Measurement of the spectral directional reflectance of forest canopies: A review of methods and a practical application

Abstract

Abstract In this paper we review methods to measure the spectral directional reflectance of forest canopies and investigate the use of a helicopter platform to gather spectral reflectance data from deciduous forest. Six sparse sampling geometries are identified: nadir, hot‐spot, solar almucantar, solar principal plane, orthogonal plane and twin azimuthal planes. A portable spectroradiometer is used from a helicopter to measure the nadir reflectance of an oak (Quercus robur) plantation at heights from 150 to 760 metres. Near‐simultaneous reflectance measurements of an area of mown grass from 150 metres and ground level are used to correct for the effect of the rotor blades on the irradiance measured from the helicopter. Variations in the amount of water vapour with height and between two sites 30 km apart are inferred from the relative depth of the atmospheric absorption feature at 940 nm.