Vegetation mapping is based on the spectral signature of plants (colour). Healthy green plants absorb intense solar radiation in the red region (energy source in the photosynthetic processes), and reflect intensely in the near infrared region.
Our multispectral sensor, combined with high resolution and detailed images, allows us to measure the amount of absorbed / reflected radiation in each region of the spectrum and thus to analyze the spatial variability (variation of vegetation vigor throughout the terrain).
This variation can be caused by nutrient deficiency, poor soil drainage, pest outbreaks and diseases or other factors harmful to agriculture, forestry, riparian, coastal, or other types of vegetation.
These differences in vegetative vigour (translated into vegetation indexes such as NDVI) allow for spatial segmentation in management units (areas sharing similar characteristics), which will allow intervention in specific areas, rather than a massive intervention in the entire land.
Infrared and Red Edge
The Red Edge band captures reflected light between visible and infrared, allowing the mapping of water stress and chlorophyll pigment concentration (among others).
The NIR band captures the light reflected in the near infrared in order to map the vigour / health of the plants.
Vegetation index NDVI
Vegetation indexes such as NDVI allow not only to map vegetation, but also to analyze it quantitatively and qualitatively.
The NDVI - Near Difference Vegetation Index - is a numerical indicator that varies (in theory) between 0 and 1.
Values close to 0 indicate diseased, dead or leafless vegetation, urban and impermeable areas, roads, dirt terrain, among others.
Values close to 1 indicate healthy vegetation, without restrictions in terms of water or nutrients, with adequate soils, etc.
Vegetation indices are obtained in two phases: a) Reflectance map processing, in order to filter the effects of incident solar radiation at the exact time of image capture and b) Raster operations between bands in order to highlight the desired information (water deficit, senescence, drought, pest or disease). For this purpose, each band is processed individually so that it can be used in the following formula:
DVI – Difference Vegetation Index
This is probably one of the simplest indexes. It is sensitive to the amount of vegetation, distinguishes soil and vegetation but does not process the difference between reflectance and radiation caused by the atmosphere and the shadows. Its formula is:
DVI = NIR – Red
NDVI – Normalized Difference Vegetation Index
This index varies from -1 to 1, indicating the vigour of vegetation, distinguishing plants from the soil and minimizing the effects caused by topography, among others. Its formula is
NDVI = (NIR – Red) / (NIR + Red)
NDRE – Normalized Difference Red Edge
Depending on the time of the year and crop progression, NDRE may be a better indicator of vegetation vigour when comparing to NDVI or other indices. In more advanced maturation states, leaves accumulate high levels of chlorophyll, and while red radiation is susceptible to high levels of absorption, the Red Edge band may be more reflective and therefore serve better the purpose of health indicator. This way, the NDRE turns out to be more suitable than the NDVI for intensive management applications for extended periods. Its formula is
NDRE = (NIR – Red Edge) / (NIR + Red Edge)
RVI - Ratio Vegetation
This index has a high value for vegetation, and low for soil, ice, water, or edified. It is an indicator of the presence of vegetation and reduces the effects of atmosphere and topography. Its formula is
RVI = NIR / Red
GRVI - Green Ratio Vegetation Index
In addition to distinguishing between soil occupation classes, as mentioned in the previous section, this index is also sensitive to the photosynthetic rates in the canopy, since the green and red reflectances are strongly influenced by the changes in the pigments of the leaves. Its formula is
GRVI = NIR / Green
These are just some of the indexes you can use - there are several for each purpose.
These indices serve as indicators for vegetation vigour, but also to identify potential outbreaks of vegetative stress problems, such as disease or irrigation problems. There are countless indexes, and each one of them can give a different perspective / analysis focused on the resolution of a certain problem.
Also, the contribution of drone mapping goes far beyond these indicators. In fact, it is possible to generate a height model (which may help to understand the origin of some problems), count trees automatically, measure areas or distances in 3D to support agricultural, forestry or environmental management.