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Abstract Radial and decentring distortion parameters have long been reported as fundamental elements in camera calibration, especially in digital close range photogrammetry. This paper reports on an investigation for the determination of both radial and decentring distortion parameters using analytical plumb-line calibration. Straight line patterns, ranging from 1, 2, 4, 10 and up to 20 lines, not strictly plumb, are examined on a set of distorted imagery. Additionally, not only the number of straight lines but also the linear image distribution is analysed. For that purpose, linear features have been distributed in different positions and orientations over the whole of the imagery. Furthermore, the relationship between the parameters mentioned and the location of the principal point is also analysed, and shows how sensitive they are to deviations from the centre of the sensor. The motivation for this paper is the measurement of the minimum number of linear features for proper calibration of non-metric digital cameras. Both radial and decentring distortion parameters are examined. Finally, some line patterns are recommended for making the plumb-line calibration technique a reliable, easy and fast procedure. Photogrammetrists could use it as a first step in the procedure of camera calibration; users who are not experts in photogrammetry could employ it as a final step.