An investigation on the three-dimensional calibration errors associated with the direct linear transformation (DLT) technique was carried out using a two video camera system. Thirty different configurations in five groupings of different numbers of control points were tested. The results showed that the accuracy improved as the number of control points increased from 8 to 24. Further, it was demonstrated that the best accuracy was achieved when the control points were evenly distributed throughout the control region. For the system under investigation, an accuracy of 1-2 mm in the x and y directions, 4-6 mm in the z direction and 6-7 mm for the resultant was obtained in a control space of 2.10 x 1.35 x 1.00 m. Additionally, the accuracy for points outside the control space and the systematic error associated with the control object were studied. Tests were done on a 32-point rectangular calibration frame, and the results indicated that the calibration errors could be reduced by 20-40% when an appropriate quadratic function was used to modify the standard DLT method.