Photogrammetric Record, 15(89): 703–714 (April 1997) LOW COST DIGITAL RECTIFICATION ON A PC By A. GEORGOPOULOS and G. N. MAKRIS National Technical University of Athens (Paper read at the Thompson Symposium held at the University of York on 20th April, 1996) Abstract Digital photogrammetric techniques have opened fresh horizons and have enabled photogrammetry to become a more user friendly tool for a wide range of applications. However cost remains a critical issue, because new digital photogrammetric workstations and their related software are considered expensive. This paper examines the problem of digital rectification and presents low cost solutions involving the use of commercial off the shelf software packages, which include graphics and painting software, image processing tools and a CAD system. Additionally, specially developed software performing vector rectification is described and dis- cussed. All the packages have been used for rectifying digital images and the results are presented. An attempt is also made to assess the accuracy and reliability of the results. Finally, several alternative digital products are presented, mainly concerning the use of the methods described in the restitution of cultural monuments. INTRODUCTION DIGITAL TECHNIQUES have caused a major change to the traditional way that photo- grammetry has earlier functioned. During the past decade it is claimed that photogrammetry has entered its digital era. This statement should, however, be met with considerable scepticism. It should rather be considered that currently photo- grammetry is experiencing its last, mature analytical phase with digital images. Full automation, which is the main argument for the implementation of digital photogrammetry, is not yet implemented in production, since computers cannot, so far, view images clearly. Analytical procedures have simplified the instrumentation necessary to transform the central projection to orthogonal projection, because many, if not all, mechanical parts have been replaced by suitable computer algorithms. This means that instead of winding handwheels and carefully rotating photocarriers, the user is required to press buttons, save files and undo actions. This progress has facilitated the use of most photogrammetric techniques by non-specialized personnel. The need for highly skilled analogue instrument operators is, in this way, reduced. Image rectification is a photogrammetric technique very widely used by non- photogrammetrists. It has always been easy to extract metric information from photographs of plane objects, simply by taking scale differences into account. Architectural applications, where most of the objects of interest present a consider- able degree of planarity, provide excellent examples of the above statement. Hence photogrammetric rectification has always been a very attractive solution in this field. Analytical rectification disposed of bulky and costly rectifiers, but produced a less satisfactory photographic product. Digital images (computer files which store an image electronically) have contributed to a solution to this problem. At the same time, 703