USAGE OF STEREO ORTHOIMAGE IN GIS: OLD CONCEPT, MODERN SOLUTION
Keywords: Photogrammetry/GIS, Visualization, Orthorectification, Stereoscopic
Abstract. The main goal of this paper is to find answer for two questions: why stereo orthoimage (orthophoto) was forgotten, and furthermore, what conditions have to be fulfilled for this particular technique to be reanimated?
Stereo orthophoto concept was first introduced by Collins, during late seventies of last century (Colins, 1968). It has ignited huge interest, and many researchers were looking forward for its development (Blachut 1976, Kraus et al. 1976). However, after around twenty years, interest in stereo-orthophoto has shrunk, and during eighties of XX century it was almost extinct. Despite of fact that digital photogrammetry provides better conditions for it now than ever, it has never got chance for reactivation. There was but few papers about stereo orthoimage, and they are proposing continuous stereo ortho-mosaic generation while preserving its metric value (Li et al 2002, Wang 2004).
Reasons of downfall of this interest were examined, and it has been concluded that the main cause was usage of this technique in conjunction with analog photogrammetry. The radiometric value of orthophotomaps and stereo components developed on photosensitive paper were very low during that time. Equipment for observation and measurement were simply very big and therefore impractical. Authors have concluded, that nowadays sum of gains outweighs its flaws and it is reason enough to go back to it. But in a time of digital vector 3D models there is a need for proper strategy for popularization stereo orthophoto. Main point of this strategy would be encapsulating stereo orthoimage method into GIS tools, optimally open source. GIS have large numbers of consumers whom regard for orthophotomap (or orthoimage) is very high. If they were to be proposed with possibility of using stereo orthoimage inside a GIS tool, as a complementary to the orthophotomap, probably they would recon how much stereoscopy helps with interpretation of an image.
Authors have planned to develop software for utilization of stereo orthoimage available from the Quantum GIS interface, and test version is to be based on anaglyph stereoscopy. Value of creating continuous stereo-mosaic composed from many 3D models was assessed. Consensus was, however, that this solution surprises us with frequent perspective changes which leads to discomfort. As an outcome it was decided to focus on single stereo orthoimage pairs. When area constrains are specified for single model, one can load them as requested by an user. In case of increased lateral and longitudinal coverage, which is more common in modern aerial photogrammetric flights, every localization will be accessible for visualization using several models, and there will be less concealed areas.
Few algorithms for generation of parallax are tested. After completion of sets from a dozen or so example models of interesting areas are gathered, there will be release. Authors are confident that stereo orthoimage may prove more useful for GIS users than true orthophotomap.