Pose Estimation of Line Cameras Using Linear Features

Pose estimation, which determines the position and the attitude of the camera, is a prerequisite for a variety of photogrammetric tasks such as image matching, surface reconstruction, ortho-photo generation, and object recognition. As digital imagery is becoming more popular in photogrammetric applications, features other than points are increasingly employed in pose estimation. For frame imagery, prior research has incorporated geometric constraints into the bundle adjustment to take advantage of high-level features, such as straight lines. This study introduces high-level feature constraints into the pose estimation problem for aerial line scanners. Traditionally, it has been assumed that robust pose estimation for a dynamic (non-frame) sensor cannot be obtained without direct measurements using Global Positioning System (GPS) and Inertial Navigation system (INS). This study attempts to challenge, in part, this argument by showing that the unknown orientation parameters for all scan lines, within a scene captured by line scanner, can be estimated by including linear features in an indirect orientation procedure that is less dependent on GPS/INS data. This indirect orientation is achieved by incorporating straight-line constraints in the bundle adjustment for aerial linear array scanners. A scene captured by an aerial line scanner is composed of sequence of scan lines, each of which may be slightly shifted against each other due to changes in the system’s trajectory. As a result, straight lines in object space do not appear as straight lines in image space. The straight-line constraint is proposed in this study to cope with this phenomenon. This constraint makes use of straightline features in object space to aid independent recovery of exterior orientation parameters of the entire scan lines as well as to increase geometric strength of the bundle adjustment. Straight lines have been adopted since imagery of manmade environments is rich with straight lines. Moreover, free-form linear features can be represented with sufficient accuracy by a sequence of straight lines (poly-lines). Experiments conducted in this study show the effectiveness of using free-form linear as well as straight-line features in the recovery of the exterior orientation parameters of aerial line scanners.