Harmonized Landsat Sentinel-2 (HLS) uses a processing chain involving several separate radiometric and geometric adjustments, with a goal of eliminating differences in retrieved surface reflectance arising solely from differences in instrumentation. Input data products from Landsat 8 and Landsat 9 (Collection 2 Level 1T top-of-atmosphere reflectance or top-of-atmosphere apparent temperature) and Sentinel-2A and Sentinel-2B (L1C top-of-atmosphere reflectance) are ingested for HLS processing. A series of radiometric and geometric corrections are applied to convert data to surface reflectance, adjust for BRDF differences, and adjust for spectral bandpass differences in section 3.1 of the Algorithm Theoretical Basis Document.
Two types of products are then generated: HLSS30 and HLSL30 (colloquially referred to as S30 and L30, respectively).
S30 and L30 tiles are produced in the Universal Transverse Mercator (UTM) projection and map to the UTM-based Military Grid Reference System (MGRS), which is currently used by Sentinel-2. Each tile is approximately 110 x 110 kilometers with a 30 meter (m) spatial resolution.
LP DAAC distributes both the S30 and L30 products:
- S30: Multi-Spectral Instrument (MSI) harmonized surface reflectance resampled to 30 m into the Sentinel-2 tiling system and adjusted to the Landsat 8/9 spectral response function.
- L30: Operational Land Imager (OLI) harmonized surface reflectance and Top-of-Atmosphere (TOA) brightness temperature, resampled to 30 m, into the Sentinel-2 tiling system.
HLS v2.0 builds on v1.4 by updating and improving processing algorithms, expanding spatial coverage, and providing validation. Particular updates are as follows:
− Global coverage. All global land, including major islands but excluding Antarctica, is covered.
− Input data Landsat 8/9 Collection 2 (C2) data from USGS are used as input; better geolocation is expected as C2 data use the Sentinel-2 Global Reference Image (GRI) as an absolute reference.
− Atmospheric correction. A USGS C version of LaSRCv3.5.5 is applied for both Landsat 8/9 and Sentinel-2 data for computational speedup. LaSRCv3.5.5 has been validated for both Landsat 8/9 and Sentinel-2 within the CEOS ACIX-I (Atmospheric Correction Inter-Comparison eXercise,
http://calvalportal.ceos.org/projects/acix).
− Quality Assurance band. The QA band is generated exclusively by, and named after, Fmask for both S30 and L30. Like in v1.4, aerosol thickness level from atmospheric correction is also incorporated into the QA band.
− BRDF adjustment. BRDF adjustment mainly normalizes the view angle effect, with the sun zenith angle largely intact. This adjustment is applied to the Sentinel-2 red-edge bands as well.
− Sun and view angle bands are provided.
− Product format. Each band in each product is delivered as individual Cloud Optimized GeoTIFF (COG) files.
− Temporal Coverage and Latency in v2.0 moves toward “keep up” processing. The intent is to continually update products with 2-4 day latency.
− HLS v2.0 provides Landsat and Sentinel-2 data products at 30 m spatial resolution. HLS v1.4 provided Sentinel-2 data at 10 meter resolution.
What is the difference between HLS granules and files? Also, what is a Cloud Optimized GeoTIFF (COG)?
Harmonized Landsat Sentinel-2 (HLS) products are produced and distributed from the Earthdata Cloud as Cloud Optimized GeoTIFF (COG). HLS granules refer to a group of files produced for an individual tile for a specific date and time. The files produced for each granule include COGs for each surface reflectance band and associated quality layer(s), as well as associated metadata files.
In every practical sense, COGs are regular GeoTIFF files. COGs are set apart from regular GeoTIFF files by the internal organization of the COG, which provides an efficient way of accessing pieces of a file rather than downloading the entire contents of a file. This is leveraged by clients that have the ability to issue HTTP GET range requests.
What are the criteria for selecting acceptable Sentinel-2 Tiles for HLSS30 v2.0 processing?
HLS images are processed if the following criteria is met:
− There’s a minimum solar zenith angle of 76 degrees (this mainly cuts out areas to the far north in winter).
− Cloud cover less than 95%.