Normalized Difference Vegetation Index
NDVI is an acronym and stands for " normalized differenced vegetation index " (also Normalized Density Vegetation Index ), in German: " normalized differentiated vegetation index ". He is probably the most widely used vegetation index and is calculated on the basis of satellite data.
Background
The index is based on the fact that healthy vegetation in the red region of the visible spectral region (wavelength of about 400 to 700 nm) and relatively small in the subsequent near-infrared range (wavelength of about 700 to 1300 nm ) is reflected relatively much radiation. In this case, the reflection is caused in the near infrared to the presence of chlorophyll and thus strongly correlated with the viability of a plant - the vital (green ), the plant, the higher the increase of the reflectance in the spectral range. Other surface materials such as soil, rock or dead vegetation, show no such characteristic difference of the reflectance of both areas. This circumstance may serve for a vegetation -covered to be distinguished from vegetation-free surfaces. A conclusion on the photosynthetic activity (vitality ) is partly possible because, although diseased plants have less green leaf mass and thus lead to a lower NDVI, NDVI, however, can not distinguish between diseased vegetation and reduced vegetation cover. Thus, a lower NDVI let into a wooded area both diseased plants suspect as a lower vegetation cover. Since the NDVI above a certain amount of green leaf surface does not increase further (see LAI Leaf Area Index), but probably can continue to grow the biomass, were for highly vegetated surfaces such as developed the equatorial rainforest indices that are more sensitive (see EVI - Enhanced Vegetation Index ).
The NDVI is calculated in many studies on data from the AVHRR sensor of the NOAA satellites, since there is the longest time series of measured data. For this, data from channel 1 ( 580-680 nm ) and channel 2 are used ( 725-1100 nm) of the sensor. It is also possible to use other data bases, such as Landsat data (channel 3 and 4 of TM / ETM ) or ASTER data to use. This is necessary for example, if a better geometric resolution is required. Global data sets can be ordered and downloaded free of charge from EOS Data Gateway Center (NASA). The resolution here is 250 m, 500 m or 1000 m per pixel, while the interval length is 16 daily mean values (for the generation of cloud-free images ) or monthly averages. These data are measured by the MODIS instrument of Earth Observing System ( EOS ).
Calculation
NDVI is calculated from the reflectance values in the near infrared and visible region of the red (red, about 620 to 700 nm) of the light spectrum:
In case of strong atmospheric disturbances (dense cloud cover) is expected to part with an approximate formula:
In this case, both channels are shifted by a spectral range: near-infrared to mid-infrared ( about 1300 to 3000 nm ) and the red to near infrared range.
By the normalization, there is a range of values between -1 and 1. Negative values indicate water areas. A value between 0 and 0.2 is nearly vegetation-free areas, while a value close to 1 indicating a high vegetation cover suggests with green plants.
More indices
- ' SAVI ' (Soil Adjusted Vegetation Indices) family
- Biogeography