DEA Surface Reflectance NBART (Sentinel-2A MSI)

Technical information

Top of atmosphere reflectance measurements

Sentinel-2 series sensors measure top of atmospheric reflectance, which is a composite of:

• surface reflectance
• atmospheric condition
• interaction between surface land cover, solar radiation and sensor view angle (BRDF effect)
• land surface orientation relative to the imaging sensor (terrain illumination).

It has been traditionally assumed that satellite imagery displays negligible variation in sun and sensor view angles. However, these can vary significantly both within and between scenes, especially in different seasons and geographic regions (Li et al. 2010, 2012).

Surface reflectance correction models

This product represents standardised optical surface reflectance using robust physical models to correct for variations and inconsistencies in image radiance values.

It delivers modelled surface reflectance from Sentinel-2A MSI data using physical rather than empirical models. This ensures that the reflective value differences between imagery acquired at different times by different sensors will be primarily due to on-ground changes in biophysical parameters rather than artefacts of the imaging environment.

This product is created using a physics-based, coupled Bidirectional Reflectance Distribution Function (BRDF) and atmospheric correction model that can be applied to both flat and inclined surfaces (Li et al. 2012). The resulting surface reflectance values are comparable both within individual images and between images acquired at different times.

For more information on the BRDF/Albedo Model Parameters product, see MCD43A1 Collection 6. 

Sentinel-2 archive

To improve access to Australia’s archive of Sentinel-2 data, several collaborative projects have been undertaken in conjunction with industry, government and academic partners. These projects have enabled implementation of a more integrated approach to image data correction that incorporates normalising models to account for atmospheric effects, BRDF and topographic shading (Li et al. 2012). The approach has been applied to Sentinel-2 imagery to create baseline surface reflectance products.

The advanced supercomputing facilities provided by the National Computational Infrastructure (NCI) at the Australian National University (ANU) have been instrumental in handling the considerable data volumes and processing complexities involved with the production of this product.

Image format specifications

band01, band02, band03, band04, band05, band06, band07, band08, band8A, band11, band 12

Format GeoTIFF

Resolution 10/20/60m based on Sentinel-2 original pixel resolution

Datatype Int16

No data value -999

Valid data range [1,10000]

Tiled with X and Y block sizes 512x512

Compression Deflate, Level 6, Predictor 2

Pyramids Levels: [8,16,32]
Compression: deflate
Resampling: GDAL default (nearest)
Overview X&Y block sizes: 512x512

Contrast stretch None

Output CRS As specified by source dataset; source is UTM with WGS84 as the datum

thumbnail

Format JPEG

RGB combination 

Red: band 4
Green: band 3
Blue: band 2

Resolution X and Y directions each resampled to 10% of the original size

Compression JPEG, Quality 75 (GDAL default)
PHOTOMETRIC colour model: YCBCR

Contrast stretch Linear
Input minimum: 10
Input maximum: 3500
Output minimum: 0
Output maximum: 255

Output CRS Geographics (Latitude/Longitude) WGS84

Accuracy and limitations

Atmospheric correction accuracy depends on the quality of aerosol data and total column water vapour available to determine the atmospheric profile at the time of image acquisition (Wang et al., 2009).

BRDF correction is based on low resolution imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS), which is assumed to be relevant to medium resolution imagery such as that captured by Sentinel-2A MSI. A single BRDF shape is applied to each Sentinel-2A tile and it does not account for changes in land cover. 

The algorithm assumes that BRDF effect for inclined surfaces is modelled by the surface slope and does not account for land cover orientation relative to gravity (as occurs for some broadleaf vegetation with vertical leaf orientation).

The accuracy of the terrain correction also depend on the quality, scale and spatial resolution of the DSM data used and the co-registration between the DSM and the satellite image (Li et al., 2013). At present, 30 m resolution SRTM DSM data were used for the correction.

The algorithm depends on several auxiliary data sources:

• Availability of relevant MODIS BRDF data
• Availability of relevant aerosol data
• Availability of relevant water vapour data
• Availability of relevant DSM data
• Availability of relevant ozone data

Improved or more accurate sources for any of the above listed auxiliary dependencies will also improve the surface reflectance result.

Quality assurance

Results from the DEA Cal/Val workflow over 17 data takes from 9 field sites were created based on both BRDF Collections 5 and 6.

The results for each collection were averaged and then compared. The comparison showed small changes in individual field sites, but overall there was no significant difference in the average results over all field sites to within 1% at most.

The technical report containing the data summary for the Phase 1 DEA Surface Reflectance Validation is available: DEA Analysis Ready Data Phase 1 Validation Project: Data Summary

References

1. F. Li, D. L.B. Jupp & M. Thankappan (2015) Issues in the application of Digital Surface Model data to correct the terrain illumination effects in Landsat images, International Journal of Digital Earth, 8:3, 235-257, DOI: 10.1080/17538947.2013.866701

2. L. Wang, F. Li, I. Alam, D. Jupp, S. Oliver and M. Thankappan, "Analysis Ready Data Sensitivity Analyses," IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium, 2019, pp. 5642-5645, doi: 10.1109/IGARSS.2019.8898667