The most common types of taping errors are discussed in the subsections that follow. They stem from instrumental, natural, and personal sources. Some types produce systematic errors, others produce random errors.
Incorrect Length of Tape
Incorrect length of a tape can be one of the most important errors. It is systematic. Tape manufacturers do not guarantee steel tapes to be exactly their graduated nominal length—for example, 100.00 ft—nor do they provide a standardization certificate unless requested and paid for as an extra.The true length is obtained by comparing it with a standard tape or distance. The National Institute of Standards and Technology (NIST)1 of the U.S. Department of Commerce will make such a comparison and certify the exact distance between end graduations under given conditions of temperature, tension, and manner of support. A 100-ft steel tape usually is standardized for each of the two sets of conditions—for example, 68°F, a 12-lb pull, with the tape lying on a flat surface (fully supported throughout); and 68°F, a 20-lb pull, with the tape supported at the ends only. Schools and surveying offices often have a precisely measured 100-ft line or at least one standardized tape that is used only to check other tapes subjected to wear.
An error,caused by incorrect length of a tape,occurs each time the tape is used. If the true length, known by standardization, is not exactly equal to its nominal value of 100.00 ft recorded for every full length, the correction can be determined as where CL is the correction to be applied to the measured (recorded) length of a line to obtain the true length,l the actual tape length, the nominal tape length, and L the measured (recorded) length of line. Units for the terms in Equation (6.3) can be in either feet or meters.
Temperature Other Than Standard
Steel tapes are standardized for 68°F (20°C) in the United States. A temperature higher or lower than this value causes a change in length that must be considered. The coefficient of thermal expansion and contraction of steel used in ordinary tapes is approximately 0.00000645 per unit length per degree Fahrenheit, and 0.0000116 per unit length per degree Celsius. For any tape, the correction for temperature can be computed as where CT is the correction in the length of a line caused by nonstandard temperature, k the coefficient of thermal expansion and contraction of the tape, T1 the tape temperature at the time of measurement, T the tape temperature when it has standard length, and L the observed (recorded) length of line. The correction CT will have the same units as L, which can be either feet or meters. Errors caused by temperature change may be practically eliminated by either (a) measuring temperature and making corrections according to Equation (6.4) or (b) using an Invar tape.
Errors caused by temperature changes are systematic and have the same sign if the temperature is always above 68°F, or always below that standard. When the temperature is above 68°F during part of the time occupied in measuring a long line, and below 68°F for the remainder of the time, the errors tend to partially balance each other, but corrections should still be computed and applied.
Temperature effects are difficult to assess in taping. The air temperature read from a thermometer may be quite different from that of the tape to which it is attached. Sunshine, shade, wind, evaporation from a wet tape, and other conditions make the tape temperature uncertain. Field experiments prove that temperatures on the ground or in the grass may be 10 to 25° higher or lower than those at shoulder height because of a 6-in. “layer of weather” (microclimate) on top of the ground. Since a temperature difference of 15°F produces a change of 0.01 ft per 100 ft tape length, the importance of such large variations is obvious.
Shop measurements made with steel scales and other devices likewise are subject to temperature effects. The precision required in fabricating a large airplane or ship can be lost by this one cause alone.
When a steel tape is pulled with a tension greater than its standard pull (the tension at which it was calibrated), the tape will stretch and become longer than its standard length. Conversely, if less than standard pull is used, the tape will be shorter than its standard length. The modulus of elasticity of the tape regulates the amount that it stretches. The correction for pull can be computed and applied using the following formula where CP is the total elongation in tape length due to pull, in feet; P1 the pull applied to the tape at the time of the observation, in pounds; P the standard pull for the tape, in pounds; A the cross-sectional area of the tape, in square inches; E the modulus of elasticity of steel, in pounds per square inch; and L the observed (recorded) length of line. An average value of E is for the kind of steel typically used in tapes. In the metric system, to produce the correction CP in meters, comparable units of P and P1 are kilograms, L is meters, A is square centimeters, and E is kilograms per square centimeter. An average value of E for steel in these units is approximately 2,000,000 kg/cm2. The cross-sectional area of a steel tape can be obtained from the manufacturer, by measuring its width and thickness with calipers, or by dividing the total tape weight by the product of its length (in feet) times the unit weight of steel (490 lb/ft2), and multiplying by 144 to convert square feet to square inches.
Errors resulting from incorrect tension can be eliminated by (a) using a spring balance to measure and maintain the standard pull or (b) applying a pull other than standard and making corrections for the deviation from standard according to Equation.
Errors caused by incorrect pull may be either systematic or random. The pull applied by even an experienced tapeperson is sometimes greater or less than the desired value. An inexperienced person, particularly one who has not used a spring balance on a tape, is likely to apply less than the standard tension consistently.