The MODIS NDVI is referred to as the "continuity index" to the existing 20+ year NOAA-AVHRR derived NDVI time series, which could be extended by MODIS data to provide a longer term data record for use in operational monitoring studies. The normalized difference vegetation index (NDVI) is a normalized ratio of the NIR and red bands,

NDVI= (r_NIR-r_Red) / (r_NIR+r_Red)

where r_NIR and r_red are the surface bidirectional reflectance factors for their respective MODIS bands. The NDVI is successful as a vegetation measure in that it is sufficiently stable to permit meaningful comparisons of seasonal and inter-annual changes in vegetation growth and activity.

The NDVI reduces many forms of noise (illumination differences, cloud shadows, atmospheric attenuation, certain topographic variations) present in multiple bands. The main disadvantage of the NDVI is related to atmospheric path radiances. The NDVI also exhibits scaling problems, saturated NDVI signals over high biomass conditions, and is very sensitive to canopy background variations, with NDVI degradation particularly strong with higher canopy background brightness (bright soils).