Sunday, October 3, 2010


A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapMODERN PRECISE THEODOLITEted for specialized purposes in fields like meteorology and rocket launch technology. A modern theodolite consists of a movable telescope mounted within two perpendicular axes — the horizontal or trunnion axis, and the vertical axis. When the telescope is pointed at a target object, the angle of each of these axes can be measured with great precision, typically to seconds of arc.

In today's theodolites, 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 MODERN PRECISE THEODOLITE 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 theodolites, costing up to $10,000 apiece, are equipped with integrated electro-optical distance measuring devices, generally infrared based, 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. This technique is called a resection solution or free station position surveying and is widely used in mapping surveying. The instruments, "intelligent" theodolites called self-registering tacheometers 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 as data collectors for this purpose.

A gyrotheodolite is used when the north-south reference bearing of the meridiaMODERN PRECISE THEODOLITEn is required in the absence of astronomical star sights. This mainly occurs in the underground mining industry and in tunnel engineering. For example, where a conduit must pass under a river, a vertical shaft on each side of the river might be connected by a horizontal tunnel. A gyrotheodolite can be operated at the surface and then again at the foot of the shafts to identify the directions needed to tunnel between the base of the two shafts. Unlike an artificial horizon or inertial navigation system, a gyrotheodolite cannot be relocated while it is operating. It must be restarted again at each site.

The gyrotheodolite comprises a normal theodolite with an attachment that contains a gyroscope mounted so as to sense rotation of the Earth and from that the alignment of the meridian. The meridian is the plane that contains both the axis of the Earth’s rotation and the observer. The intersection of the meridian plane with the horizontal MODERN PRECISE THEODOLITEcontains the true north-south geographic reference bearing required. The gyrotheodolite is usually referred to as being able to determine or find true north.

A gyrotheodolite will function at the equator and in both the northern and southern hemispheres. The meridian is undefined at the geographic poles. A gyrotheodolite cannot be used at the poles where the Earth’s axis is precisely perpendicular to the horizontal axis of the spinner, indeed it is not normally used within about 15 degrees of the pole because the east-west component of the Earth’s rotation is insufficient to obtain reliable results. When available, astronomical star sights are able to give the meridian bearing to better than one hundred times the accuracy of the gyrotheodolite. Where this extra precision is not required, the gyrotheodolite is able to produce a result quickly without the need for night observations.

Thomas Jefferson's Theodolite

This theodolite, made of brass and copper, belonged to President Thoma


s Jefferson. Jefferson, who was fascinated by the techniques of surveying, used this theodolite to determine the elevation of the Peaks of Otter in the Blue Ridge Mountains�at the

age of 72! This brass and copper theodolite, purchased by Thomas Jefferson in 1778 and preserved by the Thomas Jefferson Foundation at Monticello, was made by Jesse Ramsden. Ramsden's dividing engine revolutionized the manufacture of accurate theodolites by replacing hand scribing of theodolite circles, which was slow and prone to human error, with higher prod

uction and accuracy. Ramsden's brilliance earned him the prestigious Copley Medal, the highest award given by

the Royal Society for contributions to science. Others receiving the Copley have included Benjamin Franklin, Charles Darwin, and Albert Einstein.

W├╝rdemann Six-inch Theodolite

This six-inch theodolite was used by the Coast and Geodetic Survey in thMODERN PRECISE THEODOLITEe mid to late 19th century. With its 11-inch telescope, this instrument was tiny next to earlier theodolites.

Made by William W├╝rdemann sometime between 1849 and 1881 and designated "No. 163," this theodolite is tiny compared to Ferdinand Hassler's Great Theodolite. A thing of beauty, made of brass with silvered circles, the telescope is 11 inches long, the horizontal circle six inches in diameter, (making it a six-inch theodolite).

The vertical circle is read, with the help of a magnifier, by a single vernier. A vernier is the auxiliary scale that allows for precise reading of fractional parts of the main circle scale on a theodolite. A pair of opposite verniers read the horizontal circle, which is graduated in single minutes, to single seconds.


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