Active remote sensing

The development of remote sensing technologies increases the potential to support more precise, efficient, and ecologically-sensitive approaches to forest resource management. One of the primary requirements of precision forest management is accurate and detailed 3D spatial data relating to the type and condition of forest stands and characteristics of the underlying terrain surface. A new generation of high-resolution, active remote sensing technologies, including airborne laser scanning (LIDAR) and interferometric synthetic aperture RADAR (IFSAR) have the capability to provide direct, 3D measurements of forest canopy structure and topography. High resolution LIDAR can be used to measure attributes of individual tree crowns composing the overstory forest canopy. In addition, metrics based upon the lidar height distribution are highly correlated with critical forest structure variables, such as dominant height, basal area, stem volume, and biomass. IFSAR is a microwave remote sensing technology that is also capable of providing 3D positions of backscattering elements within a forest scene. While IFSAR typically provides measurements of lower resolution and accuracy than LIDAR, it has an all-weather capability and is acquired at a much lower cost per unit area. In addition, the use of multiple-frequency RADAR systems allows for the collection of information on different scene components. For example, long-wavelength P-band energy penetrates through the canopy and reflects mainly from the terrain surface, while short-wavelength X-band energy reflects from the first reflective surface. Therefore, the difference between the X-band (canopy) surface measurements and the P-band (terrain) surface will yield vegetation height information. In this chapter, we will describe the basic principles of these active remote sensing technologies in the context of forest canopy inventory and terrain mapping, and present an example of their application within a Pacific Northwest conifer forest.