Gyrotheodolite

It is the main instrument for orientation in mine surveying[1] and in tunnel engineering, where astronomical star sights are not visible and GPS does not work.

[4] A gyroscope is mounted in a sphere, lined with Mu-metal to reduce magnetic influence, connected by a spindle to the vertical axis of the theodolite.

These errors can be moderated by refining the initial estimate of the meridian to within a few arc minutes and correctly aligning the zero torque of the suspension.

In practice the amplitude of oscillation will only gradually reduce as energy is lost due to the minimal damping present.

This might be determined with a magnetic compass, from an existing survey network or by the use of the gyro-theodolite in an extended tracking mode.

Gyro-theodolites are not normally used within about 15 degrees of the pole where the angle between the earth's rotation and the direction of gravity is too small for it to work reliably.

When available, astronomical star sights are able to give the meridian bearing to better than one hundred times [citation needed] the accuracy of the gyro-theodolite.