BRDF Correction of Vegetation in AVHRR Imagery

Abstract

In this article we present a method for correcting AVHRR visible and near-infrared 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 imagery pairs. This imagery provides two views of the land surface close together in time but with large differences in phase angle. It is reasonable to assume that both the surface and the atmosphere will change little between orbits, and that after BRDF correction reflectances of given targets should be the same in both orbits. Before BRDF parameters can be fitted, atmospheric correction must be performed. To improve this process, average monthly atmospheric profiles and aerosol optical depths are used as radiative transfer model inputs in conjunction with a digital elevation model. Using atmosphere corrected reflectance data from 12 NOAA-14 AVHRR image pairs, BRDF parameters were extracted for predominant vegetation groups in New Zealand: indigenous forest; exotic forest; scrub; pasture; and tussock grassland. For each of these vegetation groups significant non-Lambertian reflectance behavior was observed, and BRDF correction using the derived parameters successfully minimized this variation. Measuring the spread of the corrected results from the desired 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 the near-infrared. A vegetation map specifying the proportions of the vegetation groups at any given location can be used to perform regular BRDF correction. Reflectance standardization to a fixed view and sun angle can then be performed using the pre-derived BRDF parameters and proportional BRDF correction.