Surveying and drafting instruments

Tapes. Steel tapes are commonly 25, 50, 75, 100, 200. 300 or 500 ft. long to 100 ft.. They are usually thin steel ribbons, the longer tapes being thicker and narrower to lessen liability of kinking and to stand rougher usage. The lighter tapes are graduated throughout in feet, tenths and hundredths, the heavier ones are usually marked by notched sleeves, etchings or brass rivets at every 1-, 5-, 10-, or 20-ft. point with the last foot on each end divided into tenths or hundredths. When 5, 10 or 20-ft. points alone are marked the last 5, 10, or 20-ft is usually graduated at every foot.

Steel tapes fitted with a thermometer for temperature corrections and with a spring balance for measuring the amount of pull are used chiefly in city surveying. Broken tapes can be mended by riveting on the back of a tape a piece of the old tape of the same width.

Pocket steel tapes from 3 ft. up are for accurate measuring when long tapes, marked only at the 5, 10, or 20-ft. points are used.

Levelling rods. Levelling rods are of two types, target and self-reading; the former is read by a rodman, the latter by a levelmen. The commonest forms of target rods are the Boston, the New York and the Philadelphia; the Philadelphia can be used also as a self-reading rod. Most rods are made of two or more sections, so that they may be extended. The target of the Boston rod is permanently fastened to one strip of the rod. The other target rods are always used right-side up and have movable targets. The New York rod has a vernier for reading to a thousandth of a foot; most Philadelphia rods have a scale resembling a vernier, which gives the hundredths and makes it easy to estimate thousandths, thus serving every purpose of a vernier. All these rods are usually 6 or 7.5 ft. long and are extensible for readings up to 11 or 13 ft. They are also made 3 ft. long, with extension to 5 ft. and 5 ft. long with extension to 9 ft. for use underground.

A self-reading rod has graduations (usually every tenth of a foot) painted on its face, so that it can be read through a telescope by the levelman. The figures on the face of the rod are usually of a definite height (0.06 or 0.08 ft.) and of a thickness of 0.01 or 0.02 ft., thus aiding the levelman to estimate hundredths of a foot or even closer.

Engineer's level is a telescope to which a delicate spirit level is attached parallel to the line of sight, so that when the bubble is in the centre, the line of sight is horizontal. The two common types are the Y- and dumpy-levels. In both the telescope is mounted on a vertical axis about which the telescope can swing horizontally and is levelled by 4 levelling screws.

The Y-level has its spirit level attached to the telescope which rests in two Y-shaped supports fastened to a horizontal bar rigidly connected to a vertical axis.The telescope can be taken out of the Ys, turned end for end.

The dumpy-level has its vertical axis, a horizontal bar and supports for the telescope all cast in one piece to which a spirit level is attached. The dumpy-level will stand much rougher use than Y-level.

Дата добавления: 2015-07-19