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    Re: Accuracy of position
    From: Geoffrey Kolbe
    Date: 1999 Oct 19, 3:15 PM

    Thankyou for your input Dan
    >1. Use my Garmin GPS III in averaging mode for 20 minutes to determine my
    >location to within 20 feet.  This was my assumed position.
    Gee, I have a GPS III and I did not realise you could do that!
    >2. Measured the altitude of the sun using a Davis Artificial Horizon, so the
    >angles measured were double the actual values.
    I was always worried that the clear plastic wind shields on the Davis
    Artificial Horizon would contribute some refractive error due to not being
    optically flat or parallel. Did you do some trials to test for this?
    >From your results, it would seem that the one big positive error from the
    Jupiter pretty much cancelled out the systematic negative error of the
    Plath! The population standard deviation of your results was actually about
    2.4' If we discount the Jupiter result, the standard deviation for the
    Plath comes down to about 0.68', or about the "half mile" figure that most
    navigators seem to accept as a reasonable error for a single sighting.
    (Would anyone disagree?) On this basis, to get the error in the mean down
    to the 200 yard figure, you would need to take around 16 sightings for a
    statistically significant result. (To get down to 20 yards you would have
    to take 1600 sightings!)
    This is pretty much the same trial that I ran at my home in Scotland. To
    get my position, I used a large scale Ordnance Survey map which shows my
    back yard and so I could measure off the lat. long. pretty well! I used the
    "Stormy Weather" navigational program to tell me what my altitudes should
    have been, which Paul Adamthwaite tells me seems to be accurate to a couple
    of seconds at worst! My marine sextant is an old "Husun" (Hughs & Son)
    which dates from WWII! It is a pretty tight instrument, but I do not have a
    clue how good the arc actually is. Does anyone have any experience with one
    of these old sextants?
    Thanks again,
    Dr Geoffrey Kolbe, Border Barrels Ltd., Newcastleton, TD9 0SN, Scotland
    Tel: +44 (0)13873 76253  Fax: +44 (0)13873 76214

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