More Context
These new products are the first to be released since 2021, when the HLS project released its initial L30 and S30 Reflectance (Nadir BRDF Adjusted) products offering land surface observations at 30-meter spatial resolution every two to three days.
The HLS Project is an outcome of the 2016 Satellite Needs Working Group (SNWG) assessment that documented the need among federal agencies and other data users for more frequent moderate- to high-resolution Landsat-like observations to track short-term changes in vegetation and other landscape features.
HLS project scientists developed high-resolution, analysis-ready data products for land surface monitoring and applications by capitalizing on the spectral capabilities of multiple similar instruments: the Operational Land Imager (OLI) aboard Landsat 8, the OLI-2 aboard Landsat 9, and the MultiSpectral Imagers (MSI) aboard the European Space Agency (ESA) Sentinel-2A and Sentinel-2B satellites.
HLS’s new VI products follow the 2020 SNWG solution and are created with data from the same instruments and offer the same advantages in terms of coverage, latency, and temporal and spatial resolution, but provide added benefits for agricultural and other vegetation-focused applications.
"This two- to three-day revisit time is particularly beneficial for coverage of frequently cloudy areas where the use of one platform may be insufficient for viewing certain seasonal changes or phenology events. By harmonizing the data from the instruments aboard the Landsat and Sentinel-2 satellites, users will have a better chance of getting the observations they need at a relatively high spatial resolution." -Cole Krehbiel, Project Scientist at NASA’s Land Processes Distributed Active Archive Center (LP DAAC)
In addition, Krehbiel noted that users may enhance the utility of these VI products by using them in conjunction with other HLS-derived data products, such as the Dynamic Surface Water eXtent (DSWx) and Surface Disturbance (DIST) products from the Observational Products for End-Users from Remote Sensing Analysis (OPERA) project, which detect surface change and disturbance.
"I think there could be synergistic use of these vegetation indices and [the additional HLS-derived products]. For example, when we see a surface disturbance occur, we can then look at vegetation indices data to see the before, during, and after-effects of something like a burn scar from a fire using the normalized burn ratio and NDVI, etc.” he said.
At this time, the HLS VI products provide data from December 2024 to the present. Sridhar expects the project to begin historical processing of HLS VI data in 2025, thereby allowing users to reap the full benefits of the HLS project’s decade-long archive.
New Indices Available
Here is a complete list of the new indices available:
- Normalized Difference Vegetation Index (NDVI), which is an index for quantifying green vegetation. It reflects the state of vegetation health based on how vegetation reflects light at certain wavelengths and is calculated using near infrared and red wavelengths.
- Enhanced Vegetation Index (EVI), which is similar to NDVI and can be used to quantify vegetation greenness. However, EVI corrects for some atmospheric conditions and canopy background noise and is more sensitive in areas with dense vegetation.
- Soil Adjusted Vegetation Index (SAVI), which is used to correct NDVI for the influence of soil brightness in areas where vegetative cover is low.
- Modified Soil Adjusted Vegetation Index (MSAVI), which minimizes the effect of bare soil on the SAVI.
- Normalized Difference Moisture Index (NDMI), which is used to determine vegetation water content. It is calculated as a ratio between the near infrared (NIR) and shortwave infrared (SWIR) values in traditional fashion.
- Normalized Difference Water Index (NDWI), which is used to highlight open water features in a satellite image, allowing a water body to “stand out” against the soil and vegetation.
- Normalized Burn Ratio (NBR), which is an index designed to highlight burnt areas in large fire zones. The formula is similar to NDVI, except that the formula combines the use of both NIR and SWIR wavelengths.
- Normalized Burn Ratio (NBR2), which modifies the NBR to highlight water sensitivity in vegetation and may be useful in post-fire recovery studies.
- Transformed Vegetation Index (TVI), which is a variant of NDVI that enhances its sensitivity to vegetation changes. It is particularly useful in areas with low vegetation cover, where NDVI may not be sensitive enough. TVI has applications in monitoring vegetation changes in wetlands and other areas with low vegetation cover.
Data Access
Users can access the HLSL30 and HLSS30 VI products through NASA’s Common Metadata Repository (CMR) and Earthdata Search. In addition, NASA's LP DAAC plans to integrate these products into the Application for Extracting and Exploring Analysis Ready Samples (AppEEARS) in early 2025.
Learning Resources
LP DAAC also has a GitHub page titled HLS Data Resources where users will find tutorials offering instruction on how to search for, access, process, and bulk download HLS data. LP DAAC also has plans to modify the existing tutorials and scripts to demonstrate how to access the new HLS VI products.
