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Methodology: NDVI Calculation

The Normalised Difference Vegetation Index (NDVI) is a fundamental metric we use to quantify vegetation health and density across the urban landscape. It is a simple yet powerful indicator derived from satellite imagery.

What is NDVI?​

NDVI values range from -1 to +1.

  • High positive values (close to +1): Indicate dense, healthy vegetation.
  • Values near zero: Typically represent bare soil, rock, or built-up areas.
  • Negative values: Often indicate water bodies.

The Formula​

We calculate NDVI for each pixel in a satellite image using the following formula, which compares the reflectance values of Near-Infrared (NIR) and Red light:

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Methodology: NDVI Calculation

The Normalised Difference Vegetation Index (NDVI) is a fundamental metric we use to quantify vegetation health and density across the urban landscape. It is a simple yet powerful indicator derived from satellite imagery.

What is NDVI?​

NDVI values range from -1 to +1.

  • High positive values (close to +1): Indicate dense, healthy vegetation.
  • Values near zero: Typically represent bare soil, rock, or built-up areas.
  • Negative values: Often indicate water bodies.

The Formula​

We calculate NDVI for each pixel in a satellite image using the following formula, which compares the reflectance values of Near-Infrared (NIR) and Red light:

NDVI = (NIR - Red) / (NIR + Red)

Healthy plants strongly reflect NIR light and absorb red light. This difference is what the formula captures.

Our Implementation​

For our analysis using Sentinel-2 data, we use the following spectral bands:

  • NIR: Band 8 (Near-Infrared)
  • Red: Band 4 (Red)

By applying this formula across an entire city, we can generate a detailed map showing the distribution and health of its green spaces, which is the first step in identifying areas for improvement.