VARI (Visible Atmospherically Resistant Index)
Definition
The Visible Atmospherically Resistant Index (VARI) is designed to estimate vegetation fraction using only visible bands (typically blue, green, and red), making it resistant to atmospheric effects while requiring no NIR data. It's particularly useful for RGB imagery and situations where atmospheric correction is challenging or NIR data is unavailable.
Application
VARI is widely used with consumer-grade cameras, smartphones, drones with RGB sensors, and historical aerial imagery. Applications include urban vegetation monitoring, precision agriculture (particularly with low-cost systems), phenology studies using time-lapse photography, and greenness assessment in landscapes where NIR data collection is impractical.
FAQ
What is the formula for VARI?
The standard VARI formula is: VARI = (Green - Red) / (Green + Red - Blue). This formulation uses all three visible bands to enhance vegetation detection while minimizing atmospheric and illumination effects through the blue band correction.
Why is VARI considered ""atmospherically resistant""?
VARI achieves atmospheric resistance by:
1) Using the blue band to estimate and correct for atmospheric scattering, similar to ARVI but with visible bands only;
2) Leveraging differential scattering effects across visible wavelengths;
3) The denominator term (Green + Red - Blue) helps normalize illumination variations and atmospheric path radiance.
What are the main advantages of VARI for RGB imagery analysis?
Advantages include:
1) Can be calculated from standard RGB imagery without NIR;
2) Reasonable atmospheric resistance compared to simple visible-band indices;
3) Good performance in urban and peri-urban environments;
4) Compatibility with vast archives of historical RGB imagery;
5) Suitable for citizen science and low-cost monitoring systems.
How does VARI compare to NIR-based indices for vegetation monitoring?
Comparisons show:
1) VARI generally has lower sensitivity to vegetation density than NIR-based indices;
2) It performs reasonably well for vegetation fraction estimation but less so for chlorophyll content;
3) It's more susceptible to soil background effects, especially in sparse vegetation;
4) For many applications where NIR is unavailable, VARI provides the best alternative among visible-band indices;
5) In dense vegetation, VARI correlates reasonably well with NDVI.
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