Research On Key Technologies Of Combined Wide-angle Camera For Low-Altitude Aerial Photogrammetry

In recent years,the rapid development of photogrammetry technology,especially the emergence of low-altitude aerial photography platform,has made up for the weak of traditional satellite remote sensing and aerial photogrammetry in terms of timeliness and flexibility.With the advantages of low-altitude photography to obtain the high-resolution images,its main application purpose is to carry out large-scale mapping.The flight platform and imaging sensors are the core components of low-altitude photogrammetric remote sensing system.In contrast,the development of the flight platform is more mature than the imaging system,and has a lot of research results,such as fixed-wing UAV,unmanned airships,unmanned helicopters,multi-rotor electric unmanned aerial vehicles,and even traditional manned aircraft can carry out low-altitude aerial photography;but for low-altitude aerial photography imaging system,the development is lagging behind.There are many low-altitude aerial photography measurement platform is generally equipped with a single head digital camera for aerial photography,resulting in lower aerial photography data acquisition efficiency,less effective data volume,poorer precision of data processing and mapping.Moreover,the traditional aerial photography measuring cameras,as well as multi-head combined and larger format aerial cameras available home and abroad are relatively mature,but they lack objective basis to be used for low-altitude photogrammetry applications.Therefore,research and development of camera systems which suitable for low-altitude photography conditions is very meaningful,and the combined wide-angle camera is one of the foundamential solution.Currently,the most important issues for low-altitude aerial imaging system are the efficiency and accuracy of data acquisition,the fundamental reasons are the small camera frame and field of view.The way to solve this problem is to develop wide-angle and larger format camera systems.For digital image sensors,the size is restricted by the material and manufacturing process,the field of view angle is limited by the optical performance of the optical lens,so it is very difficult and expensive to produce larger format and wide field of view imaging system for low-altitude photogrammetry system directly.However,the mosaicability of digital camera images provides another possible approach,it is possible to use multiple small-format cameras to construct a wide-angle aerial camera system with multi-sensor combined imaging technology,thereby increasing the format and field of view.The main purpose of the study is to develop combined wide-angle camera which suitable for low-altitude aerial photogrammetry.The research of the combined multi-heads and wide-angle cameras needs to take into account both theory and application aspects.On the theory aspect,photogrammetry has established complete theoretical on geometric optics.However,the photometric theory is also very important and meaningful for the design of the imaging system for low-altitude photogrammetry system,and making good use of the optical advantages of low-altitude photography is the key point.Secondly,the study of the optical and geometric properties of low-altitude photographic images is also very meaningful to the design of the combined wide-angle camera system.On the application aspect,the core issue is how to combine several small format cameras into wide-angle aerial camera which also approximately satisfies single central projection.The mathematical model and the geometric error of the imaging system should be analyzed theoretically,and feasible algorithm should be studied to achieve the goal of combined wide-angle imaging.Moreover,how to improve the data acquisition quality and processing precision is also the key to its application.The main research and contributions of this dissertation are listed as follows:(1)The optical theoretical of low altitude aerial photography has been studied and analyzed.Firstly,the optical conditions of low-altitude aerial photography were systematically analyzed,the advantages of low-altitude aerial photometry and the disadvantage of field of view of low-altitude single-lens imaging system have been discussed in detail base on the principles of photometric theory to confirm that the necessity and feasibility of using multi-head and combined imaging technology to develop combined wide-angle camera system for low-altitude aerial photogrammetry system.Secondly,the design and assembling aspects of existing airborne aerial cameras home and abroad are summarized,and the characteristics and limitations of those combined cameras for low-altitude photometry are also analyzed.Based on the analysis of the geometric structure of the existing combined cameras,some design and assembling aspects such as the number of imaging units,combination and stitching mode,field of view are discussed.Finally,the purpose to achieve a combined wide-angle camera for low-altitude aerial photogrammetry with five small-format digital camera is confirmed,and the physical structure and detail parameters of the combined wide-angle camera system have also been introduced.(2)The content related to method and process of static geometric error calibration for the low-altitude combined wide-angle camera are studied.Firstly,the mathematical model of the geometric imaging of the low-altitude combined wide-angle camera is deduced,and the spatial coordinate system between the cameras and the virtual image are defined.The strict geometries and mathematical relationship between the individual optical structures(sub-cameras)and the virtual images are established.Secondly,the error of the geometric imaging mathematical model of the combined wide-angle camera is analyzed,and a single camera calibration method based on multi-images and multi-camera joint calibration are carried out to solve the static geometric error of the system with the ground camera calibration test field images.Finally,the accuracy and reliability of the static geometric error calibration are analyzed with the experimental results.(3)A global optimization self-calibration method which takes into account both the position and orientation between the sub-images is proposed to solve the dynamic error of low-altitude combined wide-angle camera system.The purpose of the self-calibration is to compensate the dynamic errors which caused by factors such as system structure,environmental conditions changes during the aerial photography.Firstly,the mathematical model of the self-calibration model is deduced,it takes into account the changes of the position and orientation between the sub-cameras,and established the mathematical relationship between the slight change and the image parallax of images.Secondly,a fast features-based image matching method for the sub-images of combined wide-angle camera is realized,and a global optimization algorithm which based on all pixels participated image geometric matching method is proposed to accomplish self-calibration,and the principle and process of the algorithm are also discussed in detail.Finally,the precision of the self-calibration results are analyzed with the contrast experimental.(4)The integration of combined wide-angle camera and stabilization platform for low-altitude aerial photogrammetry is studied.Large rotation of roll,pitch,and yaw angles decrease image quality and result in image deformation,thereby affecting the ground resolution,overlaps,and the quality of the stereo model.These factors also cause difficulties in automatic tie point matching,image orientation,and accuracy of aerial triangulation.The issues of large-angle photography of low-altitude photogrammetry system with wide-angle cameras are discussed and analyzed quantitatively in this paper,and a simple and lightweight three-axis stabilization platform that works with a low-precision integrated inertial navigation system and a three-axis mechanical platform is proposed to solve this problem.An experiment was carried out with an airship as the flight platform.Experimental results show that the system can effectively isolate the swing of the flying platform,and confirm that stable imaging for combined wide-angle camera system can help to improve the quality of aerial photography imagery and the accuracy of its applications.(5)The production flow of low-altitude aerial photography with combined wide-angle camera system is discussed with experimental examples,the precision of self-calibration and the accuracy of aerial triangulation for different types of experimental dataset are analyzed,and related data products are also evaluated.The experimental results reflecting the capabilities of low-altitude combined wide-angle camera to carry out aerial photography application.At last,the contributions and innovation points are summarized,and the future research on combined wide-angle camera for low-altitude aerial photography in application and theoretic aspects is proposed.