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Chuck Taylor's excellent discussion of precision and accuracy
included this paragraph:

> Three sights can be very useful if they plot in a
> straight line.  Then you at least can conclude that
> they are consistent.  If they do not plot in a
> straight line, you are stuck.  You can make a case
> that two of them are consistent and the third one is
> not, but you have no way of knowing *which* one is the
> odd sight out.

In fact, you can do better than this.  Compute a couple
of predicted altitudes for the earliest and latest times
of your sights, using your estimated position.  Then either
apply corrections for refraction, dip, etc "backwards" to
these computed results, or apply them normally to your
actual observations, so that the values are commensurate.
Plot the computed altitudes against time, and find the slope
of the line between them.  Draw a line with the same slope
through your actual observations; the slope should make it
clearer which sights are good and which are bad.

This technique also helps when you have more sights.  The
point is that you don't need to guess about the slope of
the "best fit" line; you can compute that from the Almanac
if you have a reasonably good EP.

I'm pretty sure the same technique applies to lunar distances,
though the computations are messier.  I'm not sure whether to
suggest "un-clearing" computed distances, or clearing all the
observations, before plotting, but one or the other is necessary.

        -- Bill

   

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