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been re sampled from the raw satellite data to have a uniform spatial resolution of one degree of latitude and longitude Each one degree pixel is described by a time series of twelve NDVI values at monthly time increments from and by its latitude  which can be useful for discriminating among classes with otherwise similar spectral properties  Defries Tow ns hend The monthly temporal sampling procedure allows the compilation of ccoud free views of the Earths surface and also captures seasonal dynamics in vegetation The temporal information is particularly useful for classification of vegetation and land cover as seasonal changes in vegetation are one of the best indicators of vegetation type A summary of these data is provided in Table The class labels were selected to reflect fairly broad classes with extensive geographic coverage  Maps developed from this classifi cation scheme may then be used to relate land coverclasses to structurally and functionally ssignificant ecological properties From a remote sensing perspective this classification sys tem reflects a compromise between class labels that are separable from coarse resolution remote sensing data and class labels that are useful to end users such as ecologists For further details regarding the specific procedures used to compile the data the reader is referred to DeFries and Townshend and Los Justice and Tucker Labeling error occurs in land cover training data for many reasons One source arises because discrete classes and boundaries are used to distinguish among classes that have subtle boundaries in space and that have fairly small differences in terms of their physical attributes  For example  the distinction between a grassland and wooded grassland can be quite difficult to discern  Consequently pixels labeled as grassland may in fact represent open woodland areas and vice versa This source of error is especially problematic at the one degree spatial resolution of the data used here Another source of error is the difference between potential and actual vegetation  Potential vegetation refers to the type of vegetation that occurs naturally in a region based on soil  climate and geologic controls Actual vegetation refers to the vegetation present in the region  Differences arise because humans have substantially modified the Earth's surface from its natural state Labeling error often occurs because potential vegetation labels are used in the absence of other information Another source of error arises because of land cover change In areas undergoing rapid economic development  e g  the humid subtropics  information quickly  becomes out of