Normalized Difference Vegetation Index

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The global NDVI in the 12 months of 1998. You can create your own animations at http://earthobservatory.nasa.gov/Observatory/Datasets/ndvi.fasir.html .

NDVI is an acronym and stands for " N ormalized D ifference V egetation I ndex" (also Normalized Density Vegetation Index), in German: "normalized differentiated vegetation index". It is the most frequently used vegetation index and is calculated on the basis of remote sensing data.

background

Example of NDVI by Landsat 8 for the urban area of Ponta Grossa , southern Brazil

The index is based on the fact that healthy vegetation reflects relatively little radiation in the red range of the visible spectral range ( wavelength from around 600 to 700  nm ) and a relatively large amount in the subsequent near infrared range (wavelength from around 700 to 1300 nm). The reflection in the near infrared range is due to the cell structure of the leaves and is mainly determined by the mesophyll cells. The more vital (green) the plant, the greater the increase in the degree of reflection in this spectral range. Other surface materials, such as soil, rock or even dead vegetation, show no such characteristic difference in the degree of reflection of the two areas. This fact can be used to distinguish between vegetation-covered and vegetation-free areas. A conclusion about the photosynthetic activity (vitality) is possible to a certain extent, since diseased plants have less green leaf mass and thus lead to a lower NDVI, but the NDVI cannot differentiate between diseased vegetation and lower vegetation cover. Thus, a lower NDVI in a forest area can suggest both diseased plants and a lower vegetation cover . Since the NDVI does not increase further from a certain amount of green leaf surface (see Leaf Area Index (LAI) ), but the biomass can continue to increase, for heavily overgrown areas such as B. developed the equatorial rainforest indices, which are more sensitive (see EVI - Enhanced Vegetation Index ).

In many studies, the NDVI is calculated from data from the AVHRR sensor of the NOAA satellites, as this is where the longest time series of measured data is available. For this, data from channel 1 (580 to 680 nm) and channel 2 (725 to 1100 nm) of the sensor are used. It is also possible to use other data sources, such as Landsat data (channels 3 and 4 of TM / ETM + ) or ASTER data. This is necessary, for example, when a better geometric resolution is required. Global data sets can be ordered and downloaded free of charge from the 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 ​​(to generate cloud-free images) or monthly mean values. This data is measured by the MODIS instrument of the Earth Observing System (EOS).

calculation

The NDVI is calculated from the reflection values ​​in the near infrared range and the red visible range (red, approx. 620 to 700 nm) of the light spectrum:

In the case of strong atmospheric disturbances (dense cloud cover), an approximation formula is sometimes used:

In doing so, both channels are shifted by one spectral range: the near infrared to the mid infrared (approx. 1300 to 3000 nm) and the red range to the near infrared.

The normalization results in a value range between −1 and +1. Negative values ​​indicate areas of water. A value between 0 and 0.2 corresponds to almost vegetation-free areas, while a value close to 1 indicates a high vegetation cover with green plants.

Further indices

  • ' SAVI ' ( Soil Adjusted Vegetation Indices ) family

literature

  • Kerstin Viering (2014): Drones in nature conservation: scouts above the treetops . Spectrum of Science June 6, 2014