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Tapes are used in surveying to measure horizontal, vertical, and slope distances. They may be made of a ribbon or a band of steel, an alloy of steel, cloth reinforced with metal, or synthetic materials. Tapes are issued in various lengths and widths and graduated in a variety of ways.

Metallic Tapes

A metallic tape is made of high-grade synthetic material with strong metallic. strands (bronze-brass- copper wire) woven in the warped face of the tape and coated with a tough plastic for

Figure 11-35.-Pick.

durability. Standard lengths are 50 and 100 ft. Some are graduated in feet and inches to the nearest one-fourth in. Others are graduated in feet and decimals of a foot to the nearest 0.05 ft. Metallic tapes are generally used for rough measurements, such as cross-sectional work, road-work slope staking, side shots in topographic surveys, and many others in the same category. Nonmetallic tapes woven from synthetic yarn, such as nylon, and coated with plastic are available; some surveyors prefer to use tapes of this type. Nonmetallic tapes are of special value to power and utility field personnel, especially when they are working in the vicinity of high-voltage circuits.

Steel Tapes

For direct linear measurements of ordinary or more accurate precision, a steel tape is required. The most commonly used length is 100 ft, but tapes are also available in 50-, 200-, 300-, and 500-ft lengths. All tapes except the 500-ft one are band-types, the common band widths being 1/4 and 5/16 in. The 500-ft tape is usually a flat-wire type.

Most steel tapes are graduated in feet and decimals of feet, but some are graduated in feet and inches, meters, Gunters links, and chains or other linear units. From now on, when we discuss a tape, we will be talking about one that is graduated in feet and decimals of a foot unless we state otherwise.

Some tapes called engineers or direct reading tapes are graduated throughout in subdivisions of each foot. The tape most commonly used, however, is the so-called chain tape, on which only the first foot at the zero end of the tape is graduated in subdivisions, the main body of the tape being graduated only at every 1-ft mark. A steel tape is sometimes equipped with a reel on which the tape can be wound. A tape can be, and often is, detached from the reel, however, for more convenient use in taping.

Various types of surveying tapes are shown in figure 11-36. View A shows a metallic tape; view B, a steel tape on an open reel; view C, a steel tape or, a closed reel. View D shows a special type of low-expansion steel tape used in high-order work; it is generally called an Invar tape or Lovar tape.

Invar Tapes

Nickel-steel alloy tapes, known as Invar, Nilvar, or Lovar, have a coefficient of thermal

Figure 11-36.-Surveying tapes.

expansion of about one-tenth to one-thirtieth (as low as 0.0000002 per 10 F) that of steel. These tapes are used primarily in high-precision taping. These tapes must be handled in exactly the same manner as other precise surveying instruments. The alloy metal is relatively soft and can be easily broken or kinked if mishandled. Ordinarily, Invar tapes should not be used when a steel tape can give the desired accuracy under the same operating conditions. Invar tapes are used for very precise measurements, such as those for base lines and in city work. When not in use, the tape should be stored in a reel, as shown in figure 11-36, view D. Except for special locations where the ground surface is hard and flat, such as roadways or railroads beds, the Invar tape is used over special supports or stools and is not permitted to touch the ground.

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