A line must be cleared ahead when a crew is chaining (or taping) across brush-covered country. Specific tools, such as those presented in chapter 11, for the kind of job assigned must be used and handled with care. Before you start to swing, make sure that no one is within range. You may cut ordinary scrub growth in unsettled areas more or less as needed. If, however, you encounter large trees or shrubs that may be of value, you should consult your party chief for advice. Even though a tree or shrub lies directly on the chaining line, it is never absolutely necessary that it be cut down. If it is desirable that it be preserved, you can always triangulate around it or bypass it by some other method, as described in a later chapter.

The principle technical problem in clearing the line is keeping on the line. When possible, this is accomplished by the use of natural foresights; that is, by the use of bearings taken on natural objects (or, perhaps, on artificial objects) lying ahead.

Suppose there is no distinctive object lying on the line of bearing ahead. In this case, you may be able to keep on the line by BLAZING ahead. To do this, you set up the compass and sight ahead on a tree lying as far ahead as possible. You then mark this tree by blazing. (A blaze is a scar notched on a tree with a hatchet or machete.) You could also use red or white flagging as markers. You then clear a line toward the tree.

Suppose the growth is too high and thick for you to sight ahead. In this case, youll have to work ahead by looking back and aligning yourself on a couple of markers on the line already covered.

GIVING BACKSIGHTS AND FORESIGHTS

To run a line by instrument from a point of known location A to point B, for example, and given a distance and direction ahead, the instrumentman usually proceeds in the following manner:

1. Sets up the instrument (usually a transit) over point A.

2. Trains the telescope on the given direction of the line to B.

3. Sights through the telescope to keep the chainmen on line for as many consecutive foresights as can be observed from that particular instrument setup.

Suppose, for example, that the chainmen are using a 100-ft tape. After the instrument has been trained along the line of direction, the head chainman walks away with the zero-foot end of the tape, while the rear chainman holds the 100-ft end on the point plumbed by the instrument. After the head chainman has walked out the whole 100 ft, a plumb bob is dropped on a cord from the zero-foot mark to the ground.

The instrumentman sights along the line and thus determines the direction in which the head chainman must move to bring the plumb bob on to the line. The "move right" or "move left" signal is given, if needed. When the head chainman has been brought by signal to the vicinity of the line, the instrumentman signals for the final placement of the plumb bob by calling out, "To you!" (meaning "Move the plumb bob toward yourself!") or "Away!" (meaning "Move the plumb bob away from yourself!"). When the plumb bob is exactly on the line, the instrument man calls out, "Good!" or "All right!" The head chainman then marks the point indicated by the plumb bob in the correct manner. The first 100 ft have now been measured on the given line of direction.

If the distance to be measured is long, the chainmen will eventually proceed beyond the scope of the instrument as it is then set up. The instrument must then be shifted ahead to the last point marked by the head chainman. When the instrument has been set up over this point, the telescope must be reoriented to the line of direction. To do this, the instrumentman usually plunges the telescope (rotates it vertically) and backlights on a point on the line already laid off. In taking backlights, the instrumentman is guided by the rear chainman who holds on, or plumbs over, the point. When the telescope has been trained on the backsight point, it is again plunged. The telescope is now again trained in the desired direction.

If the point on the ground can be sighted through the telescope, the chainman may simply hold on the point; that is, hold a pencil point, chaining pin point, plumb bob point, or some other appropriate indicator on the point (fig. 12-4). Whatever the indicator may be, it is

essential that it be held in an exactly vertical position. For short sights, it is also essential that the shaft of the indicator be relatively slender so that the vertical cross hair can be aligned with sufficient exactness.