Lateral measurements to chain lines for locating ground features are known as offsets. For this purpose perpendicular or oblique offsets may be taken . If the object to be located (say road) is curved more number of offsets should be taken. For measuring offsets tapes are commonly used.
For setting perpendicular offsets any one of the following methods are used:
(i) Swinging
(ii) Using cross staffs
(iii) Using optical or prism square.
Chain is stretched along the survey line. An assistant holds the end of tape on the object. Surveyor swings the tape on chain line and selects the point on chain where offset distance is the least and notes chain reading as well as offset reading in a field book on a neat sketch of the object.
Perpendicular Offsets Using Optical Square and Prism Square
These instruments are based on the optical principle that if two mirrors are at angle ‘θ’ to each other, they reflect a ray at angle ‘2θ’.
Optical square consists of a metal box about 50 mm in diameter and 125 mm deep. In the rim of the box there are three openings:
(i) a pinhole at E
(ii) a small rectangular slot at G, and
(iii) a large rectangular slot at F.
A and B are the two mirrors placed at 45º to each other. Hence the image of an object at F which falls on A gets reflected and emerge at E which is at right angles to the line FA. The mirror A which is opposite to the opening at F is fully silvered. It is fitted to a frame which is attached to the bottom plate. If necessary this mirror can be adjusted by inserting a key on the top of the cover. The mirror B which is in the line with EG is silvered in the top half and plain in the bottom half. It is firmly attached to the
bottom plate of the box.
The ranging rod at Q is directly sighted by eye at E in the bottom half of the B which is a plain glass. At the same time in the top half of B, the reflected ray of the object at P is sighted. When the image of P is in the same vertical line as the object at Q, then the lines PA is at right angles to the line EB. This instrument can be used for finding foot of the perpendicular or to set a right angle. In prism square, instead of two mirrors at 45º to each other a prism which has two faces at 45º to each other is used Its advantage is it will not go out of adjustment even after long usage.
