This data set comprises the Normalized Differential Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) together with uncertainty estimates, various quality flags, solar zenith angles, number of 1 km grid cells used, and reflectances / radiances of the 4 MODIS channels used for the retrieval.
Both indices are based on the difference: reflectance observed in the near-infrared (NIR) portion of the electromagnetic spectrum minus reflectance observed in the red portion of the electromagnetic spectrum. Areas with photosynthetically active vegetation absorb much more radiation in the red portion of the electromagnetic spectrum than areas without any or with photosynthetically inactive vegetation. This is due to the pigments in the leaves, needles, etc. In contrast, absorption of NIR radiation is low for both surface types. However, the amount of NIR radiation transmitted and/or scattered is a function of the surface type / roughness. Therefore, the difference of the reflectances is small for areas without any or with photosynthetically inactive vegetation and high for areas with photosynthetically active vegetation. Note that in areas with a lot of vegetation the dependence of the NIR-radiation scattering and/or transmission on the surface type / roughness helps to overcome saturation effects in the red portion of the electromagnetic spectrum and therefore allows a better separation of high NDVI values.
The impact of clouds, aerosols, sensor noise (and other error sources not mentioned here explicitly) is mitigated using radiative transfer modeling on the one hand. On the other hand these impacts are mitigated by normalizing the above-mentioned difference with the sum of the used reflectances (NDVI) or with these reflectances, the reflectance in the blue portion of the electromagnetic spectrum and additional factors (EVI). For more details we recommend to look into the User's Guide.
The data set (MODIS collection 6) offered here (MOD13C2 and MYD13C2) has monthly temporal resolution and allows to monitor the seasonal cycle of vegetation development.
Last data set update at ICDC: January 23, 2020