The Effect Of Scanning Resolution In Digital Photogrammetric Work

Photogrammetry is one branch in surveying that has largely been affected by the development in computer technology in data acquisition, image development and processing, adjustment and further in mapping of data. Digital photogrammetric workstation is one of the yields of these developments in photogrammetry, which is gradually replacing analogue and analytical plotters. Pixel resolution is one of the main factors that affect the final results in digital photogrammetry in general and in digital photogrammetric work station in particular. In this research work, various subjects relevant to the process of digital mapping using digital photogrammetric work station were discussed. Theses subjects include: • Digital camera: The present alternative of the conventional camera and by which direct digital photogrammetric data can be acquired. • Scanners: Special precise photogrammetric scanners are used to transform hard copy images into a digital form. • Digital image processing: Image reduction and enlargement, image enhancement, edge sharpening, and so on, are now at the dispose of user’s fingertips using special softwares, rather than spending a long time in traditional manual manipulation. Two experiments were carried out in this research work, one in a mountainous area, and the other in a flat one. In the first test an aerial wide-angle hard copy stereopair at scale 1:40,000 of mountainous terrain in Switzerland was transformed to a soft copy using a different scanning resolutions by a special photogrammetric scanner. A digital photogrammetric workstation was used to measure a number of control points of known ground coordinates. The root mean square error of the measured ground coordinates of these points and their actual coordinates were computed for each scanning resolution. ERDAS software package was used to digitally rectify the scanned images using the same pixel resolution used above. The coordinates of points were measured and the root mean square errors were computed again. The result of digital rectification is required to be supported by another test in a flat terrain. In this test an aerial wide-angle hard copy stereopair at 1:20,000 scale of a flat terrain, in the center of Khartoum, was transformed to a soft copy and rectified. The results of these investigations proved that the accuracy of the measurements in digital photogrammetry is nonlinearly proportional to the scanning pixel resolution in both horizontal and vertical measurements. Mathematical models were developed to estimate the accuracy against scanning resolution and suitable plotting scales in both horizontal and vertical mapping. Production of maps using digital image rectification using software packages such as ERDAS is suitable only in a flat terrain. On the other hand, a digital photogrammetric workstation is a productive way for mapping in both flat and mountainous terrain