Underground there are no star sights and no satellite signal, so the bearing that controls the direction of development has to be derived another way. A gyro-theodolite does this by sensing the horizontal component of the Earth's rotation: a fast-spinning rotor seeks the meridian, and the instrument resolves the direction of true north to within a few seconds of arc — independent of terrain, weather or magnetic interference.
That azimuth is what keeps development on course. When two ends of a tunnel, or a holing between shafts, have to meet, the gyro bearing established at surface and again underground ties the two survey frames together so headings drive toward each other correctly and safely. Doing this the correct way minimises the risk of mis-holings — which cost money and, worse, can cost lives.
Survey House Group runs gyro observations as part of underground control: establishing and checking primary bearings, orienting shaft-bottom survey networks after coordinate transfer, and carrying surface gyro bearings forward into the workings. A gyro determines true north, so grid convergence must be applied to obtain a grid bearing — a correction we apply rigorously, because overlooking it is a common and costly error.
The instrument is set over a station and observes the horizontal component of the earth’s rotation to derive true north, then the bearing to a reference target is measured from it. Instrument constant is determined on a known azimuth before and after underground work.
Accuracy is quoted in arc-seconds and depends on instrument specification, observation procedure, latitude and setup stability. It degrades toward the poles; Southern African latitudes are favourable.
A gyro observation gives direction, not position. It will reveal that a traverse has drifted in bearing, but correcting the resulting positional error requires the survey network to be recomputed.
