NDVI = nir - red / nir + red
For vegetation monitoring, the NDVI obtained by the combination of Channels 1 (0.54-0.68 mm) and 2 (0.73-1.10 mm) (Ch2-Ch1)/Ch2+Ch1), visible and near infrared respectively of AVHRR data, is commonly used. The NDVI is representative of plant assimilation condition and of its photosynthetic apparatus capacity and biomass concentration (Groten, 1993; Loveland et al., 1991). In particular vegetation index dynamics in time are correlated with the Canopy Leaf Index (LAI) and other functional variables (Cihlar et al. 1991). These variables are strongly conditioned by the behavior of precipitation, temperature and daily radiation of the observed area (Davenport et al., 1993). Vegetation index therefore is representative of plants' photosynthetic efficiency, and it is time varying due to changes in meteorological and environmental parameters. The NDVI values range from -1 to +1 (pixel values 0-255).
The AVHRR- data is particularly suited to monitoring seasonal and inter-annual changes in land cover/land use because of its low cost and temporal and spatial characteristics. There have been a number of studies which have directly linked AVHRR-NDVI to plant phenology (DeFries 1995; Reed et al., 1994). For instance, the number of periods when the NDVI exceeded a threshold might indicate the number of growing seasons, the time integrated NDVI might indicate gross primary production and the length of the period when NDVI exceeded a threshold might indicate the length of the growing season. Seasonal and inter-annual variations can be derived form multi-temporal series of NDVI that can be associated with other ecological variables (Mora and Iverson 1995).
The NDVI is also calculated from LANDSAT-TM information by using the combinations of bands 3 (0.63-0.69 mm) and 4 (0.76-0.90 mm) (B4-B3)/(B4+B3). Healthy vegetation will have a high NDVI value. Bare soil and rock reflect similar levels of near-infrared and red and so will have NDVI values near zero. Clouds, water, and snow are the opposite of vegetation in that they reflect more visible energy than infrared energy, and so they yield negative NDVI values.
Building and Using IDV Formulas
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