Your First NDVI Map

Make an NDVI map from one satellite observation, then check what its values can and cannot tell you. Choose one of the complete, step-by-step exercises below. Both use the same open NASA data, comparison areas, quality mask, and reference results; the software is the only choice you need to make.

Question

In one summer observation, how does the vegetation signal on Central Park’s Great Lawn compare with the open water of the Jacqueline Kennedy Onassis Reservoir?

Shared data

• Area: Central Park, Manhattan, New York City. The map area is the WGS 84 bounding box (west, south, east, north) -73.982, 40.764, -73.949, 40.800.
• Observation: NASA Harmonized Landsat and Sentinel-2 (HLS) Version 2.0, Sentinel-2 component S30, MGRS tile T18TWL, 18 June 2024. The granule is HLS.S30.T18TWL.2024170T154941.v2.0.
• Bands: red reflectance B04, narrow near-infrared reflectance B8A, and Fmask quality flags. The two fixed sample rectangles are the Great Lawn (-73.9670, 40.7804, -73.9630, 40.7834) and Reservoir (-73.9640, 40.7837, -73.9605, 40.7870), in west, south, east, north order.

The files and software are free. Direct downloads from NASA require a free Earthdata Login; the exercises link to all three files and explain other access routes. No Earth Engine project or paid GIS software is required.

Choose a complete workflow

• Make the map in QGIS if you prefer a desktop GIS and a graphical interface. It walks through downloading the scene, calculating and styling NDVI, comparing the two areas, and saving a map and results table.
• Reproduce it in Python if you prefer a local script and a repeatable run record. It sets up an isolated Python environment, calculates the same NDVI raster and summaries, and records input checksums and software versions.

Both workflows mask fill, cloud, cloud/shadow adjacency, shadow, snow/ice, saturation, and the highest aerosol code. They keep water pixels so the Reservoir remains a valid comparison. The HLS S30 product description and Version 2.0 User Guide define the bands, scale factor, and quality flags.

Expected result

For these sample areas and quality mask, both workflows produce the same reference summaries:

The Great Lawn should have the higher median. The map should show the broad contrast between green vegetation and open water, with local variation from trees, shoreline mixing, shadows, and remaining atmospheric effects. NDVI is evidence about spectral greenness on this date; it does not directly measure plant health or biomass and does not establish a trend. The Spectral Bands and Digital Imagery pages explain why.

To interpret more than one date, continue with the Time Series concept and NDVI Monitoring workflow.

Sources

• Harmonized Landsat and Sentinel-2: About HLS — NASA
• HLS Data Access and Tools — NASA
• HLS S30 Product — NASA
• HLS Version 2.0 User Guide — NASA LP DAAC