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Chaining on Level Ground When taping distances on a relatively level surface and of the third or lower order accuracy, you may lay the tape on smooth ground or on a paved road or support its ends by taping stools or stakes. In horizontal chaining, the tape is held horizontally, and the positions of the pertinent graduations are projected to the ground by a plumb bob and cord. For ordinary chaining on level ground, the following procedures are generally used: 1. A range pole is set on line slightly behind the point toward which the taping will proceed. The rear chainman, with one chaining pin, stations himself at the starting point of the line to be measured. 2. The head chainman, holding the zero end of the tape and with 10 pins in his hand, then moves forward toward the distant point while guiding himself with the range pole. Assuming that the tape was already off the reel when they started, the rear chainman watches the tai1 end (100-ft mark) of the tape as the head chainman moves forward. 3. When the rear chainman sees that the tail end is about to reach his position, he calls "Chain!" At that time, the head chainman stops and looks back. The rear chainman holds the 100-ft mark at the starting point and checks the alignment; then signals the way the head chainman should move the chaining pin to be in line. While doing this, they are both in a kneeling position, the rear chainman facing the distant point, and the head chainman to one side facing the line so that the rear chainman has a clear view of the range pole. The head chainman, while stretching the tape with one hand, sets the pin vertically on line a short distance past the zero mark with the other hand. Then by pulling the tape taut and making sure that the tape is straight, the head chainman brings it in contact with the pin, The rear chainman, watching carefully for the 100-ft mark to be exactly on the point, calls "All right! The head chainman relocates the pin to exactly the zero mark of the tape and places it sloping away from the line. He then pulls on the tape again to make sure that the zero mark really matches the point where the pin is stuck in the ground. Then, he calls "All right!" or "Stuck!" This is a signal to the rear chainman to release the tape so he can continue forward for the next measurement. The process is repeated until the entire distance is measured. 4. As the rear chainman moves forward, he pulls the pin from his point. Thus, there is always one pin stuck in the ground; therefore, the number of pins in his possession at any time indicates the number of 100-ft (stations) tape lengths they have measured from the starting point to the pin in the ground. Every time the head chainman runs out of pins, he signals the rear chainman to come forward, and both of them count the pins in the rear chainmans possession. There should be 10 pins. SUPPORTING THE TAPE. When a full tape length is being measured, the two chainmen support the ends of the tape. The tape maybe laid on a level ground surface, such as a paved road or railroad rail, or suspended between stools or bucks set under the ends of the tape. For precise measurement, such as base line measurement, the tape is supported at midpoint or even at quarter points by bucks or stakes.In horizontal taping over sloping or irregular terrain, one end of the tape is held on the point at ground level, while the other end is supported high enough to make the tape horizontal. As shown in figure 12-12, the rear chainman is holding a full graduation of the tape at the point near the ground, and the head chainman, holding the zero end, projects the desired distance to the point on the ground by using the plumb bob. ALIGNING THE TAPE. Any misalignment of the tape, either horizontally or vertically, will result in an error in the measurement. Misalignment always results in a recorded distance that is too great, or a laid offline that is too short. This is obvious, since the shortest distance between two points is a straight line. Keep the tape straight and level at all times.APPLYING TENSION. A tape supported or held only at the ends will hang in the shape of a curve, called a catenary, because of its own weight. Depending on the tension or pull applied at the ends, this catenary will become shallower or deeper; and the distance between the supported ends will vary considerably. To standardize this distance, you should apply a recommended "standard" tension when you are measuring. You should attach a spring balance or tension handle to one end of the tape and measure the correct standard tension. The amount of standard tension is discussed later under "Making Tape Corrections."Maintaining a constant tension for any length of time by a hand pull is uncomfortable and can be erratic. For easier chaining, each chainman uses a pole or rod about 1/2 to 2 in. in diameter and about 6 ft long. The leather thong attached to the tension handle is wrapped around the pole at the proper height. The chainman braces the bottom end of the pole against the outside of his foot and applies tension by bracing his shoulder against the
Figure 12-12.-Horizontal taping on a slope. pole and shifting his body weight until the correct tension is read on the scale. This position can be held steadily and comfortably for a comparatively long time. Measuring distances less than a full tape length requires the use of the clamp handle (or "scissors clamp"), which is attached to the tape at some convenient point along its length. The handle permits a firm hold on the tape and furnishes a convenient attachment for a spring balance. When properly used, the handle will prevent kinking of the tape. |
 
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