A guidance control system is configured to control the positioning and spatial orientation of a digging implement mounted on a frame of an trenching machine for working a subsurface of earth to a desired trench profile. The position of a dynamic cutting edge of the digging implement is monitored and then controlled so that the sensed dynamic cutting edge position is equal substantially to the calculated dynamic cutting edge position. The guidance control system includes sensors, a processor, and accessible memory providing digital design information regarding the desired trench profile.