Currently, SWD is measured with electronic sensors and a variety of types are available.  Since no measurement is complete unless it includes an estimate (necessarily in statistical terms) of the probable magnitude of the uncertainty (Anon 1983), the uncertainty of SWD measurement also needs to be described. In the measurement of SWD, the total uncertainty can be written as the sum of the uncertainties due to precision, accuracy, protocol, and scale (Russo et al. 1993).   Consider an archer shooting arrows at a target. The closeness of the arrows together is the precsion, the closeness of the arrows to the bull's eye is the accuracy.  Protocol refers to how the two different archers shoot the arrows, for instance one may stand and the other may kneel. Scale referes to uncertanties due  to spatial or temporal scale for example one archer uses arrows that have larger tips or if two targest were of different areas.  Information on all four of these areas have been compiled for SWD sensors.
 

Precision
Accuracy
Protocol
Scale

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