Several products are derived to estimate vegetation status from the 10-day SPOT4-VEGETATION synthesis.
Vegetation Index (NDVI)
Vegetation Indexes are empiric formulae for produce quantitative measures which are often relate to vegetation biomass and conditions. The most commonly used vegetation index is the Normalized Difference Vegetation Index (NDVI):
NDVI = (NIR-RED) / ((NIR+RED)
Where NIR is the reflectance measured in the near infrared channel and RED the reflectance measured in the red channel. The higher the NDVI value, the denser or healthier the green vegetation is.
A fixed NDVI threshold value (NDVI = 0.14) was established on SPOT-VGT S10 products to distinguish between bare soil and sparse vegetation. Analysis of NDVI variations over time allows the detection of regions where the vegetation is getting greener. This information is currently incorporated in the monthly FAO Desert Locust Bulletin which gives information on potentially affected countries in desert and semi-desert areas of northern Africa, the Middle East and Southwest Asia for planning survey and control operations.
Percentage of Vegetation Cover
The Centre of Expertise of Remote Sensing and Atmospheric Processes of Vito, in close collaboration with the European Commission Joint Research Centre (JRC), has developed an algorithm to extract information from SPOT4-VEGETATION images on the percentage of vegetation cover.
Vegetation/Soil Moisture Content
The Short Wave Infrared (SWIR) channel present on SPOT4-VGT sensor provides information on the moisture present in the vegetation and in the soil. Research is underway to use SPOT4-VGT 10-day images to extract information on vegetation/soil moisture content.