Normalized Difference Vegetation Index (NDVI) is the go-to metric for analyzing vegetation health. It uses the difference between Near-Infrared (NIR) and Red light to quantify greenery.

If you are using Landsat data in QGIS, here is the fastest way to calculate it.

The Formula

The general formula for NDVI is:

NDVI = (NIR – Red)/(NIR + Red)

However, the band numbers change depending on which Landsat satellite you are using.

• Landsat 8 & 9: Use Band 5 (NIR) and Band 4 (Red).
• Landsat 4, 5 & 7: Use Band 4 (NIR) and Band 3 (Red).

Step-by-Step in QGIS

1. Load Data: Open QGIS and load your Red and NIR bands (e.g., LC08_..._B4.TIF and LC08_..._B5.TIF for Landsat 8).
2. Open Raster Calculator: Go to Raster > Raster Calculator.
3. Enter the Expression: Double-click the bands in the “Raster Bands” list to add them to the expression box.
   • For Landsat 8/9:( “Band5” – “Band4” ) / ( “Band5” + “Band4” )
   • Note: strictly follow the parenthesis order to avoid calculation errors.
4. Save Output: Select your “Output layer” location and click OK.

Visualizing & Interpreting Results

Your result will be a grayscale image with values ranging from -1 to 1. To make it readable:

1. Right-click the NDVI layer > Properties > Symbology.
2. Change “Render type” to Singleband pseudocolor.
3. Choose a Red-to-Green color ramp and click Classify.

How to read the values:

• -1 to 0: Water bodies, clouds, or snow (usually Blue/Red).
• 0 to 0.2: Bare soil, rock, or urban areas (Beige/Brown).
• 0.2 to 0.4: Sparse vegetation like shrubs or grassland (Light Green).
• 0.4 to 1.0: Dense, healthy vegetation like forests or crops (Dark Green).