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    Re: Accuracy of sextant observations at sea
    From: Peter Fogg
    Date: 2010 Nov 26, 18:58 +1100

    Antoine wrote:

    To make a long story short, after personnally using the painstaking "manual slope plotting" method up until the early 80's, with the arrival of "smart" calculators - I chose the HP41 family then - I have (successfully I think) attempted to avoid manual plotting while still remaining (sufficiently I hope) cautious about outliers.

    One of the advantages of  "manual slope plotting" is that one point along that line of best fit can be chosen and the time/altitude used for sight reduction.  It does not need to be any of the actual sights.  This can be useful, eg; for adopting a chosen azimuth of 90d or 270d via making observations over 5-minutes encompassing the time when the body, could be the sun, passes across due east or west.  Thus the resulting line of position reflects the longitude.  A few hours later, or earlier, it could be crossed with a meridian passage observation for a 2-body running fix comprising lines of latitude and longitude.

    With plentiful computing power all the sights can be reduced, thus multiple sights of each body observed, and the software can work out a fix.  I have an electronic nav calculator that does this.  But for manual sight reduction I prefer to evaluate raw sights first via the slope technique to reduce random observational error before sight reduction.  Any systemic error is resolved later.

    Regarding Manual Plotting, and with the exception of any obvious "flashing outlier" which you are expected to detect and remove anyway, I have often noticed - thanks to GPS - that what might apparently look as an 'outlier' may not always be one actually and that discarting it at this early stage might not always be the best course of action.

    Yes, I've noticed the same thing.  An apparent outlier could be the wrong time or altitude noted.  If so and corrected, the sight may then rejoin its siblings to form a nice tight pattern along the line of slope - good data from bad!  Number crunching or averaging won't achieve this, it treats all data as equally valid.  On the other hand, the observation conditions could be quite marginal and then the slope technique is almost obligatory to try and get a reasonable result out of what might not be a nice tight pattern along the line of best fit at all, at all..
     I now simply use plain averaging

     How does this averaging work in practice, given you're dealing with different times and altitudes?  How do you go about averaging them?

    Finally Antoine, you're always so exquisitely polite that you've inspired me to have a go:

    Je vous prie d’agréer, Monsieur, l'expression de mes salutations distinguées.


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