Analysis of directional effects on atmospheric correction

Atmospheric correction in the Visible and Near Infrared (VNIR) spectral range of remotely sensed data is significantly simplified if we assume a Lambertian target. However, natural surfaces are anisotropic. Therefore, this assumption will introduce an error in surface directional reflectance estimates and consequently in the estimation of vegetation indexes such as the Normalized Difference Vegetation Index (NDVI) and the surface albedo retrieval. In this paper we evaluate the influence of directional effects on the atmospheric correction and its impact in the NDVI and albedo estimation. First, we derived the NDVI and surface albedo from data corrected assuming a Lambertian surface, then by using the BRDF model parameters used in the albedo retrieval, we account for the coupling effect and retrieved an improved NDVI and albedo. The study used Climate Modeling Grid (CMG) MODIS data, which has a spatial resolution of 0.05°. We focused our analysis on four Aeronet sites located in the United States of America: KONZA EDC (a prairie region in the Flint Hills, Kansas), Howland (a forest area in Maine), Walker Branch (a forest area in Tennessee) and GSFC (at NASAʹs Goddard Space Flight Center). The results indicate that the relative errors due to the Lambertian assumption on the surface reflectance are 3–12% in the visible and 0.7–5.0% in the near-infrared, around 1% on the NDVI and less than 1% on the albedo.