| Summary: | Accuracy assessment should be a fundamental part of a programme that maps land cover from remotely sensed imagery but this activity is often constrained by the lack of high quality ground reference data. Here, two sources of volunteered data are used to illustrate the potential of amateur or neogeographical activity in map validation. Ground based photographs acquired from an internet-based collaborative project and interpreted by a set of four further volunteers provided the reference data to support evaluation of the Globcover map's representation of tropical forest in West Africa. Although the results highlight some concerns with volunteered data, notably the low levels of inter-volunteer agreement they also show that such imperfect data may be used to derive credible estimates on accuracy from both a site and non-site specific perspective. Specifically, the estimates of the producer's accuracy of forest and of forest extent derived from the free and volunteered data using a latent class model were of comparable magnitude to those derived in a formal validation by experts; the estimate of forest extent was within 1.38-9.08% of reference estimates while the difference in estimated producer's accuracy from that derived in an authoritative assessment of map accuracy was 2.82% and 0.34% for the forest and non-forest classes respectively.
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