BRDF correction of vegetation in AVHRR imagery

In this article we present a method for correcting AVHRR visible and near-i nfrared imagery for varying satellite and solar zenith angles. This method is based on the WAK BRDF model for closed canopies. The parameters required to perform BRDF correction can be derived from consecutive pass AVHRR imag ery pairs. This imagery provides two views of the land surface close togeth er in time but with large differences in phase angle. It is reasonable to a ssume that both the surface and the atmosphere will change little between o rbits, and that after BRDF correction reflectances of given targets should be the same in both orbits. Before BRDF parameters can be fitted, atmospher ic correction must be performed. To improve this process, average monthly a tmospheric profiles and aerosol optical depths are used as radiative transf er model inputs in conjunction with a digital elevation model. Using atmosp heric corrected reflectance data from 12 NOAA-14 AVHRR image pairs, BRDF pa rameters were extracted for predominant vegetation groups in New Zealand: i ndigenous forest; exotic forest; scrub; pasture; and tussock grassland. For each of these vegetation groups significant non-Lambertian reflectance beh avior was observed, and BRDF correction using the derived equal reflectance line gives a measure of the accuracy of the method. After correction, the RMS reflectance errors were approximately 0.01 in the visible and 0.02 in t he near-infrared. A vegetation map specifying the proportions of the vegeta tion groups at any given location can be used to perform regular BRDF corre ction. Reflectance standardization to a fixed view and sun angle can then b e performed using the pre-derived BRDF parameters and proportional BRDF cor rection. (C) Elsevier Science Inc., 2000.