Research On Key Technologies Of Direct Georeferencing In POS-Supported Aerial Photogrammetry

Aerial photogrammetry has become the main method of data acquisition in several aspects, such as fundamental topographic mapping, geological surveying, land resources development and so on. Traditional aerial photogrammetry needs a great number of Ground Control Points (GCPs), then, its application is limited in the region where laid for the forest, desert and mountains. Along with the use of GPS in aerial photogrammetry, the number of GCPs is reduced. Although, the GPS-supported aerial photogrammetry restricts the shape of surveying areas, it can’t be used in the strap areas such as line, river and coastline. Also, GPS-supported aerial photogrammetry still needs aerotriangulation. The positioning and orientation system (POS) integrates the global positioning system (GPS) and inertial navigation system (INS), when POS is used in aerial remote sensing, the exterior orientation elements of images can be obtained directly, that is, direct georeferencing (DG) can be performed in aerial photogrammetry. This technology breaks the limitation of aerial triangulation which is required in traditional aerial photogrammetry and GPS-supported aerial photogrammetry, and it may have a broad application foreground. But, the hardware and software of POS are imported from abroad in our country presently, and the key technologies and application criterions are depended on the foreign experiences. So, it has great practical significance to research the feasible methods, reduce its application limitation. In this way, direct georeferencing will be used in 4D producing expediently.This paper is focus on the key problems of direct georeferencing in pos-supported aerial photogrammetry. The main research contents and conclusions are as follows:1) Research on the transformation method of exterior orientation elements obtained by a POS in national coordinate reference systems. On the basis of transformation analysis on the coordinate systems related to POS, the transformation process from attitude and position data obtained by POS to exterior orientation elements used in photogrammetry is researched. On the basis of effect theories of earth curvature and meridian deviation on exterior orientation angular elements of images, the method using a compensation matrix to correct the transformation errors from attitude angles obtained by the POS to exterior orientation angular elements of images is proposed in this paper. Moreover, the rigorous formula of the compensation matrix is deduced. Three sets of actual data obtained by POS AV 510 which are different in scale and terrain are selected and used to perform experiments. The empirical results not only indicate that the compensation matrix proposed in this paper is correct and practical, moreover, transformation accuracy of exterior orientation angular elements computed based on the compensation matrix deduced in this paper is higher than that obtained by the POS.2) Research on the theoretical accuracy of direct georeferencing by position and orientation system in aerial photogrammetry. The mathematical models of theoretical accuracy in direct georeferencing by exterior orientation elements are provided in this paper, which are based on the formula of spatial forward intersection、collinearity equations and error propagation rule. Three sets of actual data which are different both in scale and terrain are used to prove the validity and feasibility of the mathematical model of direct georeferencing. The experiment verifies that theoretical accuracy of direct georeferencing basing on collinearity equations is better than which basing on spatial forward intersection. On the basis of these test results, the author suggests that ground object points can be computed through the direct georeferencing based on collinearity equations when exterior orientation elements obtained by POS system are used to stereo mapping, further more, all points in the test area may be computed together as possible as we can.3) Research on a novel method of self-calibration direct georeferencing. On the basis of delivering position errors caused by the systematic errors in a POS, the specific mathematical model of compensating the errors on the image is given. Then, the adjustment model of self-calibration direct georeferencing is also given. The results of eliminating POS systematic errors and position accuracy using the self-calibration direct georeferencing for different ground control conditions are tested using three sets of actual aerial photos. The empirical results verified that when two full GCPs are emplaced in surveying regions and their image points distribute in opposite direction, the self-calibration direct georeferencing by a POS can satisfy the requirements of aerial photogrammetric topographic mapping, but also avoids the use of a special calibration field. On the other hand, the vertical parallaxes of stereo models computed with the exterior orientation elements optimized by this novel method minish obviously, it can provide an effective method for vertical parallaxes eliminating in reconstructing stereo models.4) Research on the digital surface model (DSM) production supported by POS. The exterior orientation elements optimized by self-calibration direct georeferencing can be used to support the quickly matching of image points. After that, collect the exterior orientation elements after optimization and a large number of image points, the corresponding ground coordinates of image points can be determined by the direct georeferencing basing on collinearity equations. According to the ground coordinates of all points, the DSM can be produced by the actual methods. The DSM productions are tested using two sets of actual aerial photos. The first set was taken over a mountainous region at a scale of 1:3 000. The second set was taken over an upland region at a scale of 1:60 000. The empirical results verified that the exterior orientation elements optimized by self-calibration direct georeferencing can be used to DSM production of the related scale and terrain.