DOI: 10.5066/p904243c
title: Sentinel-2 ACOLITE-DSF Aquatic Reflectance for the Conterminous United States

description: This dynamic data release presents an aquatic reflectance product with 20-meter spatial resolution derived from Sentinel-2 satellite imagery for the conterminous United States using the Atmospheric Correction for OLI “lite” (ACOLITE). Aquatic reflectance, noted Rhow in ACOLITE documentation, is defined here as unitless water-leaving radiance reflectance and represents the ratio of water-leaving radiance (units of watts per square meter per steradian per nanometer) to downwelling irradiance (units of watts per square meter per nanometer) multiplied by π. This is also known as remote sensing reflectance (units of per steradian) multiplied by π. These data are intended for use in remote sensing of water color and differ from other satellite imagery products by using an atmospheric correction approach that is designed for aquatic applications. Level-1C top of atmosphere imagery collected with Multispectral Instrument (MSI) on the Sentinel-2A and Sentinel-2B satellites was atmospherically corrected using the dark spectrum fitting (DSF) algorithm and the Atmospheric Correction for OLI ‘lite’ (ACOLITE) version 20221114.0 processing software (https://github.com/acolite/acolite/releases/tag/20221114.0). The DSF atmospheric correction largely avoids glint and adjacency effects making the technique an improvement over other atmospheric correction for inland waters. Current default ACOLITE settings were used with the exception that;

1) output pixel size (S2_TARGET_RES) was set to 20 m, 
2) the tiling dimensions (DSF_TILE_DIMENSIONS) was set to 300 pixels, 
3) the non-water pixel mask (L2W_MASK THRESHOLD) was set to 0.05 and,
4) the Digital Elevation Model (DEM) to compute pressure (DEM_SOURCE) was set to the Shuttle Radar Topography Mission (SRTM).

The optional residual glint correcting using the SWIR bands was not applied. Users are directed to the metadata file for a complete description of the atmospheric correction process.

A Normalized Difference Water Index (NDWI) product is calculated as the difference of Band 3 (560 nm) and Band 8a (865 nm) divided by the sum of Band 3 and Band 8a, that is: NDWI = (Band 3 – Band 8a)/(Band 3 + Band a8a). The aquatic reflectance and NDWI products are stored as Cloud Optimized GeoTIFFs (COGs). Each Sentinel-2 scene includes cloud optimized geotiffs (COG) of aquatic reflectance raters for band 1, 2, 3, 4, 5, 6, 7, 8, 8a, 11, and 12, a computed Normalized Difference Water Index (NDWI) COG, and an ACOLITE settings file. The file names follow the same naming conventions as the Sentinel-2 products:

MMM_MSIAQR_YYYYMMDDHHMMSS_Nxxyy_ROOO_Txxxxx__BOO.tif

Where:
MMM is the mission ID (S2A/S2B) MSI is the Multispectral Instrument (MSI)
AQR denotes that this is an aquatic reflectance product
YYYYMMDDHHMMSS is the datatake sensing start time in universal coordinated time in Year, Month, Day, Hour, Minute, Second format
Nxxyy is the Processing Baseline number (e.g. N0400)
ROOO is the Relative Orbit number (R001 - R143)
Txxxxx is the Tile Number field is a 15 character identifier used to distinguish between different end user products from the same datatake in Year, Month, Day, Hour, Minute, Second format
BOO is Band number (B01 - B08, B8A, B11, B12) or NDWI for the NDWI product.

For Example: S2A_MSIAQR_20160109T190752_N0201_R013_T10SEH_20160109T190918_B01.tif

Users should be aware that aquatic reflectance values may differ between adjacent tiles within the 10 km of overlap between tiles. This happens because ACOLITE is applied for each tile individually. Additionally, the atmospheric correction algorithm can result in negative reflectance values, in cases where aerosol optical thickness is over estimated. Negative values have been retained in this dataset for transparency, and end users are encouraged to mask or adjust these values prior to analysis. Note that NDWI can produce negative values over water in cases where Band 8a exceeds Band 3. This can happen, for example, in cases where there is surface scum on the water as can happen in the middle of an algal bloom.

When new Sentinel-2 imagery is collected, the Level-1C product is atmospherically corrected and the aquatic reflectance and NDWI products are added to this data release.

Data files for this data release are numerous and cannot be easily displayed for browsing in the ScienceBase interface. Therefore, the data files are provided via a Globus Access Portal (linked under Related External Resources). Globus* is a fast, secure, and reliable way to move large data files and will automatically resume transfers when there are network disruptions. Learn more at https://www.globus.org/data-transfer and https://www.usgs.gov/sciencebase-instructions-and-documentation/deep-storage-data-release. Copies of the data are available on some third-party platforms including the Amazon* Web Services Open Data Sponsorship Program (https://registry.opendata.aws).

* Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.