Practice of Fast Tilt Photogrammetry for Consumption-grade UAV in Xinjiang

ABSTRACT: The image control points are measured by RTK and Phantom 4PRO, a consumer UAV in Xinjiang, respectively. Finally, the results are tested. The results show that the tilt three-dimensional results can be obtained by RTK radio mode (without coordinate correction, low accuracy) and RTK-CORS network mode. The relative coordinate difference of image control points is small, but the measured coordinate difference and the map coordinate difference are large. The plane accuracy and elevation accuracy of RTK-CORS image control points are 0.036m and 0.135M respectively, and the measured distance deviations of four kinds of obvious objects are within 2cm. Low-precision image control points can assist the rapid establishment of three-dimensional results of tilt photogrammetry. If accurate coordinates need to be concerned, it is feasible to use RTK-CORS network model to measure image control points to assist small-scale fast tilt photogrammetry of Phantom 4PRO UAV in Xinjiang.
Key words: UAV; Tilt Photogrammetry; Practice
Introduction
_UAV tilt photogrammetry takes UAV as flight platform, carries one or more tilt photography systems (digital cameras) to obtain multi-angle and multi-overlap ground multi-view images, and through post-processing, establishes three-dimensional surface model and related digital products. Compared with vertical photogrammetry, tilt photogrammetry has more advantages. In recent years, due to the maturity of UAV technology and digital imaging technology, the realization of high-speed image matching operation based on computer, and the demand of various industries for new 6D mapping products (3DM, DSM, TDOM, DOB, DEM, DLG), etc., tilt photogrammetry has developed rapidly. The aerial surveying effect is better when the aircraft is equipped with multi-lens tilt camera (such as Leica RCD-305 lens) or the UAV is equipped with ultra-light multi-lens tilt camera. For example, Xu Siqi's aerial test of dual lens tilt camera achieves 1:500 map mapping accuracy. Professional UAV tilt photogrammetry is usually equipped with 5-lens fixed or double-lens swing tilt photogrammetry system, RTK can also realize ground image-free control point measurement, but the price is expensive. The research of UAV tilt photogrammetry focuses on tilt camera test, three-dimensional modeling, topographic mapping and accuracy analysis, industry applications and other fields. The tilted image acquired by a small consumer UAV through a single lens and a specific route can also be recognized by the mainstream three-dimensional modeling software ContextCapture, Pix4DMapper, etc. After the completion of three-dimensional modeling, EPS, DP-Modeler and other software can also be used for later refinement modeling and vector extraction. In the research of aerial survey of small consumption UAV, there are many studies on vertical photogrammetry of Xinjiang UAV. For example, Zhang Chunbin and others have carried out the practice of vertical photogrammetry at six altitudes between 50m and 100m based on the Xinjiang Elf 3PRO, with the accuracy reaching centimeter level. The practice of tilt photogrammetry, such as Kong Zheng et al. [14] is based on the tilt measurement data of Phantom 2 (with DJI2 + lens) and Phantom 3 (with DJI3) in Xinjiang. The results are better than 1:500 aerogrammetric accuracy. The research route is a self-defined airline based on the survey area, and the effect of popularization in a wide range is obvious. It needs further verification.
_. The Phantom series UAVs in Dajiang are economical and suitable for inclined photogrammetry, low requirement for takeoff site and flexible operation, but they have limited power, no high precision RTK and no capability for large-scale mapping. Because of the high buildings and various landforms in urban areas, it is difficult to tilt photogrammetry. The factors such as finding geodetic control points, setting up RTK radio stations, coordinate acquisition and correction, repeating routes and the large amount of data it brings, and unmanned aerial vehicle power, etc. reduce the aerial survey efficiency. The accuracy and efficiency of tilt photogrammetry are all related to the selection of appropriate flight control software, suitable route and RTK acquisition of phase control points.
based on Phantom 4PRO UAV (Pix4DCapture flight control software) in Chuxiong City, this research carries out tilt Photogrammetry in a university. Using the southern surveying and mapping Galaxy 1RTK two surveying modes (precision mode and rough mode) to obtain three-dimensional coordinates of image control points, completes three-dimensional modeling based on Contact Capture, and carries out post-processing based on EPS. In order to explore a simple, efficient and fast method of small area oblique Photogrammetry for consumer UAV, the accuracy of the test results is checked according to the data of RTK checkpoints and the apparent distance between ground objects and steel rulers.
_1. The process, research area and research flow of tilt Photogrammetry
The main process of tilt Photogrammetry for Xinjiang UAV is as follows:
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_After triangulation and texture mapping, the digital surface model is generated. On this basis, three-dimensional modeling, ortho-rectification, vector extraction and fine modeling of terrain features are carried out to form a new 6D product for surveying and mapping.
_Research area is a new campus of a university in Chuxiong City, Yunnan Province. The terrain is relatively flat. The main buildings are six floors. The campus includes teaching building, student dormitory, canteen, student hall, sports ground and so on.