Remote sensing of land surface temperature: the directional viewing effect

Land surface temperature is an important parameter in determining the Earth radiation budget and heat and moisture flow between the surface and the atmosphere. Many vegetation, hydrologic and decomposition processes are strongly temperature dependent. Land surface temperature and emissivity products are currently being derived from satellite or aircraft remote sensing data using a variety of techniques to correct for atmospheric effects and land surface emissivity variations arising from simple, cover type differences. However, implicit in the commonly employed approaches is the assumption of isotropy in directional thermal infrared excitance. In reality surface temperature observed from a satellite or aircraft platform is a function of both Sun angle and sensor view angle. These variations, in turn, are a function of vegetation type and structure and the partitioning of radiative, sensible and latent heat exchanges between the vegetation and the atmosphere. While the Lambertian assumption may be taken as a first-order approximation, the authors' analyses and observations indicate angular variations in apparent infrared temperature will typically yield land surface temperature errors ranging from 1 to 2 degrees Centigrade unless corrective measures are applied.