Automatic Object Extraction from Aerial Imagery – A Survey Focusing on Buildings Helmut Mayer Chair for Photogrammetry and Remote Sensing, <s> The General Parts of the Strategy are: <s> The Ascender system acquires, extends, and refines 3D geometric site models from calibrated aerial imagery. To acquire a new site model, an automated building detector is run on one image to hypothesize potential building rooftops. Supporting evidence is located in other images via epipolar line segment matching in constrained search regions. The precise 3D shape and location of each building is then determined by multiimage triangulation under geometric constraints of 3D orthogonality, parallelness, colinearity, and coplanarity of lines and surfaces. Projective mapping of image intensity information onto these polyhedral building models results in a realistic site model that can be rendered using virtual “fly-through” graphics. As new images of the site become available, model extension and refinement procedures are performed to add previously unseen buildings and to improve the geometric accuracy of the existing 3D building models. In this way, the system gradually accumulates evidence over time to make the site model more complete and more accurate. An extensive performance evaluation of component algorithms and the full system has been carried out. Two-dimensional building detection accuracy, as well as accuracy of the three-dimensional building reconstruction, are presented for a representative data set. <s> BIB001 </s> Automatic Object Extraction from Aerial Imagery – A Survey Focusing on Buildings Helmut Mayer Chair for Photogrammetry and Remote Sensing, <s> The General Parts of the Strategy are: <s> Hierarchical feature based stereo matching and motion correspondence algorithms are presented. The hierarchy consists of lines, vertices, edges and surfaces. Matching starts at the highest level of the hierarchy (surfaces) and proceeds to the lowest (lines). Higher level features are easier to match, because they are fewer in number and more distinct in form. These matches then constrain the matches at lower levels. Perceptual and structural relations are used to group matches into islands of certainty. A Truth Maintenance System (TMS) is used to enforce grouping constraints and eliminate inconsistent match groupings. The TMS is also used to carry out belief revisions necessitiated by additions, deletions and confirmations of feature and match hypotheses. <s> BIB002
Appearance-Based Approaches   avoid the explicit transition from image model to object model  . With them, objects like trees can be extracted, for which the modeling is quite difficult by other means because they have a relatively varying appearance. They also can be used for the extraction of details on the rooftop or cars on roads when the resolution is close to the point where these objects cannot be extracted any more. Grouping   BIB001  BIB002  , i.e., the search for geometric/topologic regularities, allows for focusing on parts of objects and therefore limits the search space. An often encountered problem is that the regularities specified are not strict enough to ensure a reliable extraction. Grouping should therefore be accompanied by verification.   the extraction is at the same time sped up as well as improved. By using multiple scales one can start with reliable structures in coarse scale and use them to focus the extraction on the specific areas and object types in fine scale.