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    Re: camera sextant?
    From: Marcel Tschudin
    Date: 2010 Jul 8, 01:17 +0300

    Yes, there seem to be small systematic contributions but I think your
    calibration measurements are quite good. Your figure represents a
    "calibration function" which tends to be a quadratic, not linear, as
    you have drawn it. They seem to be quadratic as George has indicated
    it in his figures showing their possible extrapolation. In my opinion
    your "outliers" are "normal" points indicating only the curvature.
    Part of the "problem" is here that we deal with digital photos.
    Differences can only be 0, 1, 2 etc. pixels. A possible way to improve
    this would consist in taking several measurements of a position in a
    photo and use the mean value. If we accept the "calibration function"
    to be quadratic, then I would assume that most measured HEIGHTS (your
    data points) would be within +/-1.4 pixels, corresponding to POSITIONS
    being measured to +/-1 pixel.
    The 1 pixel difference to your previous measurements may have various
    reasons. I tend to attribute  to your learning curve ;-) It could
    however be an indication to use for this sort of calibration
    (calibration by calculated positions) observations made at different

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