average row
observations.

> 1. The body is very likely to be changing
> altitude in a nonlinear fashion.

Just vice versa: very UNLIKELY.
Namely: ONLY when near the meridian on high altitude.
In all other cases it is linear for all practical purposes.

> 2. High vessel speed can also lead to non-linearity in the data.

No, it cannot. This was the subject of my very first
message in this Averaging discussion, and everyone seemed
to agree. It is only on jet-propulsion airplanes
that the speed of the vessel can cause any noticeable
non-linearity problem.

   

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