What is NDVI?
The land is defined as the “ensemble of the soil constituents, the biotic components in and on it, as well as its landscape setting and climatic attributes” (Vlek et al, 2010).
Land degradation is the long-term loss of ecosystem function and services, caused by disturbances from which the system cannot recover unaided (UNEP, 2007).
Degradation may also be considered in terms of specific components of the land that are affected. For example, vegetation degradation implies a reduction in productivity, declining species diversity, and degeneration in the nutritional value of plant populations.
With the use of different technologies, people can be monitoring the quality and development of plants. Remote sensing allows us to use different band combinations to get the answers to various necessities. One of them is the use of vegetation indexes.
Many vegetation indexes exist but the most common is NDVI. Normalized Difference Vegetation Index (NDVI) was created by Rouse in 1974 and it serves to determine the quality and vegetation development.
But how does it work? First, we should know that reflectance is the proportion of energy that is reflected by an object. The spectral reflectance of the vegetation differs considerably in the near-infrared region (λ=700 to 1300 nm) and the visible red range (λ=550-700 nm) of the electromagnetic spectrum (Govaerts B.& Verhulst N., nd).
BLUE and RED band of the spectrum are absorbed by the chlorophyll. The Plant and the properties of the leaves reflect the energy that is irradiated by the near-infrared.The plants in our eyes look green because they have low reflectance in NDVI.
NDVI has three very important characteristics: its mathematical simplicity, its easy interpretation, and it's power to normalize the spectral response of vegetation systems.
The ideal vegetation index is one that is particularly sensitive to vegetation cover, insensitive to the brightness and color of the soil and little affected by atmospheric disturbances, environmental factors, and lighting observation geometry.
The green and vigorous vegetation reflect less radiation in the visible red band, chlorophyll absorption region in the near-infrared band. When the vegetation undergoes stress, the values of the R band increase and those of the IR band decrease.
The NDVI is calculated from reflectance measurements in the near-infrared (NIR) and the red (R) as indicated by the following formula:
The NDVI is a non-dimensional index, and therefore its values range from -1 to 1 (Menesses C., 2011).
The values less than 0.1 correspond to the bodies of water and the bare earth, and the values that are above 0.1 correspond to the photosynthetic activity of scrubland, forest, jungle, among others (Menesses C.,2011).
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