Vegetation Index is obtaining a single value indicating the density of vegetation by applying mathematical operations to reflection values with different wavelengths in electromagnetic spectrum.
Plants absorb or reflect rays in various wavelengths in a manner unique to them according to their cell structures, leaf and surface widths, amount of water in their bodies and their positions in their natural environments. Each plant species has a unique cell structure and posture in nature. Thus it is possible to distinguish plant species from each other.Generally plants absorb light in the wavelength of 0.4-0.7 μm which is called the visible light region. While they absorb a very little portion of the infrared rays, they reflect a very large portion of them.
Plant Behavior in the Visible Region (0.4-0.7 μm)Icon
Pigmentation of the leaves is dominant factor in the visible region (0.4-0.7 μm). The reflection is controlled there by leaf pigments such as chlorophyll, carotene, xanthophyll and anthocyanin.
Reflectance in the blue and red regions of the visible spectrum is very low. Because the chlorophyll in the leaf absorbs most of the energy from these wavelength bands centered between approximately 0.45 and 0.65 μm.Low absorption of the plant in these wavelengths causes the normal, healthy leaves to be visible to our eyes. If a plant is under stress and chlorophyll production is reduced, it will have a higher reflection especially in the red zone of the spectrum.
Plant Behavior in the Near Infrared Region (0.7-1.3 μm)Icon
Since absorption is very little and reflection is very high in this region of the spectrum, reflection ratio varies based on the plant species (30-70%). A high amount of the remaining energy is transferred into the leaf.
Absorption is minimum in this region of the spectrum. Reflection from plant between wavelengths of 0.3 and 1.3 μm is mostly affected by the internal structure of plant leaves.Reflection measurements in this region allows to distinguish even the plant species seen as same at visible wavelengths because the plant species show significant differences in internal structure.
Plant Behavior in the Infrared Region (> 1.3 μm)Icon
In the mid-infrared region (1.3-2.7 μm) water content of plants affect spectral reflectance. Strong water absorption bands formed near 1.4 μm, 1.9 μm, 2.7 μm is dominant in the spectral sensitivity of green vegetation.
Reflection curve shows concavities in these sections.The degree of absorption of solar radiation by the vegetation is a function of the water content of the leaves. Reflection and water content of the leaf is inversely proportional and water content is proportional with leaf thickness. As the moisture content of the leaves decreases, reflectance in the middle infrared region increases. As the leaves loose moisture, the change in their cell structures affect the near infrared reflectance.
The methods used in analysis module for vegetable index are; NDVI (Normalized Difference Vegetation Index), OSAVI (Optimized Soil Adjusted Vegetation Index) and SAVI (Soil Adjusted Vegetation Index).
Normalized Difference Vegetation Index (NDVI)Icon
Normalized vegetation index is formulated to increase the contrast between near infrared (NIR) and visible red (R) bands or to examine plant asset by collecting the information of two bands in one band. NDVI is cleared of topographic and atmospheric effects for images with different times. Being normalized also eliminates different solar angle effects.
NDVI = (NIR-R)/(NIR+R)
NDVI values not only provide information about green vegetation but also determine the areas with weak or no vegetation.
NDVI value varies between the values of -1.0 and +1.0 depending on the vegetation area. Vegetation index closer to the value 1 indicates strong vegetation and closer to the value 0 indicates disappearing vegetation and a negative value indicates that the field has absolutely no vegetation.For example if the value obtained is 0.1 or lower, it corresponds to rocky areas; between 0.2 and 0.3, it corresponds to grass or meadow; between 0.6 and 0.8, it corresponds to tropical rain forests.
Soil Adjusted Vegetation Index (SAVI)Icon
Vegetation index with soil reflectance effects reduced to minimum by using correction factor.
SAVI = (NIR-R)*(1+L)/(NIR+R+L) L= 0.5
Correction factor value varies depending on the amount of green vegetation. Usually, it can be accepted that L=1 in regions with no vegetation, L=0.5 in regions with moderate vegetation. SAVI value provides values between -1.0 and 1.0.Vegetation index closer to the value 1 indicates strong vegetation and closer to the value 0 indicates disappearing vegetation and a negative value indicates that the field has absolutely no vegetation.
Optimized Soil Adjusted Vegetation Index (OSAVI)Icon
OSAVI = (1+0.16)*(NIR-R)/(NIR+R+0.16)
For vegetation index operations (?)