BCC Geospatial Center of the CUNY CREST Institute
Sentinel-5P
What is the HLS (Harmonized Landsat Sentinel-2)?
HLS is a combined dataset created by harmonizing images from Landsat 8 & 9 (NASA/USGS) and Sentinel-2 (ESA). The goal is to provide frequent, high-quality images of Earth’s surface, with matched resolution, projection, and spectral characteristics.
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Why it's useful: You get images every 2–3 days, instead of waiting 8–16 days from just one satellite.
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Who uses it: Scientists, farmers, urban planners, conservationists — anyone who wants to track changes on land quickly and clearly.
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What is Landsat?
Landsat is a joint program between NASA and the USGS that’s been observing Earth for over 50 years — it’s the longest continuous record of Earth’s surface.
Landsat 8 & 9: Eyes on the Land
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What they do: These satellites take pictures in visible and infrared bands, tracking land changes like vegetation health, drought, urban expansion, and more.
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Resolution: 30 meters (about the size of a baseball diamond).
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Revisit time: Every 8 days (when combined).
How is Landsat Different from Sentinel-2?
While both Landsat and Sentinel-2 are "color cameras" (optical satellites), they have some key differences that make them unique and complementary:
Spatial Resolution:
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Landsat's main optical bands are primarily at 30-meter resolution. It has one sharper black-and-white band (Panchromatic) at 15 meters.
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Sentinel-2 offers finer detail with several bands at 10-meter resolution, plus some at 20m and 60m. So, Sentinel-2 can see slightly smaller objects on the ground.
Revisit Frequency (How Often it Visits):
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A single Landsat satellite (like Landsat 8 or 9) passes over the same spot only once every 16 days. With two satellites working together, you get a pass about every 8 days.
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Sentinel-2, with its two satellites, offers much more frequent visits, typically every 5 days, and even every 2-3 days in many areas. This means Sentinel-2 is better for watching fast-changing events.
Number & Placement of Bands (Slight "Color" Differences):
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Landsat 8/9 has 11 spectral bands (9 optical, 2 thermal).
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Sentinel-2 has 13 spectral bands.
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While many bands are similar (like Blue, Green, Red, NIR, SWIR), their exact wavelengths and widths are slightly different. For example, Sentinel-2 has more dedicated "Red Edge" bands which are very sensitive to early plant stress, and a Coastal Aerosol band. Landsat also has two unique thermal bands for temperature mapping that Sentinel-2 doesn't have.
Thermal Infrared Bands:
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Landsat has dedicated thermal infrared bands (TIRS instrument) that measure the actual temperature of the Earth's surface (land surface temperature, water temperature).
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Sentinel-2 does not have thermal infrared bands. For temperature, we look to Sentinel-3.
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How it Works (The "Harmonization")
Think of HLS like a recipe blender that takes ingredients from two different kitchens—Landsat and Sentinel-2—and blends them into one smooth, consistent dish that’s ready for science.
Even though both satellites are like "color cameras" looking at Earth, they’re made by different agencies and have slightly different specs. HLS fixes these differences so you can use their data together seamlessly.
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Clearing the Air (Atmospheric Correction):
Just like sunglasses remove glare, HLS removes haze, dust, and cloud effects to see the true color of Earth’s surface. This turns raw images into clear ones that show real surface reflectance. -
Hiding the Clouds (Cloud & Shadow Masking):
Clouds are like photo-bombers in satellite images. HLS detects clouds and their shadows and hides them so you only see the useful parts of the image. -
Aligning the Maps (Co-registration & Gridding):
Landsat and Sentinel-2 don’t always look at Earth from the exact same angle or position. HLS lines up the images perfectly on a common map grid, so every pixel matches. -
Fixing the Lighting (View Angle & Sunlight Adjustment):
Think of how a photo changes when the sun is at a different angle. HLS adjusts for these sun and satellite angle effects so images look consistent, no matter when they were taken. -
Matching the Colors (Spectral Band Adjustment):
Even though both satellites capture red, blue, etc., their cameras are slightly different. HLS fine-tunes these bands to make Landsat’s and Sentinel-2’s colors behave as if they were taken by the same camera.
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