Mapping & Monitoring
The Need for Validation
Remote sensing is the process of collecting and interpreting information about an object without being in physical contact with the object. For estimates of net primary production from space, scientists do not measure changes in CO2 concentrations, or C gas exchanges. Rather, light that has been reflected off an area of the earth's surface is measured, and then inferences regarding canopy light harvesting as an input to approximate rates of production in that area. It is necessary to validate these estimates using ground based measurements, in order to define how accurate the estimates are likely to be. These tests help assess the reliability and creditability of the products, as well as define any limitations the products have.
Product Comparisons and Validations
The two graphs below show comparisons of the MODIS NDVI product to NDVI derived from finer resolution Landsat and IKONOS imagery. The MODIS product was found to systematically overestimates NDVI relative to both Landsat and IKONOS products. This could be a result of sensor differences or variability introduced through changes in scale.
Left - Comparison of MODIS NDVI to Landsat ETM+ NDVI. Right - Comparison of Landsat ETM+ NDVI to IKONOS NDVI
A comparison of the MODIS FPAR product to WHRC field-based measurements of FPAR demonstrates that MODIS overestimates FPAR in boreal fire disturbed regions. This has been documented by other investigators in other study sites, and is an area of active research. Part of the answer is likely related to differences in field versus satellite measurements of ground cover.
