MODIS Land Product Validation
The MODIS land science team incorporates a number of validation methods to characterize and measure the uncertainty factor among its products. They may include comparing MODIS products with the following reference data sources: in situ data derived from several validation test sites, operational scientific network reference data, and derived data from higher resolution airborne or satellite sources. Ideally, these reference data are collected or derived from a large number of validation sites that are well distributed in space and time. Additionally, intercomparison with similar products derived from other sensors facilitates relative validation, which is an indirect yet simple method to assess the spatial and temporal consistency between products.
Validation Core Sites
Core sites are intended to facilitate land product validation activities across all biome types. They provide long-term data, and most of them support an infrastructure to deliver in situ measurements. A strategy attempted in the past and that continues today is to focus on product validation activities at a set of sites that are representative of a range of global biome types. In compiling these Core Sites, the aim is to have consensus among instrument teams and validation investigators. Several core validation sites support a flux tower infrastructure to derive extensive temporal measurements of terrestrial biophysical variables over different land-cover types. Such core sites are usually built upon existing programs of long-term measurements with supporting infrastructure to generate in situ data. Given such a long-term measurement strategy, these data products can add significant value to multiple science disciplines, and render them useful to more than a single satellite sensor team. A separate yet equally important effort is the post-launch characterization and calibration of Earth satellite sensors. Characterizing the performance of an instrument throughout the life of its mission by including specially designed onboard calibration devices is integral to the accurate retrieval of at-sensor radiance. The MODIS and VIIRS Calibration characterization support teams help coordinate vicarious calibration techniques at standardized test sites.
MODIS land products are assigned one of four stages in the validation hierarchy; these stages are based on definitions established by the Land Product Validation (LPV) subgroup, which is part of the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV). This four-stage validation hierarchy defines the data product evolution in terms of its accuracy, spatial and temporal consistency, and uncertainty characterization. Publication of the product validation results in peer-reviewed literature is a requirement to ensure its evolution beyond the first stage. The MODIS science team works closely with the WGCV’s subgroup on Land Product Validation (LPV) on validation activities and the development of protocols.
For validation status information for individual MODIS land products, see the information under the Validation tab on the MODIS Land Team web site.