INVESTIGATIONS INTO DIRECT LINEAR TRANSFORMATION (DLT) APPROACH FOR REDUCTION OF DATA FROM NON-METRIC CAMERAS

Abstract

Non-metric camera applications are gaining fast popularity in t-he field of close - range photogrammetry. Direct linear transformation (DLT) is one of the widely used techniques of data reduction from non-metric camera imageries. Present work involves in the investigation of various accuracy aspects of the DLT technique. The investigations were carried out using mathematically generated photograms and measured photo-coordinates, for which a control field was established and photographed using Yashica-365, a non-metric camera, having focal length 80mm and format size as 55x55 mm. Computer programs were developed for DLT and generation of photogram pairs, so as to facilitate investigations. Investigations using mathematically generated photograms, involved studies for various parameters, affecting accuracy, such as camera orientation geometry, depth extension of object space, number of control points and their distribution in 3-D object space etc., whereas, investigations using measured photo-coordinates involved studies related to control points and number of unknowns carried in the solution. Investigations showed that sufficient number of well distributed control points.are required for accurate results. Higher base to distance ratio and small depth extension of object space were found to be resulting in higher accuracy. It was also found that using 11 parameters as unknowns in the place of 9 does not introduce by error in the mathematical formulation of DLT. Investigations resulted in the conclusion that DLT techniques can provide sufficiently high accuracy, for various applications in the field of close -range photogrammetry.