Abstract
In an ideal world (for remote sensing) the nature of the surface would be completely specified by the spectral signature. In the real world, however, the compexity of natural surfaces, effects of the atmosphere and ambiguity of the spectral signatures act to limit remote sensing without ground truth to applications that demand little from the radiometric quantities in the data. Ground surveys are complementary to the synoptic overview provided by satellites, helping to link the image data to the surface context. This paper reviews the purposes and problems of such ground surveys and examines in particular the nature of the relationship between the object of inquiry and the spectral signature. The investigator must select object variables that are both appropriate to the application and well matched to the spectral signature. This problem is discussed in the context of vegetation canopies.

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