Vegetation Indicators

In Terrascope, the following Sentinel-2 value-added vegetation indicator data are provided:

The vegetation indicators provide information on the land surface that is quantifiable with measurements, and are independent of the sensor characteristics (spectral bands, illumination and observation geometry). It is therefore a more direct estimate of true land surface conditions. Below, a short introduction to the vegetation indicators and processing is given, more elaborate descriptions on the methodology and processing can be found in the BIOPAR V200 ATBD.

BIOPAR definitions

The Normalized Difference Vegetation Index (NDVI) is a simple vegetation index, which provides information on the biomass’ greenness. Even though not being a physical property of the vegetation cover, it’s simple formulation makes it widely used for ecosystem monitoring.

The Leaf Area Index (LAI) is defined as half the total area of a canopy’s green elements per unit horizontal ground area. The satellite-derived value corresponds to the total green LAI of all the canopy layers, including the understory, which may represent a very significant contribution, particularly for forests. The LAI distributed by Terrascope is also known as GAI, which stands for Green Area Index, and is related to the green part of the vegetation only (i.e. not only the leaves, but excluding the non-green parts).

The Fraction of Absorbed Photosynthetic Active Radiation (fAPAR) quantifies the fraction of solar radiation absorbed by leaves for the photosynthetic activity. It depends on the canopy structure, vegetation element optical properties, atmospheric conditions, and angular configuration.

The Fraction of Vegetation Coverage (fCOVER) corresponds to the fraction of ground covered by green vegetation. Practically, it quantifies the vegetation’s spatial extent.

The Canopy Water Content (CWC) is the water mass per unit ground area and is a key vegetation indicator in agriculture and forestry applications.

The Canopy Chlorophyll Content (CCC) is defined as the total chlorophyll content per unit ground area in a contiguous group of plants. It is well suited for quantifying canopy level nitrogen content and gross primary production estimation.

Vegetation Indicator retrieval methodology

The NDVI is calculated from two individual TOC reflectance measurements as follows:

\[NDVI = \frac{(NIR - RED)}{(NIR + RED)}\]

with NIR and RED the spectral TOC reflectances measured in the near-infrared and red wavebands, respectively. In Terrascope, two 10 m spectral TOC reflectances are used, resulting in the following formula:

\[NDVI = \frac{(B_{08} - B_{04})}{(B_{08} + B_{04})}\] with B04 and B08 the TOC reflectances in the MSI bands with center wavelenghts of 0.650 and 0.842 \(\mu\)m, respectively.

The other defined vegetation indicators are calculated based on a methodology developed by the Environnement Méditerranéen et Modélisation des Agro-Hydrosystèmes (INRA-EMMAH), see Figure 1. It mainly consists in generating a comprehensive vegetation characteristics database (fAPAR, fCover, LAI, CCC and CWC) and the associated top of canopy (TOC) reflectances using Radiative Transfer Models (RTM). Artificial Neural Networks (ANN) are then trained to estimate the canopy characteristics from the TOC reflectances along with the corresponding angles defining the observational configuration.

Biopar ANN

Figure 1: Schematic representation of INRA-EMMAH’s neural network method to derive vegetation indicators.

Two sets of neural networks were trained, one set based on the TOC reflectances of 8 bands with output at 20 m resolution, and another set based on the TOC reflectances of 3 spectral bands with output at 10 m resolution. The results of both approaches are available in Terrascope.

Product data layers

For the Vegetation Indicator products, the following layers are generated:

  • The actual parameter (NDVI or BIOPAR)
  • The scene classification. These files are delivered together with an XML file containing the parameter’s metadata.

In addition, a quicklook is provided.

The NDVI is only delivered at 10 m resolution, whereas CCC and CWC are only available at 20 m resolution. LAI, FAPAR, and FCOVER are available at both 10 m and 20 m resolution.

Table 1 provides the Vegetation Indicators’ technical information, such as their physical range and the rescaling coefficients. The latter should be applied to the data to translate them to physical values (PV), as the data are stored as BYTE. To rescale the BYTE output layers, the following formula should be applied:

\[PV = DN * slope + offset\]

with PV the Pyshical Value and DN the BYTE value.

Table 1: Characteristics of the NDVI and BIOPAR images and rescaling information. Physical min and max are the physical range that is retained in the output, the Digital Numbers (DN) are the value of the physical min and max after rescaling to BYTE. The ‘slope’ and ‘offset’ are the coefficients to use to recompute the physical values from the BYTE output images.

product units Physical min Physical max DN min DN max offset slope No data
NDVI - -0.08 0.92 0 250 -0.08 0.004 255
FAPAR - 0 1 0 200 0 0.005 255
LAI m2 m-2 0 10 0 250 0 0.04 255
FCOVER - 0 1 0 200 0 0.005 255
CCC g cm-2 0 600 0 250 0 0.05 255
CWC \(\mu\)g cm-2 0 0.55 0 250 0 0.026 255

The scene classification is copied from the TOC reflectance product as described in the Level-2A TOC reflectance ATBD. As mentioned before, the scene classification layer is provided at 20 m resolution. Table 2 specifies the meaning of the pixel values in this layer.

Table 2: Meaning of the values in the Scene Classification layer.

Value Meaning
0 No data
1 Saturated or defected
2 Dark area pixels
3 Cloud shadow
4 Vegetation
5 Bare soil
6 Water
7 Unclassified
8 Cloud medium probability
9 Cloud high probability
10 Thin cirrus
11 Snow

The folder structure used on the Terrascope platform is /data/MTDA/TERRASCOPE_Sentinel2/<product>/<year>/<month>/<day>/<tile_ID>/<resolution>. tile_ID has the following convention: <sensor>_<date>_<UTM_tile>_<product>_<version>, for example S2B_20190504T105629_31UDS_FCOVER_V200.

See the BIOPAR V200 ATBD for more methodological details on the Vegetation Indicators. An extensive validation study was also performed and is reported in the Quality Assessment Report.

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