Using volunteered data in land cover map validation: Mapping tropical forests across West Africa

Accuracy assessment should be a fundamental part of a land cover mapping programme but often constrained by the lack of ground data. Here, two sources of volunteered data are used to illustrate the potential of neogeographical activity in map validation. Ground based photographs acquired by an internet-based collaborative project and interpreted by a set of volunteers provided the reference data to support evaluation of the Globcover map's representation of tropical forest in West Africa. The results highlight concerns with volunteered data, notably the low levels of agreement but also show that imperfect data may be used to derive useful information on map properties and accuracy.