The difficulties inherent in assessing the accuracy of large-area spatial data sets, their subsequent analysis by GIS, and their importance to landscape characterization make the development of methods for robust accuracy assessment a critical challenge. In December 2002, the U.S. EPA sponsored a symposium focused on evaluating the scienctific elements relevant to performaning of accuracy assessments for remote sensing derived data sets and GIS data analysis and integrated products. The result is this peer-reviewed text from an international group of scientists representing federal, state and local governments, academia, and non-governmental organizations.