Validation of MODIS land surface reflectance and vegetation indices with multi-scale high spatial resolution data

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS), on board the Terra platform launched in 1999, is a highly evolved successor to the AVHRR for providing data on terrestrial biospheric dynamics and process rates. A shift from the AVHRR to MODIS sensor will involve improved sensor characteristics and atmospheric contamination correction, which will inevitably result in more accurate global vegetation studies. However, the success and accuracy of MODIS spectral measures for improved global vegetation monitoring will also depend on how well the vegetation algorithms themselves are able to depict actual spatial and temporal vegetation differences. The MODIS VI products use MODIS surface reflectance, corrected for molecular scattering, ozone absorption, and aerosols, and adjusted to nadir with use of a BRDF model, as input to the VI equations. Two vegetation (VI) algorithms are to be produced globally for land, at launch. One is the standard normalized difference vegetation (NDVI), which is referred to as the continuity index to the existing NOAA-AVHRR derived NDVI to provide a long term data record for use in operational monitoring studies. The other is an enhanced vegetation (EVI) with improved sensitivity into high biomass regions and improved vegetation monitoring through a de-coupling of the canopy background signal and a reduction in atmosphere influences. The authors attempt to validate the MODIS land surface reflectance and derived vegetation (VI) products, including 250 m NDVI at 16 day; 500m NDVI and EVI at 16 day; 1 km NDVI and EVI at 16 day. Their validation will be implemented at a number of validation sites by scaling fine-grained surface measurements to several levels of MODIS pixel size (250m, 500m, and 1000m), then assessing the differences between these surfaces and the MODIS products.