502.6 says "Signs shall be 60 inches (1525 mm) minimum above the finish floor or ground surface measured to the bottom of the sign." 

But which floor or ground surface?  The parking surface or the surface directly under the sign, if different (such as a curb or landscaping)? 

The only way for uniformity would be from the parking surface, right?  And the 1991 Standards referred also seemed to refer to the parking surface, in a way:  "Such signs shall be located so they cannot be obscured by a vehicle parked in the space."

FYI, ANSI 502.7 says "signs shall be 60 inches (1525 mm) minimum above the floor of the parking space, measured to the bottom of the sign."