The height reference unit is a functional plug-in subassembly or the guti direction computers (see 1.1) intended to receive a single, rotary, manual input or height by means of a mechanical gearing actuated by a rotary knob. The inpu is then converted to two electrical outputs: one designated REFERENCE, corresponding to the difference between the height component of origin coordinates and the height component or reference coordinates; the other signal designated REPLOT, corresponding to the differences between the height component of origin coordinates and the height component or the coordinates of a point being replotted.