ACCURACY ASSESSMENT OF PRECISE INDIRECT GEOREFERENCING FOR IMAGES OF UNMANNED AIRCRAFT SYSTEMS

Abstract

Unmanned Aircraft Systems (UASs) can collect high resolution and high quality images for local mapping. Before the UAS images can be used for accurate mapping tasks in local areas, the precise position and orientation of the UAS images should first be determined. Direct georeferencing by POS (Position and Orientation System), a combination of GPS (Global Positioning System) and IMU (Inertial Measurement Unit), is the best choice; however, most commercial UASs cannot carry highly accurate IMUs because of the limited payload. Therefore, this study will discuss the accuracy of indirect georeferencing for UAS images. One approach for indirect georeferencing is general aerial triangulation (AT) by using well-distributed ground control points (GCPs). The other one is GPS-supported AT with GPS observations as airborne controls. In this paper, the camera is calibrated by the field method, and the accuracy of these two approaches for indirect georeferencing is presented. Based on 20 horizontal check points and 29 vertical check points, this study shows the stereoscopic viewing accuracy of general AT for UAS images, collected by Canon EOS 5D Mark II camera with 24 mm F/1.4L II USM lens at a flying height of 550 m, is about 0.26 m (ca. 1.73 pixels) in planimetry and 0.27 m (ca. 1.80 pixels) in height. GPS-supported AT produced the stereoscopic viewing accuracy about 0.44 m (ca. 2.93 pixels) in planimetry and 0.55 m (ca. 3.67 pixels) in height. The test results show that the accuracy of these two indirect georeferencing approaches of fixed-wing UAS images can be used for updating local 1/5,000 topographic maps in Taiwan.