STADIA (AIL. stadia, station, from Lat. stadium, from Gk. ard&ov, standard of length, furlong). An attachment fitted to the telescope of a transit and used in surveying to measure dis tances. Stadia surveying is a modification of transit surveying, and is effected by the use of the stadia or gradienter attachment. The tele scope of the ordinary transit (see ENGINEERING INSTRUMENTS) contains a vertical and a hori zontal wire dividing the field of vision into four quadrants; but the stadia telescope contains ex tra horizontal wires mounted on an independent diaphragm, so that the distances between the re spective wires are adjustable. The principle of its operation is simple. If the space between two auxiliary wires, one on each side of the hori zontal wire of the telescope, is so adjusted as to subtend a foot on a rod at a distance of 100, the space will subtend two feet on a rod at two hun dred feet, and so on. Thus by proper adjustment the approximate distances to various near-by points can be calculated at once by reading the rod. If from any convenient point in a field the corners are all visible, and the stadia instrument be set at this point, the distance to each corner can at one be read and also the angles between these lines. Thus with two sides and the in
cluded angle of each triangle, the areas of the different parts and that of the whole field can be calculated. This plan is known as the method of radiation. The gradienter attachment is a micrometer screw which takes the place of the tangent screw to the axis of the telescope. A mounted scale registers the number of turns of a micrometer screw. The value of the thread is usually such that one revolution of the screw moves the horizontal wire of the telescope over a space of one foot on a rod at the distance of a hundred feet. If the micrometer screw is divided into one hundred spaces, it is clear that when the screw is turned through 25, 50, or 75 spaces, or 1, or - of a space on the scale, the wire of the telescope will pass over i 1, or of a foot on the rod, and so on. See SURVEYING.