In addition to land cover, there are several vegetation characteristics that can be measured using passive and active remote-sensing instruments. These include
• primary productivity,
• vegetation health and vigor, and
• vegetation structure.
Measurements of these characteristics are based on the fact that reflectance, transmittance, and scattering of energy in a canopy is greatly affected by the structure of the vegetation and how the vegetation components (leaves, branches, trunk) interact with the spectrum of energy being used by a particular remote-sensing instrument.
Vegetation indices have been used extensively for global studies to monitor changes in vegetation health and cover and have been effective in mapping droughts, desertification, phenology, net primary productivity, and deforestation around the world. The most common vegetation index, the 'normalized difference vegetation index' (NDVI), is based on the principle that healthy green vegetation absorbs most of the incident red wavelengths of light and reflects most of the near-infrared wavelengths. The formula for NDVI is
where NIR is the radiance value from the near-infrared band and red is the radiance value from the red band.
Two other common vegetation indices that use a similar principle as NDVI are the 'soil adjusted vegetation index' (SAVI), which was developed to reduce the effect of background material (i.e., soil, sand, snow) and the 'enhanced vegetation index' (EVI) which is less sensitive to atmospheric scattering effects. Vegetation index data sets are usually available as temporal composites, such as 10-day or monthly. In a composite product the 'best' index values from the composite period are provided. Using this approach it is possible to reduce the negative effects of clouds and haze.
Vegetation structure and biomass data sets are often created using data acquired from radar and lidar sensors. Although radar has been used to measure vegetation properties such as biomass, leaf area index, and forest structure most of this has been experimental so data are somewhat limited. This is an area of active research and as new instruments are developed, operational methods using radar instruments may be available in the not too distant future.
Commercial lidar instruments are available for mounting in airplanes that can quickly provide vegetation height information and this can be correlated to tree volume and biomass using allometric tables. Unfortunately, using these instruments is expensive and it is often not feasible to cover large areas. Research using airborne and satellite lidar instruments to measure vegetation structure directly is underway and early results look promising.
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