MultiSpectral and NDVI Imagery
The big thing that many agronomists and farmers are after, is the ability to monitor crop health. This is normally done using a Multi-Spectral Sensor and creating an NDVI (Normalized Difference Vegetation Index) reflectance map.
Here is a short definition of NDVI if you are not already familiar - Live green vegetation absorbs visible light (solar radiation) as part of photosynthesis. At the same time plants scatter (reflect)solar energy in the near infra-red. This difference in absorption is quite unique to live vegetation and provides a measure of the greenness of the vegetation. NDVI is an index which measures this difference, providing a measure of vegetation density and condition. It is influenced by the fractional cover of the ground by vegetation, the vegetation density and the vegetation greenness. It indicates the photosynthetic capacity of the land surface cover.
So what a Multi-spectral camera can do is pick up the greenness, density or health of your crops. Areas that appear red/yellow will normally require some form of attention and may require more water, extra fertiliser, etc.
The image below gives you an idea of the differnt wavelengths a plant may reflect during differnt stages of health.
So by using this lightwave data as collected by the NDVI sensor, we can begin to do some analysis to determine problem areas. Some of the more common reflectance maps and algothims used are:
NDVI (Normalized Difference Vegetation Index) - NDVI contrasts the red and near-infrared bands of light reflected from plant leaves. It is a general indicator of canopy density and is frequently used to distinguish live green vegetation from soil.
NDRE (Normalized Difference Red Edge) index - NDRE is sensitive to chlorophyll content in leaves, variability in leaf area, and soil background effects.
NIR (Near Infra-Red) Reflectance - Near Infrared intensity render. Vegetation will appear brighter, while non-vegetation will appear darker.
OSAVI (Optimized Soil-Adjusted Vegetation Index) - OSAVI maps variability in canopy density and is not sensitive to changing soil brightness as NDVI. This index is best used in areas with relatively sparse vegetation where soil is visible through the canopy.
CIR Composite (Color Infra-Red) Composite - Combining NIR, Red, and Green bands. Healthy vegetation reflects a high level of NIR and appears red. Dormant vegetation is often green or tan, while sandy soils appear light tan and clay soils dark tan or bluish green.