Linking UAV and Spaceborne Imaging Spectroscopy for Multi-scale Calibration and Validation of a Mediterranean Saline Surface

The increasing availability of spaceborne imaging spectroscopy data requires validation approaches capable of linking measurements acquired at different spatial scales. This study examines the temporary Sal ’e Porcus saline pond in western Sardinia, Italy, as a candidate reference surface for cross-scale spectral comparison. Surface reflectance was characterised using ASD FieldSpec ground spectroscopy, UAV-borne hyperspectral imagery acquired with a Specim AISA Kestrel 10 sensor, and temporally proximate PRISMA and EnMAP observations. UAV surface reflectance was derived by empirical-line calibration using field-measured radiometric targets. The four spectra were linearly interpolated onto a common 1 nm wavelength grid over their overlapping interval within 400–1000 nm and compared using correlation, linear regression and residual statistics. At the analysed sampling location, all dataset pairs showed strong spectral co-variation (Pearson’s r = 0.961–0.995). The strongest linear correspondence was obtained between UAV and ASD spectra (r = 0.995, slope = 1.001, R² = 0.990 and residual RMSE = 0.007). Comparisons involving satellite observations retained the broad spectral pattern but showed pair-dependent slopes and larger residual dispersion. Because the acquisitions were non-simultaneous and differed in spatial support and spectral response, these discrepancies cannot be attributed solely to spatial scale or sensor performance. The findings support the potential of UAV imaging spectroscopy as an intermediate observational scale and justify further investigation of Sal ’e Porcus as a candidate CAL/VAL surface.