The photogrammetric approach, also known as stereoscopic feature extraction, is one step closer to the source of the data than GIS-based digitizing. Instead of compiling feature data from an orthophoto or another source of information, stereo extraction uses the original images along with the triangulation metadata. The triangulation metadata consists of exterior orientation parameters that are required for stereo pair generation. Exterior orientation data could also come from airborne GPS/IMU data – which may be necessary for mapping remote areas where collecting ground control points is not an option.
Stereo feature extraction is inherently 3D. The vector data are measured in X, Y, and Z by manipulating a floating cursor while viewing a stereo pair of left and right images. As a result, the point, line, and polygon feature data all have Z values associated with each vertex. Because GIS developed in a 2D paradigm, many early stereo feature extraction packages were coupled with CAD environments as a platform. Basically a stereo viewer would be added for stereo image viewing and 3D measurement, and the feature data would render in the application's native “viewer”. In more recent times applications have been developed for GIS packages in addition to CAD environments, as well as a number of native applications that do not require platform technology.
CityGML is intriguing, as the geometric properties are but one aspect of urban information modeling and are viewed as an inter-related component of a much broader system. But persistence models aside, tools are still required for the original content generation.
More on specific 3D feature extraction tools tomorrow...