Question

Normalized Difference Vegetation Index to monitor crop health is determined from

• A. Reflectance of red band
• B. Reflectance of blue and red band
• C. Reflectance of shortwave and red band
• D. Reflectance of near-infrared and red band

Hint:

Remember:
- High absorption of Red + High reflection of NIR = Healthy Vegetation (High NDVI).
- Low absorption of Red + Low reflection of NIR = Stressed or Sparse Vegetation (Low NDVI).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The Normalized Difference Vegetation Index (NDVI) is a widely used remote sensing index that evaluates the presence of live green vegetation.
It exploits the distinct spectral reflectance characteristics of healthy green leaves in different regions of the electromagnetic spectrum.

Step 2: Key Formula or Approach:
The mathematical formulation for the NDVI calculation is:
\[ \text{NDVI} = \frac{\rho_{\text{NIR}} - \rho_{\text{Red}}}{\rho_{\text{NIR}} + \rho_{\text{Red}}} \]
where $\rho_{\text{NIR}}$ is the reflectance in the near-infrared band, and $\rho_{\text{Red}}$ is the reflectance in the red band.

Step 3: Detailed Explanation:
Healthy green leaves contain chlorophyll pigments that strongly absorb solar radiation in the visible red spectrum ($0.4$ to $0.7\text{ }\mu\text{m}$) for photosynthesis.
At the same time, the internal cellular structure of healthy leaves (specifically the mesophyll tissue) strongly reflects and scatters radiation in the near-infrared (NIR) spectrum ($0.7$ to $1.1\text{ }\mu\text{m}$).
By comparing these two bands, NDVI can easily distinguish healthy green leaves from other land features like soil, water, or dead vegetation.
The values of NDVI range from $-1$ to $+1$:
- Active green vegetation typically has high positive values ($0.3$ to $0.8$).
- Bare soil has low positive values ($0.1$ to $0.2$).
- Water, clouds, and snow have negative values.

Step 4: Final Answer:
The NDVI is determined from the reflectance values of the near-infrared and red bands.
Therefore, the correct option is (D).