The Cylindrical filter option in the Construct Filter Feature dialog box (Insert | Feature | Constructed | Filter) filters data in a spiral scan or a circular scan that spans more or less than a full circle. The deviations in this case are radial. As PC-DMIS does not consider the data to be periodic (meaning it does not form a complete, closed circle), a number of points equal to the filter width at the beginning and at the end of the scan are absent from the filter set.