This method is used when a large obstruction, such as a building, is in the line of sight between two points. The solution establishes a line parallel to the original line at a distance clear from the obstacle, as shown in figure 13-18. The instrument

is set up at B, and a 90 angle is turned from line AB. The distance BB is carefully measured and recorded. The instrument is moved to B, and another 90 angle is turned. BC is laid off to clear the obstacle. The instrument is moved to C, and a third 90 angle is turned. Distance CC, equal to BB, is measured and marked. This establishes a point C on the original line. The instrument is moved to C, and a fourth 90 angle is turned to establish the alignment CD that is the extension of AB beyond the obstacle.

When the distance to clear the obstacle, BB or CC, is less than a tape length, you can avoid turning four 90 angles as follows: Erect perpendicular offsets from points A and B in figure 13-18 so that AA equals BB. Set up the instrument at B, and measure angle ABB to be sure that its 90. Extend line AB to C and then to D, making sure that point C clears the obstacle. Then, lay off perpendicular offset CC equal to AA or BB and perpendicular offset DD equal to CC. Then, line CD is the extension of line AB. The total distance of the line AD is the sum of the distances AB, BC, and CD.

You also compute the diagonals formed by the end rectangles and compare the result to the actual measurement, if you can, as a further check.