【正文】 ng can satisfy the accuracy standards of topographic surveying. So it can be deduced that so long as aerotriangulation meets the accuracy standards, the exterior orientation elements derived from that can be used absolutely for producing 4D product. YParallaxes in reconstituted stereo models In the work of photogrammetry, another issue we should pay attention to is the feasibility of directly reconstructing the stereo model for terrain mapping YUAN Xiuxiao/On Stereo Model Reconstitution in Aerial … 241with exterior orientation elements used, that is Yparallax of model point is not beyond 20 m[1113]. So three stereo pairs depicting different terrain types are chosen from four tests respectively to reconstruct stereo model using the obtained exterior orientation elements[15], and Table 4 is the Yparallax on stereo model’s corresponding points. From Table 4, it can be seen that when using the exterior orientation elements obtained by the method of aerotriangulation to reconstruct stereo model, no matter what type of terrain is applied and how much the scale is, the model points’ maximum Yparallax lies in one pixel and the RMS of that is below half a pixel, which satisfies the accuracy requirement of terrain mapping that vertical parallax’s RMS of each model should not be beyond 20 m. But when we use the exterior orientation elements provided by POS, the vertical parallaxes of each model point are all slightly larger, and the smaller the photo scale, the larger the vertical parallax, pletely not meeting the requirement.Table 4 YParallaxes of stereo models reconstructed by exterior orientation elements Yparallax/μm Maximum Minimum RMS Image Model Pts Maximum height difference /m Regular GPS POS Regular GPS POS Regular GPS POStest 1 282/281 23 274/273 32 343/344 27 test 2 14/13 26 29/28 47 275/274 34 test 3 1017/1018 31 1013/1014 22 234/233 54 238/237 23 test 4 1076/1075 32 1115/1114 28 1105/1104 28 4 Conclusion It can be shown from the experiment that, if the elements of the exterior orientation obtained from aerotriangulation meet the accuracy standards, they can be used directly for image orientation and stereo model reconstitution. However, because of systematic errors in POS exterior orientation elements, it is currently difficult to meet the standards of photogrammetry, especially when extracting 3D spatial information. It was found that, in the time of digital photogrammetry, much work can be done automatically by puters, with the reliance on GCPs lessened gradually, thus simplifying operational photogrammetry. On the whole, standard AT, which is the most established and widelyused approach to obtain image orientation parameters, is still the main body of photogrammetry。 GPS (global positioning system)。 for difficult areas, noncharted areas or areas that are not accessible, GPSsupport AT without GCPs can be adopted to acquire the basic spatial information for producing national base maps。=+. 4) Theoretical accuracy of EO is obtained from the unknown covariance matrix calculated according to the law of error propagation,0(,)iiissmQXYZσ ? ωκ== . 1) For test 1, densified points’ horizontal accuracy and vertical accuracy are better than meter, totally meeting the accuracy requirement: meter in planimetry and meter in height[11] of 1:500 Topographic Maps Specifications for Aerophotogrammetric Office Operation for flat land. 2) For test 2, densified points’ horizontal accuracy is better than meter and vertical accuracy better than meter, totally meeting the accuracy requirement: meter in planimetry and meter in height[11] of 1:500 Topographic Maps Specifications for Aerophotogrammetric Office Operation for mountain land. 3) For test 3, densified points’ horizontal accuracy is better than meters and vertical accuracy better than meter, totally meeting the accuracy requirement: meters in planimetry and meters in height[12] of 1:5 000 Topographic Maps Specifications for Aerophotogrammetric Office Operation for mountain land. 4) For test 4, densified points’ horizontal accuracy is better than meters and vertical accuracy better than meters, totally meeting the accuracy requirement: meters in planimetry and meters in height[13] of 1:50000 Topographic Maps Specifications for Aerophotogrammetric Office Operation for high mountain land. It can be seen from Table 2 that for the images of different land types at different scales, the densified points obtained from standard AT and GPSsupport AT both satisfied the requirement of 4D product, and that the accuracy of exterior orientation elements obtained by these two methods are generally similar. The larger the photo scale is, the higher the accuracy of the photo’s linear elements we can get。 POS (position and orientation system)。 POS photogrammetry can be used for the production Geospatial Information Science 11(4): 235242 242 of orthophotos and update of 4D products in small regions. However, POS has a promising prospect in the field of large