North American Datum 1983 and WGS84
The initial definition of NAD83(1986) was intended to match GRS80 and WGS84, and many older publications indicate no difference. Subsequent more accurate measurements found a difference typically on the order of a meter over much of the US. Each datum has undergone refinements with more accurate and later measurements.
One fundamental difference is that NAD83 is defined to remain essentially constant over time for points on the North American Plate, whereas WGS84 is defined with respect to the average of stations all over the world. Thus there is a change over time as to the difference between the systems. For much of the US the relative rate is on the order of 1 to 2 cm per year.
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