The theodolite is an instrument used for measuring angles in surveying, where the angle required is indicated by the position of a moving sighting arm or alidade on a stationary scale. The more common form in the sixteenth century was the 'simple' theodolite, where there is a single circular scale for measuring horizontal angles. The more complex 'altazimuth' theodolite has scales for vertical as well as horizontal angles, the vertical scale usually being a semicircle. The altazimuth instrument would not have been called a theodolite in the period, but was given a more individual name, such as the 'topographical instrument' described by Leonard Digges.

A magnetic compass will generally be included for orientation, but the essential feature of the theodolite is that the angle is read from the position of an alidade, not of a magnetic needle. Thus the divided circle does not move during the operation, but its orientation is maintained by reference to the magnetic needle. The usual division of the circle is into degrees, but other possibilities are the winds (eight sectors with Mediterranean wind names each divided to 45 degrees), the points of the marine compass and the shadow square. Often shadow square divisions are combined with degrees.

The simple theodolite became popular in the 16th century, as an instrument for general surveying and triangulation. The altazimuth instrument was described, but little used.

Jim Bennett
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