Sometimes you need to run a straight line between nonintervisible points when events make the use of the above methods of bypassing an obstacle impractical. If there is an intermediate point on the straight line from which both of the end points can be observed, the method called BALANCING IN (also called BUCKING IN, JIGGLING IN, WIGGLING IN, or RANGING IN) may be used.

A problem often encountered in surveying is to find a point exactly on the line between two other points when neither can be occupied or when an obstruction, such as a hill lies between the two points. The point to be occupied must be located so that both of the other points are visible from it. The process of establishing the intermediate point is known as wiggling in or ranging in.

The approximate position of the line between the two points at the instrument station is first estimated by using two range poles. The range poles are lined in alternately in the following manner. In figure 13-19, view A, set range pole 1 and move range pole 2 until it is exactly on line between pole 1 and point A. You do this by

sighting along the edge of pole 1 at the station A until pole 2 seems to be on line. Set range pole 2 and move pole 1 until it is on line between pole 2 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 poles and both stations from either viewing position. After finding the approximate position of the line between the two points, you set up the instrument on this line. The instrument probably will not be exactly on line, but will be over a point, such as B, (see fig. 13-19, view B). With the instrument at B, you backsight on A and plunge the telescope and notice where the line of sight C passes the point C. Estimate this distance CC and also the distance that B would be away from C and A. Estimate the amount to move the instrument to place it on the line you need. Thus, if B is midway between A and C, and C misses C by 3 feet to the left, B must be moved about 1.5 feet to the right to reach B. Continue the sequence of backlighting, plunging the telescope, and moving the instrument until the line of sight

passes through both A and C. When you do this, the telescope is reversed, but the instrument is not rotated. This means that if the telescope is reversed for backlighting on A, all sightings on A are made with the telescope reversed. Mark a point on the ground directly under the instrument. Then, you continue to use this method with the telescope direct for each backsight on A. Mark a second point on the ground. The point you need on the line AC is then the midpoint between the two marked points.

The method outlined above is usually time consuming. Even though the shifting head of the instrument is used in the final instrument movements, you may have to pick up and move the instrument several times. The following method often saves time. After finding the approximate position of the line between the two points, you mark two points B and B, (fig. 13-19, view C), 1 or 2 feet apart where you know they straddle the line AC. Set up over each of these two points in turn and measure the deflection angles a and b. Also measure the horizontal distance a, between points B and B. Then you can find the position B on the line AC by using the following equation:

in which a is the proportionate offset distance from B toward B for the required point B, and a and b are both expressed in minutes or in seconds.