Distance measurement The commonly used distance measurement methods include tape measure distance and photoelectric ranging. The tape measure distance is measured along the ground by using a band ruler which can be rolled up, which belongs to the direct measure distance. Photoelectric ranging is to transmit and receive modulated light wave between two points, and to measure distance according to its propagation speed and time, which belongs to electronic physical ranging.

Photoelectric ranging belongs to indirect ranging.

**Tape Measurement Distance**

Type of tape measure and measure distance tool

The tape measure has two kinds of steel tape and tape length, 3m, 5m, 20m, 30m, 50m several kinds.

**Distance measurement**

Generally, the first amount of n whole-length, in the ground with the brazing or marking, the last amount of long, the total length of: NX length + redundency in order to prevent errors in the measurement and improve the measurement accuracy, it needs to be measured back and forth. The difference between the length of the measurement and the length of the measurement, divided by the sum of the length, is the fraction of the molecule 1, called relative error, or relative precision.

The relative precision of ruler is generally not less than 1/3000. Between the two endpoints of the scale field is the standard length l, with the identified ruler, the L ', (l-l ')/L is corrected by the temperature correction number per meter. Multiplied by nominal length, that is to be changed to (d) the length correction of the steel Tape gauge distance .

(1) when the ruler correction value is greater than 1/10000 of the ruler length, should be added to the length of the correction;

(2) when the distance between the temperature and the standard temperature difference between positive and negative 10 degrees, should be added temperature correction;

(3) along the ground to measure the ground slope is greater than 1.5%, should be corrected by heightening difference.