Image interpretation keys for validation of global land-cover data sets

Photointerpretation keys originated from the need to efficiently train aerial photographic interpreters. Keys provide interpreters with a baseline of signature information that can be widely applied to images illustrating differing land covers, features, facilities, processes, and activities. Keys were produced for the international Geosphere Biosphere Programme Data Information Systems (IGBP-DIS) DISCover Validation effort. This effort had two objectives: to provide a prototype set of image interpretation keys that could be produced efficiently and cost effectively, and to create a foundation of enduring reference materials that could be refined and enlarged to facilitate the validation of future global land-cover products. The methodology of key development and production for this effort originated as an ideal, but has by necessity been shaped by pragmatic considerations. Factors including cost, time, and availability of imagery and ancillary data limited broad applicability of the keys developed. Research included determination of the most effective band combination, signature development and application, consistent implementation of a minimum mapping unit, use of signature elements in the evaluation of key use, and development of a product useful for cross applications. We continue to expand the present set of keys and develop a digital infrastructure for a global reference library of key materials. These materials will expedite future validation efforts and lead to greater confidence in the accuracy of future global land-cover products.