Kamis, 10 Juli 2008

Modern theodolites

Modern theodolite Nikon DTM-520
Modern theodolite Nikon DTM-520

In today's theodolite, the reading out of the horizontal and vertical circles is usually done electronically. The readout is done by a rotary encoder, which can be absolute, e.g. using Gray codes, or incremental, using equidistant light and dark radial bands. In the latter case the circles spin rapidly, reducing angle measurement to electronic measurement of time differences. Additionally, lately CCD sensors have been added to the focal plane of the telescope allowing both auto-targeting and the automated measurement of residual target offset. All this is implemented in embedded software.

Also, many modern theodolite are equipped with integrated electro-optical distance measuring devices, allowing the measurement in one go of complete three-dimensional vectors -- albeit in instrument-defined polar co-ordinates -- which can then be transformed to a pre-existing co-ordinate system in the area by means of a sufficient number of control points. The technique is called free station position surveying and is widely used in mapping surveying. The instruments, "intelligent" theodolite called self-registering tachometers or "total stations", perform the necessary operations, saving data into internal registering units, or into external data storage devices. Typically, ruggedized laptops or PDAs are used for this purpose.

Gyro theodolites

The gyro theodolite is a special type of theodolite used for surveys which do not have sky visibility (mining, underground survey). This instrument gives the orientation of true north (the direction of Earth's rotational axis), which is set as a reference for future underground observations.[4]

The gyro-theodolite is composed of a theodolite with a gyroscope on it. True north is stored in the gyroscope for future use by orienting the theodolite approximately to the north (within a few minutes of arc); this minimizes losses when the directionality is later read. A power supply activates the rotation of the gyro spinner, which maintains orientation as the theodolite is transported. To read, the theodolite is set at rest. The gyro will then oscillate through the north meridian until reaching the direction of true north; this is a point of minimum potential energy, similar to the bottom point of a pendulum. When the gyro has stabilized, the reading on the horizontal circle of the theodolite is equivalent to true north, and can therefore be used as a reference point.

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