Conversion of Nadir, Narrowband Reflectance in Red and Near-Infrared Channels to Hemispherical Surface Albedo

Measured nadir-viewed narrow-band reflectances in red and near infrared channels simulating AVHRR Channels 1 and 2 are compared with measured integrated solar spectrum hemispherical albedos. Reflectances and albedos were measured from an aircraft over four widely different vegetation surfaces in southeastern Tasmania (Australia). A linear function is used to simultaneously correct for narrow band to broad band conversion errors and to convert nadir viewed reflectances to hemispherical albedos. While the application of the conversion equation presented is limited to the vegetation types of the study, the equation is independent of vegetation type. Results indicate that the reflectance from the rough surface of a forest canopy is not Lambertian and that the use of a directional to hemispherical albedo conversion using a Lambertian assumption may lead to errors as great as 35%. | Measured nadir-viewed narrow-band reflectances in red and near infrared channels simulating AVHRR Channels 1 and 2 are compared with measured integrated solar spectrum hemispherical albedos. Reflectances and albedos were measured from an aircraft over four widely different vegetation surfaces in southeastern Tasmania (Australia). A linear function is used to simultaneously correct for narrow band to broad band conversion errors and to convert nadir viewed reflectances to hemispherical albedos. While the application of the conversion equation presented is limited to the vegetation types of the study, the equation is independent of vegetation type. Results indicate that the reflectance from the rough surface of a forest canopy is not Lambertian and that the use of a directional to hemispherical albedo conversion using a Lambertian assumption may lead to errors as great as 35%.