Modeling hyperspectral bidirectional reflection distribution function (BRDF) data of smooth cordgrass (Spartina alterniflora) using a Sandmeier field goniometer

The bidirectional reflection distribution function (BRDF) is a theoretical concept that describes the relationship between a target's irradiance geometry and the viewing angle of the sensor relative to the target. The BRDF can significantly affect the radiometric accuracy of remotely sensed data, particularly in off-nadir views. The goal of this research was to collect hyperspectral (400–2500 nm sampled approximately every 3 nm) and multi-angular (every 15° along the zenith arc and every 30° along the azimuth base) resolution BRDF data at one-hour diurnal intervals of Smooth Cordgrass ( Spartina alterniflora) communities using a NASA Sandmeier Field Goniometer (SFG). The BRDF sampling took place at the North Inlet-Winyah Bay National Estuarine Research Reserve, SC during late summer (high live biomass and low dead biomass) and late winter (low live biomass and high dead biomass) in two biophysically different canopies (high and low marsh). It was believed that sensor viewing angle, time of day (Sun angle), time of year (plant phenology), and canopy biophysical characteristics contributed to the BRDF spectral variation in Spartina. These data were used to visualize changes in BRDF response from 400–2500 nm through the creation of two- and three-dimensional graphic plots and animations that provided a quantitative and qualitative assessment of BRDF patterns as they relate to changing Sun and sensor angles, phenology cycles, and canopy biophysical characteristics. This research is important because it (a) fulfills the need among the BRDF community to increase detailed in situ validation studies; (b) establishes a hyperspectral and multi-angular resolution BRDF database for Spartina ; (c) documents temporal BRDF patterns of Spartina acquired in two contrasting phenological cycles; (d) investigates the physical mechanisms that appear to drive the observed BRDF based on observed biophysical characteristics; and (e) increases the limited knowledge base on BRDF characteristics in vegetation with erectophile leaf orientation in an intertidal environment. Future research will attempt to use BRDF measurements to improve the ability to accurately map coastal vegetation and more precisely identify areas of stress within fragile estuarine habitats. Further examination of these relationships may lead to the discovery of previously unknown underlying physiological processes that contribute to the BRDF.