is set up at B, and a 90° angle is turned from lineAB. The distance BB´ is carefully measured andrecorded. The instrument is moved to B´, andanother 90° angle is turned. B´C´ is laid off toclear the obstacle. The instrument is moved to C,and a third 90° angle is turned. Distance CC´,equal to BB´, is measured and marked. Thisestablishes a point C on the original line. Theinstrument is moved to C, and a fourth 90° angleis turned to establish the alignment CD that is theextension of AB beyond the obstacle.When the distance to clear the obstacle, BB´or CC´, is less than a tape length, you can avoidturning four 90° angles as follows: Erect perpen-dicular offsets from points A and B in figure 13-18so that AA´ equals BB´. Set up the instrument atB’, and measure angle A´B´B to be sure that it’s90°. Extend line A´B´ to C´ and then to D´,making sure that point C clears the obstacle.Then, lay off perpendicular offset C´C equal toAA´ or BB´ and perpendicular offset D´D equalto C´C. Then, line CD is the extension of line AB.The total distance of the line AD is the sum ofthe distances AB, B´C´, and CD.You also compute the diagonals formed by theend rectangles and compare the result to theactual measurement, if you can, as a furthercheck.Line Between Nonintervisible PointsSometimes you need to run a straight linebetween nonintervisible points when events makethe use of the above methods of bypassing anobstacle impractical. If there is an intermediatepoint on the straight line from which both of theend points can be observed, the method calledBALANCING IN (also called BUCKING IN,JIGGLING IN, WIGGLING IN, or RANGINGIN) may be used.A problem often encountered in surveying isto find a point exactly on the line between twoother points when neither can be occupied or whenan obstruction, such as a hill lies between the twopoints. The point to be occupied must be locatedso that both of the other points are visible fromit. The process of establishing the intermediatepoint is known as wiggling in or ranging in.The approximate position of the line betweenthe two points at the instrument station is firstestimated by using two range poles. The rangepoles are lined in alternately in the followingmanner. In figure 13-19, view A, set range pole1 and move range pole 2 until it is exactly on linebetween pole 1 and point A. You do this byFigure 13-19.-Setting up on a line between two points.sighting along the edge of pole 1 at the stationA until pole 2 seems to be on line. Set range pole2 and move pole 1 until it is on line between pole2 and point C. Now, move pole 2 into line again,then pole 1, alternately, until both are on line AC.The line will appear to pass through both polesand both stations from either viewing position.After finding the approximate position of theline between the two points, you set up theinstrument on this line. The instrument probablywill not be exactly on line, but will be over a point,such as B´, (see fig. 13-19, view B). With the in-strument at B´, you backsight on A and plungethe telescope and notice where the line of sightC passes the point C. Estimate this distance CC´and also the distance that B´ would be away fromC and A. Estimate the amount to move theinstrument to place it on the line you need. Thus,if B´ is midway between A and C, and C´ missesC by 3 feet to the left, B´ must be moved about1.5 feet to the right to reach B. Continue thesequence of backlighting, plunging the telescope,and moving the instrument until the line of sight13-16