Digital ortho-images - A powerful tool for the extraction of spatial- and geo-information

Digital ortho-images provide great advantages in comparison to their analogue counterparts, especially with respect to flexibility, production of derived products and combination with other data sets. Today, production of digital ortho-images has become more commonplace due to the development of more powerful computers with sufficient resources, easier acquisition of input data, increased generation of digital data, development of many commercial ortho-image production systems, and new application areas, particularly in connection to GIS and digital mapping. After a review of past developments and the current status, the paper focuses on different extensions of ortho-images. These extensions permit the extraction of spatial information based on orthorectified stereo pairs, stereomates, monoplotting, and image analysis techniques. Emphasis is placed on the extraction of 3D information using orthorectified stereo pairs. An overview of practical usages of ortho-images in a GIS is also presented. Currently, digital ortho-images are primarily used either for visualisation or as a backdrop. However, ortho-images offer many, to a large extent unexploited, possibilities, especially for the update and generation of databases, and automated mapping. The paper concludes with an outline of future trends in research and development.