Normalized Difference Vegetation Index (NDVI)

Revision as of 15:28, 17 December 2010

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 photo- synthesis. Leaf cells have also evolved to scatter (i.e., reflect and transmit) solar radi- ation 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 (wavelength longer than about 700 nanometers) is not sufficient to be useful to synthesize organic molecules: a strong absorption here would only result in over-heating the plant and pos- sibly damaging the tissues. Hence, live green plants appear relatively dark in the PAR and relatively bright in the near-infrared. ^[1]

Since early instruments of Earth Observation, such as NASA’s ERTS and NOAA’s AVHRR, acquired data in the red 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\[ NDVI=\frac{NIR-RED}{NIR+RED} \]

where RED and NIR stand for the spectral reflectance measurements acquired in the 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. Subsequent work has shown that the NDVI is directly related to the photosynthetic capacity and hence energy absorption of plant canopies^[2][3]

Exercise 26: Calculate NDVI

1. Open the GRASS shell and type:
   r.mapcalc ndvi=”1.0*(etm40-etm30)/(etm40+etm30)”
   Confirm with Enter
2. Go to the Browser and click the Refresh button.
3. Load raster map ndvi to map canvas.
4. Go to the modules list: r.colors
5. Change colors to Normalized Difference Vegetation Index.

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References

1. ↑ Gates, David M. (1980): Biophysical Ecology, Springer-Verlag, New York, 611 p.
2. ↑ Sellers, P. J. (1985): Canopy reflectance, photosynthesis, and transpiration. International Journal of Remote Sensing, 6, 1335-1372.
3. ↑ Myneni, R. B., F. G. Hall, P.J. Sellers, and A.L. Marshak (1995): The interpretation of spectral vegetation indexes. IEEE Transactions on Geoscience and Remote Sensing, 33, 481-486.