Sentinel-2 Level-2A Surface Reflectance

Date modified: 21 April 2021

Product overview

Background

The Sentinel-2 mission is part of the European Union Copernicus programme for Earth observations. Sentinel-2 consists of twin satellites, Sentinel-2A (launched 23 June 2015) and Sentinel-2B (launched 7 March 2017). The two satellites have the same orbit, but 180° apart for optimal coverage and data delivery. Their combined data is used in the Digital Earth Africa Sentinel-2 product.

Together, they cover all Earth’s land surfaces, large islands, inland and coastal waters every 3-5 days.

Sentinel-2 data is tiered by level of pre-processing. Level-0, Level-1A and Level-1B data contain raw data from the satellites, with little to no pre-processing. Level-1C data is surface reflectance measured at the top of the atmosphere. This is processed using the Sen2Cor algorithm to give Level-2A, the bottom-of-atmosphere reflectance (Obregón et al, 2019). Level-2A data is the most ideal for research activities as it allows further analysis without applying additional atmospheric corrections.

The Digital Earth Africa Sentinel-2 dataset contains Level-2A data of the African continent. Digital Earth Africa does not host any lower-level Sentinel-2 data.

Specifications

Table 1: Sentinel-2 Level-2A product specifications

Specification

Product name

Sentinel-2 Level-2A

Number of bands

15

Cell size - X (metres)

10, 20, 60 (band-dependent, see Table 2)

Cell size - Y (metres)

10, 20, 60 (band-dependent, see Table 2)

Coordinate reference system

EPSG: 6933

Temporal resolution

Every 5 days

Temporal range

2017-05-29 00:00:00 – present

Parent dataset

Sentinel-2 Level-1C (external to DE Africa)

Update frequency

Ongoing

The specifications listed in Table 1 refer to the period where both Sentinel-2 satellites have been operational. Standalone Sentinel-2A data exists for the period of 2015 – 2017, before Sentinel-2B was launched, but it has a reduced temporal resolution.

Figure 1: Sentinel-2 Level-2A geographic extent

Sentinel-2 data extent

Digital Earth Africa Sentinel-2 data is available for the regions shaded in blue. Specific temporal and geographic extents can be explored as an interactive map on the Digital Earth Africa Metadata Explorer.

Table 2: Sentinel-2 Level-2A measurements

Band ID

Description

Spatial resolution (m)

Value range

Data type

No data value

B01

Coastal aerosol

60

1 - 10000

uint16

0

B02

Blue

10

1 - 10000

uint16

0

B03

Green

10

1 - 10000

uint16

0

B04

Red

10

1 - 10000

uint16

0

B05

Red edge 1

20

1 - 10000

uint16

0

B06

Red edge 2

20

1 - 10000

uint16

0

B07

Red edge 3

20

1 - 10000

uint16

0

B08

Near infrared (NIR) 1

10

1 - 10000

uint16

0

B8A

NIR 2

20

1 - 10000

uint16

0

B09

Water vapour

60

1 - 10000

uint16

0

B11

Short-wave infrared (SWIR) 1

20

1 - 10000

uint16

0

B12

SWIR 2

20

1 - 10000

uint16

0

AOT

Aerosol optical thickness

20

1 - 65535

uint16

0

WVP

Scene average water vapour

20

1 - 65535

uint16

0

SCL

Scene classification layer

20

0 - 255

uint8

0

The Digital Earth Africa Sentinel-2 Level-2A dataset has fifteen bands of data, which can be grouped into the multispectral bands, and the auxilary quality assurance bands.

  • Multispectral observations — 12 bands: Sentinel-2 satellites carry a multispectral instrument that measures in the visible, near infrared, and short-wave infrared ranges. These are indexed as B01 - B12. The band B10 is not included in the DE Africa Sentinel-2 Level-2A product as it does not contain surface information.

    Surface reflectance values are scaled between 1 and 10000. Note different spectral bands have different native resolutions.

  • Quality assurance — 3 bands: The Sentinel-2 Level-2A product also includes three auxilary data bands which are used to improve the accuracy of surface reflectance-derived products. These corrections were applied as part of the Sen2Cor conversion from Level-1C (top-of-atmosphere) to Level-2A (bottom-of-atmosphere) data.

    • Scene Classification Layer (SCL) distinguishes between cloudy, clear and water pixels. The algorithm identifies areas as clouds, snow, cloud shadows, vegetation, bare soil and water. The resulting SCL is used for cirrus cloud correction in the processing chain. It is also commonly applied in further analysis as a cloud mask, which removes unwanted cloudy pixels from the multispectral observations. SCL bit values are defined in Table 3.

    • Aerosol Optical Thickness (AOT) is a measure of haze, or visibility through the atmosphere. In Sentinel-2 Level-2A data, this is derived using the Dark Dense Vegetation pixel method, which relies on calibrating reflectance against dark vegetation and using the estimated atmospheric thickness to correct the rest of the measurements.

    • Water Vapour (WVP), also referred to in literature as Integrated Water Vapour (IWV) or Water Vapour (WV), is the total water vapour integrated over an atmospheric column. It is calculated using an atmospheric pre-corrected differential absorption algorithm (Schläpfer et al, 1998). Like AOT, it is a key component of atmospheric correction.

Table 3: SCL bit values

Bit value

Classification

0

No data

1

Saturated or defective

2

Dark area pixels

3

Cloud shadows

4

Vegetation

5

Bare soils

6

Water

7

Unclassified

8

Cloud medium probability

9

Cloud high probability

10

Thin cirrus

11

Snow or ice

Sentinel-2 Level-2A metadata can also be viewed on Digital Earth Africa’s Metadata Explorer.

Processing

Each observation from the parent dataset, Level-1C, is processed by the European Space Agency (ESA) through the Sen2Cor algorithm. First, cloud detection is performed using SCL, then AOT and WVP are calculated. These are applied to give the resultant bottom-of-atmosphere Level-2A observations.

More details on Sentinel-2 Level-2A processing can be found in Gascon et al, 2017.

Media and example images

Image 1: The Eye of Africa, Mauritania. 2021 Sentinel-2, true-colour (RGB).

Credit: Contains modified Copernicus Sentinel data 2021, processed by Digital Earth Africa.

Sentinel-2 true-colour image

Image 2: Bazaruto Archipelago, Mozambique. 2020 Sentinel-2, true-colour (RGB).

Credit: Contains modified Copernicus Sentinel data 2020, processed by Digital Earth Africa.

Sentinel-2 true-colour image

References

Obregón, M.A., Rodrigues, G., Costa, M.J., et al. (2019). Validation of ESA Sentinel-2 L2A aerosol optical thickness and columnar water vapour during 2017-2018. Remote Sensing, 11(14), 1649. https://doi.org/10.3390/rs11141649

Schläpfer, D., Borel, C.C., Keller, J., et al. (1998). Atmospheric precorrected differential absorption technique to retrieve columnar water vapor. Remote Sensing of Environment, 65(3), 353-366. https://doi.org/10.1016/S0034-4257(98)00044-3.

Gascon F., Bouzinac C., Thépaut O., et al. (2017). Copernicus Sentinel-2A calibration and products validation status. Remote Sensing, 9(6), 584. https://doi.org/10.3390/rs9060584

License

Access to Sentinel data is free, full and open for the broad Regional, National, European and International user community. View Terms and Conditions.

Acknowledgements

The Sentinel-2 Level-2A product is provided by the European Space Agency.

Data access

Amazon Web Services S3

Sentinel-2 Level-2A is available in AWS S3, sponsored by the Public Dataset Program.

Table 4: AWS data access details

AWS S3 details

Bucket ARN

arn:aws:s3:::deafrica-sentinel-2

Product name

s2_l2a

The bucket is in the AWS region af-south-1 (Cape Town). Additional region specifications can be applied as follows:

aws s3 ls --region=af-south-1 s3://deafrica-sentinel-2/

The data is organised per tiles using Military Grid Reference System. The file paths follow the format

<utm_zone>/<latitude_band>/<grid_square>/<year>/<month>/<scene_ID>.

Table 5: AWS file path convention

File path element

Description

Example

utm_zone

Grid zone designator

10

latitude band

Latitude band are lettered C- X, omitting the letters “I” and “O”

S

square

Pair of letters designating one of the 100,000-meter side grid squares inside the grid zone

DG

year

Year the data was collected, YYYY

2018

month

Month of the year the data was collected (without leading zeros)

3

scene_ID

All files corresponding to a scene

ccc27a76-71ab-5da9-9d88-4f2c3e874ef3

OGC Web Services (OWS)

This product is available through DE Africa’s OWS.

Table 6: OWS data access details

OWS details

Name

DE Africa Services

Web Map Services (WMS) URL

https://ows.digitalearth.africa/wms?version=1.3.0

Web Coverage Service (WCS) URL

https://ows.digitalearth.africa/wcs?version=2.1.0

Layer name

s2_l2a

Digital Earth Africa OWS details can be found at https://ows.digitalearth.africa/.

For instructions on how to connect to OWS, see this tutorial.

Open Data Cube (ODC)

The Sentinel-2 Level-2A dataset can be accessed through the Digital Earth Africa ODC API, which is available through the Digital Earth Africa Sandbox.

ODC product name: s2_l2a

The ODC product name is derived from Sentinel-2 Level-2A. Some typefaces obscure the fourth character, which is a lowercase L.

Specific bands of data can be called by using either the default names or any of a band’s alternative names, as listed in the table below. ODC datacube.Datacube.load commands without specified bands will load all bands; see ODC documentation.

Table 7: Sentinel-2 ODC band names

Band name

Alternative names

B01

band_01, coastal_aerosol

B02

band_02, blue

B03

band_03, green

B04

band_04, red

B05

band_05, red_edge_1

B06

band_06, red_edge_2

B07

band_07, red_edge_3

B08

band_08, nir, nir_1

B8A

band_8a, nir_narrow, nir_2

B09

band_09, water_vapour

B11

band_11, swir_1, swir_16

B12

band_12, swir_2, swir_22

SCL

mask, qa

AOT

aerosol_optical_thickness

WVP

scene_average_water_vapour

Product names and band names are case-sensitive.

For examples on how to use the ODC API, see the DE Africa example notebook repository.

Technical information

Additional information about the Sentinel-2 instrumentation can be found on the ESA Sentinel-2 User Guide.