Calculating NDVI with QGIS: A Step-by-Step Guide

Remote sensing is a powerful tool in the field of geospatial analysis, and one of the most useful indices derived from satellite imagery is the Normalized Difference Vegetation Index (NDVI). This blog will walk you through the process of calculating NDVI from Landsat satellite imagery using QGIS. The NDVI is a simple yet effective measure that helps in assessing vegetation health and coverage.

Understanding NDVI

Before diving into the calculation process, it’s essential to understand what NDVI is and why it’s significant. NDVI is calculated using the following formula:

NDVI = (NIR – Red) / (NIR + Red)

Here, NIR refers to the near-infrared band, and Red refers to the red band of the electromagnetic spectrum. The NDVI values range from -1 to 1:

• Values closer to 1 indicate dense green vegetation.
• Values around 0 indicate barren areas or sparse vegetation.
• Negative values typically represent water bodies.

Vegetation reflects near-infrared light and absorbs red light, which is why high NDVI values are associated with healthy vegetation.

Required Data for NDVI Calculation

To calculate NDVI, you will need Landsat satellite imagery. Specifically, for Landsat 8, you will require:

• Band 4: Red portion of the spectrum.
• Band 5: Near-infrared portion of the spectrum.

It’s crucial to have these bands ready and accessible in your QGIS environment. If you need guidance on downloading Landsat data, there are tutorials available online.

Setting Up QGIS for NDVI Calculation

Once you have the necessary data, open QGIS and load the required bands. Here’s how to do that:

1. Open the QGIS application.
2. Locate the Landsat data folder in the QGIS browser.
3. Import Band 4 (Red) and Band 5 (NIR) into your project.

After importing, the bands will appear in grayscale as they are single-band images. This is normal and does not affect the calculation process.

Using the Raster Calculator

The next step is to use the Raster Calculator in QGIS to compute the NDVI. Follow these steps:

1. In the bottom left corner of QGIS, type “Raster Calculator” to locate the tool.
2. Select the QGIS Raster Calculator from the options presented.
3. Enter the NDVI calculation formula: (Band5 – Band4) / (Band5 + Band4).

Ensure that parentheses are correctly placed, as the order of operations is crucial in this calculation.

Setting Output Parameters

After entering the NDVI formula, you must set the output parameters:

• Select a reference layer to ensure the output cell size matches your input bands.
• Choose an output file format (e.g., TIFF) and name your NDVI output file (e.g., NDVI.tiff).

Click “Run” to execute the calculation. The processing time may vary depending on your data size.

Visualizing NDVI Results

Once the NDVI calculation is complete, it’s important to visualize the results effectively. To do this:

1. Double-click on the NDVI layer to open the Layer Properties.
2. Go to the “Symbology” tab.
3. Change the render type to “Singleband pseudocolor.”
4. Select an appropriate color ramp (e.g., brown to blue-green).

Adjust the classification parameters to enhance the visualization of vegetation health. For example, set negative values to represent water bodies and positive values to highlight vegetation.

Interpreting NDVI Maps

After applying the color ramp and adjustments, you should see a map that indicates areas of vegetation and non-vegetation. The color coding will help you quickly identify:

• Healthy vegetation (high NDVI values).
• Barren land or urban areas (low NDVI values).
• Water bodies (negative NDVI values).

This visualization can be crucial for various applications, including agriculture, forestry, and environmental monitoring.

FAQs About NDVI and QGIS

What is the significance of NDVI?

NDVI is widely used to assess vegetation health, monitor changes in land use, and study ecological conditions. It helps in understanding the vitality of vegetation in a given area.

Can NDVI be calculated using other satellite data?

Yes, NDVI can be calculated using various satellite data, including Sentinel-2 and MODIS. The key is to have access to the appropriate spectral bands.

Why is the order of bands important in the NDVI formula?

The order of bands matters in the subtraction part of the formula because it affects the resulting value. The near-infrared band should always be subtracted from the red band to produce accurate NDVI results.