Errors in base line measurements. 1. Tape Non-Standard Length

1. Tape Non-Standard Length. If the tape is саrefully handled the comparison made by the bureau of standards wIIl be suff'iciently precise to render the error from.this source negligible for the duration of a field season. Experience has shown that taре changes in length over period of years even though it is not used, hence, a tape should be standardized in the same season in which it is lied.

2. Variations in temperature. The effect of temperature is the most serious source of errors in precise linear measurements.

For example, an error of 0.5° F in the temperature of a steel tape introduces an error of 1/300,000 in the measured length, which alone is greater than the total probable error permitted in precise work. The magnitude of the error due to variations in temperature is greatly reduced by the use of an invar tape.

Favourabie conditions are those in which the air and the ground are at nearly the same temperature. These conditions are obtained on cloudy days or at night, and it is at such times that important base lines are usually measured.

3 Tape not horizontal. This.source of error is rendered negIigible as foIIows: the difference in elevation of adjacent posts is determined by a line of profile levels run with a transit or an engineer's level. The gradient for each tape length is then readily computed.

The corrections for grades up to 5 per cent may be computed by the approximate formula.

Steeper grades may be used if necessary, but the corrections should then be computed by the exact formula. The same correction is applied to measurements of base lines along highway or railroad, the grade of the supporting.surface being obtained by levelling.

4. Wind. If a strong wind is blowing normal to the base line and if the tape is not supported throughout its length there will be a lateral displacement of the unsupported portions of the tape, thus producing an effect similar to that of a sag.

Дата добавления: 2015-07-19