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Number of control points in DLT

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  • Number of control points in DLT

    When I was at the University of Iowa, we used up to 68 control points for the
    calibrations in DLT. I prefer using more than 16 control points for two
    reasons: (a) using more control points allows me to eliminate some
    inacurrately digitized or incorrect defined points; and (b) using more control
    points allows me to do extrapolations if I have to.

    As Dr. Brian Davis pointed out, we need to eliminate those inaccurately
    digitized or incorrectly defined control points. My own DLT program package
    can do this automatically or let users to do this themselves. Most of time,
    this "optimization" of the calibration can reduce overall calibration error by
    1-2 mm. This improvement sometimes is indeed greater than that by increasing
    the number of calibration points.

    I found that using more control points significantly improves the accuracy of
    extrapolation. Thoretically, we should avoid extrapolation as much as we can.
    In reality, we have to more or less do some extrapolations. With the number
    of control points between 60-68, I did not see significant decrease in the
    accuracy in the estimated 3-D coordinates even the point was over 30 cm
    outside the calibration volume. I do feel that PEAK PERFORMANCE 24 contropl
    point calibration frame is not sufficient for extrapolation although it may
    provide reasonable interpolation accuracy.

    Overall, I agree with Rick. We should use more than 16 control points for DLT
    calibration. I don't know the exact number of control points that should be
    used as minimum, but I think that using more control points definitely has
    some advantages and digitizing 50 or more control points does cost signicantly
    more time than that for 16 control points.

    Bing Yu, Ph.D.
    Assistant Professor
    Division of Physical Therapy
    The University of North Carolina at Chapel Hill