Geometric Accuracy Assessments of Orthophoto Production from UAV Aerial Images

Abstract

Orthophoto mosaic is assembled from aerial perspective images through a process called orthorectification, which eliminate photographic tilts and terrain relief effects. These orthorectified images have been resampled from the original ones that may have been prepared from a DTM which does not accurately model the surface. Meanwhile, some proprietary software such as Agisoft utilizes spatially dense 3D point clouds that are generated from a so called Structure from Motion technique to generate the orthophoto. The software provides a black-box method to regard these clouds as DSM, and it utilizes this surface model to project pixels from the original images. This paper investigates geometric accuracy of the produced orthophoto mosaic according to the American Society of Photogrammetry and Remote Sensing (ASPRS) standards. To minimize scale differences among images, a 35mm fixed-lens camera is mounted on a fixed-wing UAV platform. Flight missions are carried out at around 250m flying height controlled by a navigational grade sensor on board to provide spatial resolution of about 27mm. A number of orthophoto mosaics are produced by varying numbers of GCPs, flight paths configuration and terrain relief differences. The geometric accuracies are assessed through a provision of ICPs on each test field area. Coordinates deviations between the ICP and the corresponding orthophotos are framed into a RMSE figures. Within a 95% confidence level, it is revealed that a suitable orthophoto map scale is up to around 1:500. It is recommended that a cross flight configuration to achieve better results.