【正文】 errain mapping standards: meters in plane and meters in elevation[12]. 4) For test 4, the horizontal accuracy of direct georeferencing is better than meters and vertical accuracy better than meters, which well meet the requirement specified in 1:50 000 downland terrain mapping standards: meters in plane and meters in elevation[13]. It can be learned from Table 3 that the aerotriangulation approach performs better than POS in obtaining exterior orientation elements when direct georeferencing is applied to images with different land types and scales. This result indicates that using the exterior orientation elements acquired by the aerotriangulation method to perform direct georeferencing 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。181。22, X YXY181。m Hori. Verti. Hori Verti X Y XY Z Xs/m Ys/m Zs/m φ/(″) ω/(″) κ/(″)test 1 Std. 23 39 49 33 GPS 4 4 67 67 POS Std. 39 69 116 86 GPS 4 4 151 151 test 2 POS GPS 4 4 30 30 3 POS test 4 Std. 15 19 10 14 GPS 4 4 25 25 POS Note: 1) Std., GPS and POS stand that the exterior orientation elements are obtained by the method of standard AT with dense GCPs on the border,GPSsupported AT with 4 full GCPs on the corners and POS system, respectively (same to the following tables) . 2) Because the distribution of GCPs cannot meet the requirement of standard AT, for test 3 we cannot carry out the bundle block adjustment withdense GCPs on the border (same to the following tables). 3) RMS of check points’ residual is calculated from the error iΔ (i=X,Y,Z) between the adjusted coordinates and the ground measured coordinatesof n check points, that is 2iin181。 State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, 129 Luoyu Road,Wuhan 430079, China. Email： Geospatial Information Science 11(4): 235242 236 production costs, triggering the revolution in the field of photogrammetry[2]. Nevertheless, GPSsupported AT is advantageous for aerial photogrammetric operation primarily over vast and difficult areas, at small and medium mapping scales, not for striplike zone and urban largescale mapping[3]. In the 1990s, people started to investigate employing GPS/INS integ