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Figure 12.2. Gyrothedolite
The instrument pictured above is typically used by the military.
Their principal application is in mining, tunnelling and gunnery, but in
conventional surface surveys they can be used to provide initial
orientation, in place of magnetic bearings or astronomical azimuths.
It basically consists of a gyroscope attached to the bottom of a
Hilger & Watts micrometer theodolite. The gyroscope enables the
direction of true north to be determined with a standard error of about
±15" within a very short time. A very important component of the
gyrotheodolite is a 'meridian indicator' (also known as a Precision
Indicator of the Meridian) which is connected to the gyroscope and
external connection to the power supply. It is basically a meter
graduated directly in angular units, and the reading is thus a maximum
when the gyro is pointing north and a minimum when it is pointing
east or west.
The theodolite remains permanently attached to the gyro
assembly. The gyroscope unit is suspended in vertical gimbals, which
lie in the line defined by the downward extension of the (permanently)
attached theodolite. The actual gyroscope is floated in liquid, and it is
called a floating gyroscope to distinguish it from the pendulous type in
which the gyroscope unit is suspended and hangs under gravity.
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