A. Error in temperature variations,General steel tape of the thermal expansion coefficient of α=1.25x10-5, the temperature difference per meter per degree is only one out of 10,000, but the same steel tape in a larger temperature difference in the environment will produce a larger length change, affecting the measurement results.

The error of temperature variation has been considered in the equation of length.

B. Pull Error Pull size will affect the length of the ruler, if the measurement without spring to measure the pull, will produce errors.

The elastic modulus of steel is e=2x106 kg/cm2, according to Hooke's law, the length error of ± 1.8 mm will be produced when the 30 meters length of the 5 kg rally error.

C. Error of Ruler level When measuring the horizontal distance, the steel tape should be kept to the same level as possible, otherwise the error of distance growth will be generated. For a length of 30 meters, the two ends of the ruler with a height difference of 0.4 meters will produce an error of about 2.6 mm, the relative error is 1/11200.