Theodolite
Collier's New Encyclopedia (1921)
a most important instrument for measuring horizontal and vertical angles, but particularly adapted for accurately measuring the former. Its principle is identical with that of the altitude and azimuth instrument; the construction and purpose of the two, however, differ, the latter being employed for astronomical purposes, while the theodolite is used for land surveying; but the better instruments of this class may be employed for observing the altitude of celestial bodies. The vertical circle is not generally, however, of sufficient size, nor so graduated as to be available for very accurate astronomical observations. When a point is to be viewed with the telescope, the telescope is moved so that the image of the point coincides with the intersection of the cross wires. The vertical limb is divided into degrees, and is capable of being read by means of the vernier and the microscope to thirds of a minute. A pair of plates, constituting THEODOLITE at their edge the horizontal limb of the iustrument, are free when unclamped, to move independently of each other. The plate carries a magnetic compass and two spirit levels at right angles to each other, by means of which the circle may be brought accurately into the horizontal plane by raising or depressing it by means of the screws. The plate is furnished with two verniers, diametrically opposite to each other, the degrees marked on which are read off by the microscope. By the motion of the telescope, on the horizontal axis of the vertical limb, altitudes and vertical angles can be measured, while, by its motion on the vertical axis, the angular distances between two objects can be ascertained by the readings on the horizontal circle. Before using a theodolite, it should be properly adjusted; that is, the different parts should be brought to their proper relative positions. The theodolite is in adjustment when the following conditions are fulfilled: (1) When the intersection of the cross wires is in the axis of the telescope. (2) When the axis of the attached level is parallel to the axis of the telescope. (3) When the axes of the levels on the horizontal limb are perpendicular to the axis of the horizontal limb. (4) When the axis of the vertical limb is perpendicular to the axis of the horizontal limb.
What Has Changed in the Last Century
Collier's described the theodolite of 1921 as a brass circle read by vernier and microscope "to thirds of a minute," with a magnetic compass and spirit levels to be trued up by hand before every job. Within a few years compact glass-circle optical theodolites had replaced the open verniers, and after the invention of electronic distance measurement in the late 1940s the instrument merged with a rangefinder to become the total station, which from the 1970s recorded angles and distances digitally instead of in a field book. Today a surveyor is as likely to use a robotic total station that tracks a prism unattended, or satellite GNSS positioning, as to peer through an eyepiece. The four adjustment conditions Collier's lists are still taught, and the optical principle is unchanged — what has gone is the manual reading, the levelling screws worked by hand, and the compass needle.
Written for this edition, 2026.