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    Re: Systematic error and its resolution
    From: Frank Reed CT
    Date: 2007 Apr 6, 23:03 EDT
    Geoffrey, you wrote:
    "In conclusion, I was able to get reliable fixes
    far more quickly by moving away from the
    traditional fix using three objects about 120°
    apart with its resultant cocked hat, and adopting
    a strategy of taking sightings on four objects,
    each near one of the Cardinal points. I know that
    if the box formed by the position lines is
    substantially square, the errors are far more
    likely to be dominated by systematic rather than
    random errors. With a tradition
    three-position-fix you never know if random or
    systematic errors are dominating, so you can
    never be sure just how accurate your fix is."
    Very nice. A few months ago, I wondered in a message on the list about the statistics of this. Clearly, with three sights you can't eliminate systematic error unless you know in advance somehow that it is present. Your method with four sights at cardinal points makes it obvious graphically that we're dealing with systematic error. I think it's probably true (and probably obvious) that you would get even better results with more sights, but I haven't looked into the statistical details. In the general case, I think the approach would be to vary the fixed correction until the box surrounding the crossing points of the various LOPs is as small as possible. This should work even in cases at arbitrary azimuths and with random errors, too.
    42.0N 87.7W, or 41.4N 72.1W.

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