Seeing through shadow: Modelling surface irradiance for topographic correction of Landsat ETM+ data

Despite advances in remote sensing, retrieving surface properties at high resolutions in complex terrain is a major challenge. Slope and aspect as well as the topography surrounding a target impact surface insolation and lead to variability in calculated surface reflectance even for homogeneous land cover. Retrieval of surface reflectance is particularly problematic in case of topographic shading, where the total irradiation at the surface is a combination of diffuse irradiation and terrain-reflected irradiation from nearby slopes. To facilitate the retrieval of surface reflectance from high-resolution optical remote sensing, we have explored the feasibility of using a three dimensional radiative transfer code to simulate gridded surface irradiance for a ∼37km2 area in the New Zealand Southern Alps. We have tested the sensitivity of simulated irradiance and calculated surface reflectance both in- and outside shaded areas to atmospheric aerosol content, surface albedo, atmospheric boundary layer structure and different solar spectra. Retrieved surface reflectance has been shown to be highly sensitive to atmospheric aerosols and surface albedo, particularly for areas shaded by topography. Not considering atmospheric aerosols in topographic correction can increase derived surface reflectance by well over 50%, while terrain-reflected irradiance can contribute 40% to surface reflectance in shaded areas, even for wider valleys. Both factors should therefore be considered in topographic correction of satellite imagery, even for relatively aerosol-free atmospheres and low surface albedo. Topographic correction for the whole scene was performed with the model settings resulting in the smallest RMSD between surface reflectivity in shaded and unshaded areas of similar land cover. Topographic correction based on 3D radiative transfer simulations has proven to effectively remove topographic effects and almost equalise derived mean reflectance in- and outside shaded areas. While the effective removal of shadows likely requires a higher dynamic range than Landsat’s ETM+ can offer, we suggest further evaluation of this approach in future studies at other sites and with other sensors.