Generating UAV accurate ortho-mosaicked images using a six-band multispectral camera arrangement

Abstract

Site-specific weed management in a very early phenological stage of crop and weed plants requires the ultra-high spatial resolution imagery provided by unmanned aerial vehicles (UAV) flying at low altitudes. These UAV images cannot cover the whole field, resulting in the need to take a sequence or series of multiple overlapped (end-lap and side-lap) images. As consequence, a large number of UAV images are acquired for each plot. These multiple overlapped images must be oriented by calculating the bundle adjustment and ortho-rectified throughout an aerial-triangulation procedure to create an accurately georeferenced ortho-mosaicked image of the entire field. As the spatial accuracy of ortho-mosaicked image is mainly dependent on camera calibration and percentage of overlapping, this paper describes the effect of flight parameters and sensor arrangement using an UAV flying at 30 m altitude and six-band multispectral camera (miniature camera array composed of a master channel and five more channels or bands) on the workflow of the mosaicking process. The objective is to develop a procedure to generate an accurate ortho-image. This procedure would be then used in multispectral UAV imagery taken on crops for weed seedling mapping. The main phases of UAV workflow were: sensor calibration, mission planning, UAV flight and processing images. Regarding the sensor calibration, six different geometric calibrations were calculated, one for each image registered corresponding to every band. Referring to mission planning, two end-lap and side-lap settings (60%-30% and 70%-40%) were considered. Calibration and image overlapping are related to the flight duration and accuracy resolving aerial-triangulation. Different aerialtriangulations were calculated for different scenarios taken into account several combinations of calibrations and the overlapping values. The spatial accuracy of the different generated ortho-mosaicked images using both overlapping values and calibration were evaluated by the ASPRS (American Society for Photogrammetry and Remote Sensing) test. The results showed that the best setting of flight to keep the spatial accuracy in the bundle adjustment was 40- 70% overlapping and using the master channel for the aero-triangulation and its own calibration parameters. Generally, high values of end-lap and side-lap improved the accuracy of block photogrammetric adjustment, although they increased the flight length and decreased the area overflow. In addition, the framework to process and mosaic the images taken by this type of sensor was defined.