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    Re: Rejecting outliers
    From: George Huxtable
    Date: 2011 Jan 8, 21:57 -0000

    Gary wrote-
    "But for calculating the slope the DR doesn't need to be particularly 
    accurate as the slope is determined only by your latitude and the azimuth 
    of the body, which you can measure to sufficient accuracy. If you scan the 
    Motion Of the Body table from H.O. 249 at:
    you will see that the rate of change in altitude is very insensitve to 
    errors in latitude and azimuth. For example, looking at the MOB table for 
    my data where my DR latitude was 14� 25' and the azimuth was 103� true, the 
    tabulated values for the change in altitude for one minute of time changed 
    only 0.1' for a 2 degree azimuth change and by only 0.2' for a 5 degree 
    change in latitude. So for a five minute observation period, if my DR 
    latitude had been wrong by five degrees the slope would be off by only one 
    minute of altitude. Similarily, the measured azimuth would have had to have 
    been off by four degrees to produce a one minute error in the slope. 
    Scanning H.O. 249, volume 2 shows that to produce an error of four degrees 
    in the azimuth my DR longitude would have had to have been off by ten 
    degrees, 600 nm!"
    Gary's conclusion is right, though his example was not a typical one. 
    Observing from within the tropics a body that's nearly due East, that body 
    will be climbing more-or-less vertically , and so its rate-of-change of 
    altitude will be near the maximum possible, of 15� per hour, and will 
    therefore be very insensitive to changing the parameters.
    But it's not the same the whole World over. From higher latitudes, when 
    observing bodies that are climbing diagonally across the sky, the 
    rate-of-change of altitude will be less, and it will have a greater 
    dependence on latitude and azimuth.
    Even so, it will often allow the rate of change to be calculated more 
    precisely than it can be measured, depending on the precision of the 
    observation and the accuracy with which the position is known.
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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