Vicarious Radiometric Calibration of UAV-Data in Mountainous Area Using Semi-Empirical Line Approach and In-Situ Spectroradiometric Measurements

In the present study, the ground-based vicarious calibration based on in-situ spectroradiometric measurements and a semi-empirical line approach (SELA) using pseudo-invariant targets (PITs) has been applied to convert the raw data (DN(λ)) acquired with a UAV-multispectral imaging system (Phantom DJI Pro 4+) to the ground-surface reflectance factors (GSRF(λ)) in the visible, red-edge and NIR wavebands. To achieve so, spectral measurements were acquired using an ASD considering four types of homogeneous bright and dark PITs (i.e. white ceramic tiles, concrete surface, asphalt, and very dense vegetation cover). Statistical regressions were established (p<0.05) and the calibration equations were developed. An accuracy assessment was undertaken by comparing the calibrated reflectances to those measured in the field. The results revealed that the established calibration equations were achieved with an excellent coefficient of determination (0.94 ≤ R
²
≤ 0.97) for all considered bands. While the GSRF were retrieved with an RMSE ≤ 5.4% for visible, and around 3% for red-edge and NIR. The magnitudes of these RMSE are in agreement with the expected absolute radiometric calibration accuracies reported by the literature, providing good quality-assurance to retrieve biophysical and/or geophysical parameters comparable in time and consistent with other calibrated datasets. Undoubtedly, the SELA can be used with confidence ensuring acceptable accuracies for radiometric calibration and atmospheric corrections of imagery data acquired by UAV-mounted imaging system.