Vegetation Indices

Vegetation Indices (VI) are used to measure the presence of green vegetation on the land surface. NEODAAS provides daily MTCI (MERIS Terrestrial Chlorophyll Index) and MGVI (MERIS Global Vegetation Index) products at ~1 km and ~300m resolution as well as MODIS NDVI and EVI at ~1 km.

MODIS NDVI and EVI

The Normalized Difference Vegetation Index (NDVI) is defined as the difference between the visible (red) and near-infrared (nir) bands, divided by their sum. The NDVI gives a measure of vegetation amount and condition. It is associated with vegetation canopy characteristics such as biomass, leaf area index and percentage of vegetation cover.

NDVI = (NIR - RED) / (NIR + RED)

The Enhanced Vegetation Index (EVI) was developed to optimize the vegetation signal correcting for some distortions in the reflected light caused by the particles in the air as well as the ground cover below the vegetation. The EVI data product also does not become saturated as easily as the NDVI when viewing rainforests and other areas of the Earth with large amounts of chlorophyll.

EVI = G*(NIR-RED)/(NIR+C1*RED-C2*BLUE+L)

where the coefficients adopted in the MODIS-EVI algorithm are: L=1, C1=6, C2=7.5, and G(gain factor)=2.5.

Both indices have a practical range of 0 (no vegetation) to 1 (maximum cover). At the moment no cloud clearing is possible on individual images, but where images overlap in composites the pixel with the highest vegetation index is selected, which is more likely to be cloud free. For more information, see the MODIS Land Team website at http://modis-land.gsfc.nasa.gov/ or for algorithm details, see the ATBD at http://modis.gsfc.nasa.gov/data/atbd/atbd_mod13.pdf

Figure 1: MERIS 7-day MGVI composite (10th-16th September 2009).

MERIS TOA/BOA vegetation indices

Meris Global Vegetation Index (MGVI) is the MERIS Top Of Atmosphere (TOA) Vegetation Index and can be used to assess the presence on the ground of green vegetation. The MGVI Level 2 product is calculated using three MERIS bands (blue/red/near infrared) and corresponds to the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). It has a practical range of 0 to 1.

Meris Terrestrial Chlorophyll Index (MTCI) is the MERIS Bottom Of Atmosphere (BOA) Vegetation Index and can be used to determine the physiological status of the vegetation. The MTCI L2 product is calculated using three red/near infrared bands of Envisat MERIS data and represents the amount of chlorophyll per unit area of ground which is related to the photosynthetic rate. It does not have a theoretical maximum value, but practical values range from 0 to about 4.2.

Cloud clearing is integrated into these algorithms.

For more information visit the Meris website at http://envisat.esa.int/instruments/meris/ or for algorithm details see the ATBDs at http://envisat.esa.int/instruments/meris/atbd/atbd_mgvi_jrc.pdf and http://envisat.esa.int/instruments/meris/atbd/atbd_2_22.pdf