For example, granite, and the sedimentary rock called arkose, derived from it, have similar spectral properties. The maps we use as the standards are largely extrapolations, or more correctly, abstractions. But, the sensor, whether or not it can resolve them, sees all. When quantifying accuracy, we must adjust for the lack of equivalence and totality, if possible. Another, often overlooked point about maps as reference standards, concerns their intrinsic or absolute accuracy.
Maps require an independent frame of reference to establish their own validity.
For centuries, most maps were constructed without regard to assessment of their inherent accuracy. In recent years, some maps come with a statement of confidence level. As the size of crop fields increases, the error decreases further. After about four bands, the accuracy increase flattens or increases very slowly. Note that the highest accuracy associates with crop types because fields, consisting of regularly-space rows of plants against a background of soil, tend to be more uniform .