Terrestrial applications of multiangle remote sensing

Multiangle remote sensing is an emerging technology that enables important applications of terrestrial (land) remote sensing, in ecology and land cover mapping as well as in a variety of disciplines in the Earth Sciences. Advances have been realized in three major areas: the measurement or characterization of canopy structure and surface roughness; the separation of the contributions of the upper canopy and the background in forest and shrub-dominated environments; and improvements in the accuracy of classifications of land cover in environments as dissimilar as deserts and ice sheets. The focus of the chapter is on land surface applications of solar wavelength multiangular data acquired from the air and space; it avoids discussion of methods for retrieval of essentially radiometric quantities such as shortwave albedo, bidirectional and hemispheric reflectance factors (Chapter 9), retrieval of land surface temperature (Chapter 4), theoretical radiative transfer modeling studies, and those based uniquely on field measurements. The chapter introduces existing instruments providing multiangle data; a review of work performed under the broad headings Empirical and Synergistic Approaches, Radiative Transfer, The RAMI Exercise, Canopy Openness, Clumping Index, Structural Scattering Index, Geometric-Optical and Hybrid Models, Direction and Wavelength, The Background in Canopy Reflectance Modeling, Land Cover and Community Type Mapping, Snow and Ice, and Dust Emissions; and finally, brief discussions under the headings Near-Simultaneous and Accumulated Sampling, Angular Sampling, and Scale and Multiangle Observation. The emphasis is on the added value that existing solar wavelength multiangular data can provide to applications. There has been a wide array of approaches, all of which have resulted in studies demonstrating progress and many of which show the advantages possible over the use of purely nadir-spectral techniques, particularly for accessing measures of canopy structure.