POS-supported Automatic Image Measuring

Automatic image measuring is to identify and to measure conjugate points in two or more overlapping photographs automatically. In digital photogrammetry, this process is done by image matching. Image matching is an ill-posed problem because it violates several if not all of the conditions a well-posed problem must meet. A straightforward way to make image matching well-posed is to restrict the space of possible solutions and provide good initial parallax.Initial parallax is related to the searching space. The better the initial parallax, the smaller the searching space. So in image matching, the initial parallax should be determined firstly.In traditional automatic aerial triangulation, the initial value is decided by gray-correlation. When there is large rotation angle between stereo pairs and when the image is of isolated island in the sea or dense lakes or desert areas, this method can not decide accurate initial parallax. The incorrect initial value will lead to failure of image matching. When transfering conjugate points between strips, gray-correlation always cann't decide the initial parallax because there is a relative small overlapping area for searching. Usually, several conjugate points are determined by the operator before image matching. This greatly reduces the automatic level of aerial triangulation.With the development of photogrammetry technology, new sensors are constantly introduced to the application of photogrammetry. Integrating the Global Positioning System (GPS) with an Inertial Navigation System (INS) allows for direct image georeferencing, offering a possibility of relaxing the demand for aerial triangulation (AT) in airborne surveying/mapping. A lot of experiments have shown that the orthograph image produced with the EO (exterior orientation parameter) gained by DGPS/IMU can reach a high precision. For low contrast areas such as desert, it is very useful to introduce DGPS/IMU data as an additional parameter to aerial triangulation.Since POS can get EO of the image, using the EO value and the corresponding elevation to predict the location of conjugate points can get a better initial parallax. Thus, the limitation of traditional method in deciding initial parallax, such as gaining incorrect initial parallax, large searching space, etc, may be hopefully improved. Then the operator's interference in automatic image measuring will reduce and the efficiency, reliability of image matching will increase.This paper studies the use of POS data in automatic image measuring. Using POS data to decide the initial parallax can avoid the dependence of image condition. So the parallax decided will be better than that of the gray-correlation's which is depended on image condition. In this paper, three methods are introduced to decide the initial parallax. They are gray-correlation, edge-grads statistics and POS data. Several different experiments are carried out of different scene to evaluate the applicability and strongpoint of POS-supported automatic image measuring.