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    Usefulness of the slope technique
    From: Peter Fogg
    Date: 2009 Aug 9, 10:50 +1000


    George Huxtable wrote:
    A simple plot of Jermy's data "Moonlines, Moon away from LAM" shows
    that the observation at 8.54.56 is a real misfit, and should be discarded.

    Not so fast, George!  With the luxury of many sights that generally accord then, and only then, the discarding of an apparent dodgy one may be contemplated.  However, if sights of one body need to be recorded within 5-minutes of time, either because the straight-line assumption of the body's apparent rise/fall becomes stretched (ho ho) if extended further, or because the sunset/sunrise window of opportunity is limited, or because atmospheric conditions preclude, etc, then that sight/data point is too potentially valuable to be so casually discarded.  It then becomes more sensible and much more practical to examine the supposed outlier to see whether it has the potential to be useful.

    This is a fairly typical example of an apparent transcription or misreading-of-instrument error.  Once identified and corrected it can join the others in providing useful information.

    It seems most likely that there was a recording or transcribing error, and
    that it should have been noted as 8.52.56, but it's not safe to guess at
    such things,

    Not safe?  Safer, I propose, than including it without correction.  Much safer, it seems obvious, than adopting it without any knowledge of how sound a sight it is likely to be, compared to its peers.  Remember that if only one sight is made of one body and then adopted, then that one sight could just as well be that bad one as any of the others.  How would you know? 

    This standard procedure (the uncritical adoption of random sights) has more in common with hopeful guessing than use of the slope technique, it seems to me.

    In this case the outlier has become identified as such due to sight reduction having been performed on all sights.  With electronic computational power available this is feasible, but without it becomes a real chore; a 'pushing-a-rock-uphill' method of identifying outliers, or dodgy sights.

    In practice, without computer assistance, it may be much simpler to draw up a simple plot of the sights taken over time and compare them with the body's apparent rise/fall.  Then a picture is provided of the pattern of sights, identification of outliers becomes obvious, and the best apparent match of the pattern of sights can be made to the slope.  Then only one sight reduction needs to be made of that best match, in terms of altitude/time, and a Line of Position produced that is free, in practical terms, of erratic error.  Typically this best match provides a better LOP than any of the actual sights.  Such is the practical power of the slope.
     
    and the only safe procedure is to delete that line. 

    Nah.  



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