The SWING OFFSET is used for locating points close to the control lines (fig. 13-21). Measurement of a swing offset distance provides an accurate determination of the perpendicular

distance from the control line to the point being located. The swing offset is somewhat similar to the range tie (explained later), but as a rule, requires no angle measurement. To determine the swing offset distance, a chainman holds the zero mark of the tape at a corner of the structure while another chainman swings an arc with the graduated end of the tape at the transit line AB. When the shortest reading on the graduated end of the tape is observed, the swing offset or perpendicular distance to the control line is obtained at points a or b. In addition, the horizontal distances between the instrument stations (A and B) and the swing offset points (a and b) maybe measured and marked. A tie distance and angle a or f may be measured from either instrument station to the corner of the structure to serve as a check.

The method of PERPENDICULAR OFF-SETS from a control line (fig. 13-22) is similar to swing offsets. This method is more suitable than the swing offset method for locating details of irregular objects, such as stream banks and winding roads. The control line is established close to the irregular line to be located, and perpendicular offsets, aa, bb, cc, and so on, are measured to define the irregular shape. When the offset distances are short, the 90 angles are usually estimated; but when the distances are several hundred feet long, the angles should be laid off with an instrument. The distances to the offset points from a to i are measured along the control line.