For precise levelling the GPM3 is fitted onto the NA2 or NAK2 telescope objective and locked in position with the knob.
By turning the micrometer knob the parallel-sided glass plate is tilted, thereby causing the line of sight to be raised or lowered. The range of displacement is 10mm.
On the scale, seen in the GPM3's eyepiece, the amount of displacement is read directly to 0.1mm and estimated to 0.01mm.
10”/2mm tubular level accuracy and with a double run with PPM 0.3mm per 1km.