Required Consistency between Biome Definitions and Signatures with the Physics of Remote Sensing. II: Theoretical Arguments

The insights gained from present land cover classification activities suggest integration of multi-angle data into classification attempts for future progress. Land cover types that exhibit distinct signatures in the space of remote sensing data facilitate unambiguous identification of cover types. In this two-part series, we develop a theme for consistency between cover type definitions, uniqueness of their signatures and physics of the remote sensing data. In the first part (Zhang et al., 2001), empirical arguments in support of the consistency principle were presented. This part provides a theoretical justification of the consistency requirements. Radiative transfer best explains the physics of the processes operative in the generation of the signal in the optical remote sensing data. Biome definitions given in terms of variables that this theory admits and the use of the transport equation to interpret biome signatures guarantee the consistency requirements. It is shown in this paper that three metrics of the biome angular signature in the spectral space – location, angular signature slope and length indices – are related to eigenvalues and eigenvectors of the transport equation. These variables allow a novel parameterization of canopy structure based on the partitioning of the incident radiation between canopy absorption, transmission and reflection. Consistency between cover type definitions and uniqueness of their signatures with the physics of the remote sensing data is required not only to reduce ambiguity in land cover identification, but also to directly relate land cover type to biophysical, and biogeochemical processes in vegetation canopies.