Nap-of-the-Object Photogrammetry And Its Key Techniques

With the development of society and technology,we are increasingly demanding to observe the world and acquire refined geospatial information of objects of interest.Compared with other technical,photogrammetry has the advantages of low-cost,convenient data processing,high precision,noncontact imaging and directly with texture.Existing methods often use oblique photogrammetry or close-range photogrammetry to obtain high-resolution and high-quality images of scene of interest.Due to the inherent characteristics of these two methods,there are still problems when use them to acquire refined images of unconventional ground(such as landslides,dams,high slopes,etc.)or artificial objects(such as building facades,tall ancient buildings,landmark buildings,etc.)In recent decades,the rapid development of UAV technology has provided a new approach of data acquisition for photogrammetry.The combination of UAV with traditional aerial or lowaltitude photogrammetry,close-range photogrammetry,and oblique photogrammetry has drawn increasingly attention and research.However,those methods still encounter problems of data acquisition efficiency and quality when applied to obtain high resolution images of unconventional ground and artificial objects.Therefore,the method of Nap-of-the-Object Photogrammetry(No OP)is researched in this thesis on the basis of the requirements of high precision 3D reconstruction and information extraction.The No OP can achieve automatic acquisition of sub-cm or even mm level resolution images of those objects by making full use of the advantages of UAV platforms in data acquisition,and can obtain fine 3D reconstruction results of those objects through high precision aerial triangulation.The main work of this thesis is as follows:(1)This thesis introduces the method of No OP,and explains its concept and meaning.The research background of No OP is the demand of high precision 3D reconstruction and information extraction.The No OP take full use of the advantages of UAV platforms in data acquisition.It has the characteristics of nap-of-the-object fly and automatically imaging towards the target surface.The core strategy of No OP is the “coarse-to-fine” and “from nothing to existing” high precision data acquisition strategy.It can achieve automatically and efficiently obtaining of high-resolution images for unconventional ground or artificial objects,which lays the foundation of refined reconstruction.(2)This thesis researches the method of three-dimensional UAV path planning with initial terrain guidance.Path planning includes calculating the position and attitude of UAV of imaging the target,which is the basis of safe fly to obtain image.According to the characteristics of No OP's objects of interest,this thesis divides them into regular and irregular targets,and develops appropriate path planning method for them.At the same time,the method of planning convergent photography flying path is proposed to improve the accuracy of No OP in the depth direction.The proposed path planning method can achieve automatic and efficient nap-of-the-object imaging using UAV,and ensures the efficiency and quality of image acquisition.(3)The aerial triangulation method of No OP is researched in this thesis.The proposed method can achieve high-precision aerial triangulation processing of No OP data by fully exploiting the constraints of spatial relationships.Firstly,the type of the imaging targets is determined using the GPS of images,and the distance threshold for grouping images using spatial distance constraints is adaptively determined.The images are then grouped to avoid exhaustive matching.Then the spatial overlap constraints are used to further remove the image pairs with small or narrow overlapping area,after which the tie points matching is performed and the matched images are used to construct free network in each group.At last,the exterior parameters from each group are combined using the least square method,and be used as the initial value for the overall GPS-supported self-calibration bundle adjustment.Three sets of typical No OP data are used to validate the effectiveness of the proposed aerial triangulation method.Experiment results demonstrate that the proposed method can effectively process the PNOF data and recovering the high-precision position and attitude of images.(4)The accuracy of No OP is analyzed and evaluated.Firstly,the uncontrolled positioning accuracy of the data acquisition platform used in No OP is explored.Experiment results show that the RTK-UAV has high absolute positioning accuracy,which can provide accuracy guarantee for the No OP's target of interest where is inconvenient to set ground control points.Then,the effectiveness and necessity of oblique imaging of No OP are also analyzed through two experiments,which also valid the correctness of the calculation method for the placement angle in oblique path planning.Finally,the relative accuracy of No OP is evaluated through two sets of data,ant it is found that the relative accuracy can be ensured even using ordinary UAVs.(5)Combined with applications,the practical application value and significance of No OP in the fields of geological survey,disaster emergency,water conservancy engineering,cultural relics protection and high-precision 3D reconstruction of buildings are explained.