外文翻译 - 图文(3)

YUAN Xiuxiao

Abstract This paper describes the operational issues and basic technical requirements of modern aerial photogrammetry.The accuracy of photogrammetric point determination and the y-parallax at corresponding model points is analyzed when stereo models are reconstituted by using the exterior orientation elements of aerial images. Real aerial photographs, at image scales from 1:2 500 to 1:6 0000，with DGPS/IMU data taken from various topographies in China were processed by our POS-supported bundle block adjustment program WuCAPS. The empirical results verified that the accuracy of the exterior orientation elements from bundle block adjustment meets the requirements of the specifications of topographic mapping.However, the accuracy of the exterior orientation elements determined by POS fails to meet the requirements of the specifications of topographic mapping. Keywords aerial triangulation (AT); GPS (global positioning system）；POS（position and orientation system）；stereo model reconstitution；ground control points (GCPs)；accuracy Introduction

Aerial photogrammetry is the science and technology for obtaining 3-dimensional spatial information about the Earth's surface from aerial images．Photogrammetric point determination, which locates ground objects by using images, is the basis for object recognition in remote sensing. And the key point of this issue is the rapid and accurate determination of an image's position and behavior at the instant of imaging. This goal was met by aerial triangulation based on well distributed GCPs.

With the development of spatial positioning technology, remote sensing technology, and computer science, aerial triangulation evolved and progressed towards digital mapping without GCPs．In the early 1950's, photogrammetric scientists began studying how to utilize various auxiliary data to reduce the number of GCPs required. However, the methods haven't become practical due to technological limitations

[1]．Until 1970's, with the emergence of American Global Positioning

System (GPS), people got to adopt carrier phase differential GPS（DGPS）technology to determine an exposure station's positions(that is three linear elements of aerial photos) during aerial photographic process, which was used to perform aerial triangulation (called CJPS-supported AT for short) that can decrease photogrammetric reliance on GCPs, shorten the mapping cycle; and reduce production costs, triggering the revolution in the field of photogrammetry

[2].Nevertheless, GPS-supported AT is

advantageous for aerial photogrammetric operation primarily over vast and difficult areas, at small and medium mapping scales, not for strip-like zone and urban large-scale mapping

[3]．In the 1990s,people started to investigate employing GPS/lNS

integrated system (also called POS) to acquire a photo's position and attitude (i.e.，to obtain exposure station's position by GPS，and images’attitude elements by IMU), for the purpose of photo orientation, and the final goal is to replace block aero triangulation procedure

[4?9]．

Modern digital photogrammetry will play an important role in automated productions of 4D products(DEM, DOM, DLQ DRG) and updating of spatial databases. This paper will introduce current operational applications of aerial photogrammetry and related technical requirements, in particular, geometric positioning accuracy obtainable in the photogrammetric information chain from photo orientation to stereo-model reconstitution, aiming to investigate their practicability for 4D products production. It is hoped that findings from this study will provide guidance for operational aerial photogrammetry in the context of national, land surveying, mapping, and fundamental geographic information acquisition. 1 Current patterns of the modern aerial photogrammetry

Nowadays, there are primarily three patterns for aerial photogrammetry, namely, standard aerial photogrammetry GPS-supported aerial photogrammetry and POS-supported aerial photogrammetry. Their main procedures are shown as Fig. l.

From Fig.l，we can learn that the main difference between these three patterns lies in the ways of aerial photo acquisition and photo orientation. For standard AT, it is through block aerotriangulation with a large number of GCPs to get a model orientation points’ coordinates to complete image orientation. For GPS-supported AT, in aerial photo acquisition process,dynamic GPS positioning is used instead of GCPs to determine the positions of exposure center and meanwhile obtain the model's

orientation points’coordinates, which are then used to rectify the image's orientation. For POS-supported AT, images and their corresponding orientation elements (six exterior orientation elements of images) are both acquired, in order to realize geometric inversion of photography by storing their spatial positions and attitude at the moment of exposure． 2 Related technologic requirements 2 .1 Aerial photography

In modern aerial photography, in order to improve the quality of obtained images, besides adding flight control systems to aerial camera (such as ASCOT,CCNS4, Aerial TRACKER system), the methods include sticking a GPS receiver with the camera firmly when adopting GPS aerial photography and mounting POS system on the camera in DGPS/IMU aerial photography. According to the different patterns of aerial photography, we can formulate an answerable plan as shown in Fig.2. 2.2 Ground control plan

In digital photogrammetry workstations, aerotriangulation is carried out by the most theoretically rigorous procedure of bundle block adjustment, but for the sake of obtaining the best pass points' coordinates and the exterior orientation elements of photos,ground control plan should be designed, as shown in Fig.3，for different patterns of aerial photogrammetry.

2.3 Digital mapping

Theoretically, after getting the accurate exterior orientation elements of images, measurable stereo models can be reconstructed using model restoration,by which we can do surveying and mapping of terrain and objects automatically. However, the

current process of producing 4D product is：single photo interior

?relative orientation of stereo pair?single model absolute

orientation?surveying and mapping on stereo models．The method of model

orientation

restoration is only adopted in the direct georeferencing of POS-supported aerial photogrammetry.

3 Experiments and analysis

There are two ways of aerial photogrammetric positioning. One is called block aerotriangulation, regarding image points’coordinates, GCPs’coordinates and/or the exterior orientation elements of images as weighted observed values, and combined bundle block adjustment is performed to solve the images’ orientation parameters and target points’spatial coordinates, so as to supply orientation control points for stereo model mapping and do highly accurate geometric positioning. For aerial photogrammetry of different scales and topographic types, topographic maps specifications for aerophotogrammetric office operation has defined respective aerotriangulation method, ground control plan, and also concrete standards for pass point accuracy. This method is established and widely used. The other is called direct georeferencing, under the supposition that highly accurate image elements of exterior orientation were available, space intersection is carried out to calculate corresponding object point's object space coordinates by using photo coordinate system's coordinates of conjugative image points in stereo pairs. This approach directly determines the object's position, so 4D products can be produced. Then the paper will mainly discuss how well the positioning accuracy can be achieved and the stereo model Y-parallax when using image exterior orientation elements obtained in various ways. 3.1 Data

Experiments were implemented on 4 groups of actual images from different areas as shown in Table 1.All negatives were scanned with a resolution of 21 m, and in order to get the tie points, POS-supported bundle block adjustment software WuCAPS was used for images of test 1，test 2 and test 4, and homemade JX-4 digital photogrammetry workstation was used for images of test 3．The GCPs are all measured manually in the stereoscopic mode, and the accuracy (RMS) of all image points is statistically better than 16.0 m according to the results of relative orientation modual with the function of gross error elimination by WuCAPS．After that, we use Applanix POS/AV system's postprocess software POSPac

[10] to do test field

calibration and the integrated process of DGPS and IMU data, then by applying coordinate system transformation and system error rectification,six exterior

orientation elements of each image,which were provided by the POS system, can be obtained.

3.2 Performance of exterior orientation elements

In order to analyze the performance of exterior orientation elements obtained by different methods,standard AT with dense GCPs on the border and GPS-supported AT with four full GCPs in corners were firstly implemented, the six exterior orientation elements of each image can be obtianed, and their theoretic accuracy can be estimated. Then we assumed the results of standard AT as the \and estimated the performance of exterior orientation elements provided by POS．The results are shown in Table 2．

From Table 1，we see that the images of test 1，test 2 can be used for producing 4D product at the scale of 1:500~1:2 000, and images of test 4 for that at the scale of 1:5000~1:10 000. In the principle of Topographic Maps Specifications for Aerophotogrammetric Office Operation

[11?13],test 1，test 2, test 3，and test 4 belong to

flat land, mountain land, high mountain land, and lowland, respectively. From the results in Table 2, some conclusions can be summed up as follows.

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