The Calculation of the Directional Reflectance of a Vegetative Canopy

The need for the identification of vegetative canopies and the detection of stresses in vegetative canopies by remote sensing techniques has continued to grow in economic importance. The management of natural resources such as forests and wetlands, and the prediction of yields and assessment of pest damage of agricultural crops require both timely and economical survey techniques in order to supply the fundamental information for the formulation and execution of effective management strategies. One of the most promising remote sensing techniques for rapid and economical mapping of vegetative canopy types is the airborne multispectral optical mechanical scanner using automatic spectral pattern recognition summarized by (Lowe, 1968). This technique is capable of utilizing very subtle but systematic spectral reflectance differences for the mapping of vegetative canopy types. The major weakness of this technique is the difficulty in relating subtle reflectance differences to the elemental causative factors which could be recognized and classified by botanists on the ground. Unless some insight is achieved in connecting causative factors with detected effects, there is no foundation for claiming that a specific cause is uniquely coupled with a detected effect. Certain detected effects could be due to spurious causes which may be transient and be fundamentally unconnected with the condition of interest to the remote sensor user even though the occurrence of the detected effect appears to be associated with this condition at one time and location. A mathematical reflectance model of a canopy, which is based upon the spectral and geometric character of the individual pieces of the canopy, can connect the plant biology causes to the remotely-sensed reflectance effect.