Cost-effective reflectance calibration method for small UAV images

Abstract

We introduce a cost-effective reflectance calibration method for small unmanned aerial vehicle (sUAV) images using ethylene-vinyl acetate (EVA) greyscale reference panels. The goal is to test if such light-weight and low-cost panels can provide sufficient calibration accuracy to support UAV survey projects. The universal calibration equations to convert red-green-blue (RGB) digital number (DN) values of UAV images to surface reflectance values were constructed based on the relationship between RGB values measured by a colour digitizer and surface reflectance values measured by a spectrometer. We compared the calibration results for UAV ortho-mosaic images acquired at three different illumination conditions in late autumn to the results derived from high-cost commercial panels. The comparison showed high degree of agreement between our method using the EVA panels with the traditional methods using the commercial panels. The Mann–Whitney U test verified our method was statistically more significant at all illumination conditions tests. In addition, the calibration results applied for two different sensors and three different flight altitudes acquired in early summer were satisfactory. This method is transferable to various illumination conditions and flight altitudes as long as the effects of shades and the bidirectional reflectance distribution function (BRDF) are minimal. We expect our research could expedite sUAV image calibration by lowering its cost and levelling its availability.