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Making Tape Corrections A 100-ft tape should, in theory, indicate exactly 100.00 ft when it is in fact measuring 100.00 ft. However, a tape supported only at the ends has a sag in it, so when it indicates 100.00 ft, actually the distance measured is less. Even a tape supported throughout on a flat surface can be slightly longer under tension than it is without tension. Also, a tape will be longer when it is warm than when it is cold. CALIBRATING A TAPE. All tapes are graduated under controlled conditions of tempera-ture and tension. When they are taken to the field, these conditions change. The tape, regardless of the material used to make it, will be either too short or too long. For low accuracy surveys, the amount of error is too small to be considered. As accuracy requirements increase, variations caused by the temperature and sag must be computed and used to correct the measured distance. In the higher orders of accuracy, the original graduation is checked for accuracy or calibrated at intervals against a standard distance. This standard is usually two points, a tape length apart, that have been set and marked using a more precise tape or a tape already checked. The standard may be just the precise or checked tape (known as the king or master tape). This tape is kept in a safe location and is not used for making field measurements, but only to check the accuracy of the field tapes.For the highest orders of accuracy, the tapes are sent to the National Bureau of Standards, U.S. Department of Commerce, Washington, DC, 20234, for standardization under exact conditions of tension, temperature, and points of support. A tape standardization certificate is issued for each tape, showing the amount of error under the different support conditions and the coefficient of expansion. The certificate (or a copy) is kept with each tape. For field operations, the tapes are combined in sets; one is selected as the king tape, while the others are used as field tapes. The standard tension for a tape supported throughout is 10 lb, and the standard temperature is 68F. Standard length is, simply, the nominal length of the tape. A 100-ft tape, for example, at a temperature of 68F, supported throughout, and subject to a tension of 10 lb, should indicate 100 ft when it is measuring exactly 100 ft. To CALIBRATE a 100-ft tape means to determine the exact distance it is actually measuring when it indicates 100 ft, while being supported throughout, at a temperature of 68F and under a tension of 10 lb. In addition to the National Bureau of Standards, many state and municipal authorities provide standardizing service. RECOGNIZING TAPE OR STANDARD ERROR. Suppose now that you send a 100-ft tape to the Bureau of Standards to be calibrated; the bureau will return a certificate with the tape. Assume that the certificate states that when the tape, supported throughout at a temperature of 68F, and under a tension of 10 lb, indicates 100 ft, it actually measures 100.003 ft on the standard tape. The tape, then, has a STANDARD ERROR (also called TAPE ERROR) of 0.003 ft for every 100 ft it measures. This tape "reads short." Depending on the order of precision of the survey, you may have to apply this as a correction to measurements made with this particular tape. CORRECTING FOR STANDARD ERROR. Whether you add or subtract the standard error depends upon the direction of the error. The tape in the above example indicates a distance that is shorter than it actually measures; in other words, when you use this tape to lay off a distance of 100 ft, the line is actually 100.003 ft. The decision to add or subtract the error depends upon whether you are measuring to determine the distance between two points or to set a point at a given distance from another. Assume first that youre measuring the distance between two given points, and the distance as indicated by the tape is 362.73 ft. First, what is the total tape error? Obviously, it is 0.003 times the number of tape lengths. In this case, it is 0.003 x 3.6273 = 0.0108819 ft, which rounds off to 0.01 ft. The next question is: Do you add this total correction to, or subtract it from, the recorded distance of 362.73 ft? Well, if you remember that the tape reads short, you will realize the reasonable thing to do is ADD the total standard error to the recorded distance. The correct distance between the two points, then, is 362.74 ft. Suppose now that with the same tape, you are to set a point 362.73 ft away from another point. Your correction here will be applied in the opposite direction. Since the tape reads short, the laid tape distance of 362.73 ft is LONGER than 362.73 ft by the amount of the total correction for standard error (0.01 ft). Therefore, you must SUBTRACT the total tape error. To lay off a distance of 362.73 ft with this tape, you would actually measure off a distance of 362.72 ft. Suppose now that the Bureau of Standards calibration certificate states that when a tape indicates 100.00 ft under standard conditions, it is actually measuring only 99.997 ft. Again, the standard error is 0.003 ft per 100 ft, but this tape "reads long"; that is, the interval it indicates is LONGER than the interval it is actually measuring. Suppose you measure the distance between two given points with the tape and find that the tape indicates 362.73 ft. The total standard error is again 0.01 ft. Because the tape reads long, however, the distance it indicated was longer than the distance it actually measured. Therefore, the total standard error should be subtracted, and the distance between the given points should be finally recorded as 362.72 ft. Suppose you are using this same tape to set a point 362.73 ft away from another point. Again, the total standard error is 0.01 ft. Because the tape reads long, however, a measurement of 362.73 ft by the tape will actually be LESS than 362.73 ft. Therefore, the total correction for standard error should be added, and you should measure off 362.74 ft by the tape. |
 
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