Terra Earth Observation SatteliteFigure 1. Artist's rendering of the Terra spacecraft. Image cerdit (NASA)
MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) satellite.Terra MODIS aquires data in 36 spectral bands, or groups of wavelengths |
Normalized Difference Vegetation Index (NDVI)Live green plants absorb solar radiation in the photosynthetically active radiation (PAR) spectral region, which they use as a source of energy in the process of photosynthesis. Leaf cells have also evolved to scatter (i.e., reflect and transmit) solar radiation in the near-infrared spectral region (which carries approximately half of the total incoming solar energy), because the energy level per photon in that domain (wavelengths longer than about 700 nanometers) is not sufficient to be useful to synthesize organic molecules. A strong absorption at these wavelengths would only result in overheating the plant and possibly damaging the tissues. Hence, live green plants appear relatively dark in the PAR and relatively bright in the near-infrared. By contrast, clouds and snow tend to be rather bright in the red (as well as other visible wavelengths) and quite dark in the near-infrared. The pigment in plant leaves, chlorophyll, strongly absorbs visible light (from 0.4 to 0.7 µm) for use in photosynthesis. The cell structure of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1 µm). The more leaves a plant has, the more these wavelengths of light are affected, respectively. Since early instruments of Earth Observation, such as NASA's ERTS and NOAA's AVHRR, acquired data in visible and near-infrared, it was natural to exploit the strong differences in plant reflectance to determine their spatial distribution in these satellite images.
The NDVI is calculated from these individual measurements as follows: NDVI = (NIR-VIZ) / ( NIR + VIZ ) where VIS and NIR stand for the spectral reflectance measurements acquired in the visible (red) and near-infrared regions, respectively . These spectral reflectances are themselves ratios of the reflected over the incoming radiation in each spectral band individually, hence they take on values between 0.0 and 1.0. By design, the NDVI itself thus varies between -1.0 and +1.0. It should be noted that NDVI is functionally, but not linearly, equivalent to the simple infrared/red ratio (NIR/VIS). The advantage of NDVI over a simple infrared/red ratio is therefore generally limited to any possible linearity of its functional relationship with vegetation properties (e.g. biomass). The simple ratio (unlike NDVI) is always positive, which may have practical advantages, but it also has a mathematically infinite range (0 to infinity), which can be a practical disadvantage as compared to NDVI. Also in this regard, note that the VIS term in the numerator of NDVI only scales the result, thereby creating negative values. NDVI is functionally and linearly equivalent to the ratio NIR / (NIR+VIS), which ranges from 0 to 1 and is thus never negative nor limitless in range. But the most important concept in the understanding of the NDVI algebraic formula is that, despite its name, it is a transformation of a spectral ratio (NIR/VIS), and it has no functional relationship to a spectral difference (NIR-VIS). In general, if there is much more reflected radiation in near-infrared wavelengths than in visible wavelengths, then the vegetation in that pixel is likely to be dense and may contain some type of forest. Subsequent work has shown that the NDVI is directly related to the photosynthetic capacity and hence energy absorption of plant canopies. (Wikipedia) Using two independent satellite imagery products, MODIS Terra and SPOT Vegetation this paper documents change to land cover, primarily vegetation from 1998 through 2007 in the regions of Darfur most impacted by the genocide since 2003. Its findings show a steadily increasing return of natural vegetation coverage and vigor, likely grasses and shrubs, in formerly agrarian and livestock grazing ranges, since 2004. This environmental recovery is demonstrably not a result of increased rainfall, but of the abrupt change in land use directly related to the systematic violence committed by Sudanese government and militia forces against the peoples of Darfur. In an agriculture-based society, this vegetation rebound resulted from the loss of livestock and the inability to farm, caused by human displacement and the destruction of subsistence resources from 2003 to 2007. (Yale University Genocide Studies Working Paper No. 36 Author: Russell Schimmer) Yale University Genocide Studies Program, Remote Sensing Project |