What's the difference between third-class leveling and fourth-class leveling?

First, the instruments are different: the measuring instruments for measuring the third level are better than the ordinary level (S3 level can be used for the fourth level).

Second, the measurement methods are also different. Single-sided ruler is enough for ordinary leveling, and double-sided ruler is needed for third-and fourth-class leveling, and the "back, front and back" measurement is adopted.

Thirdly, the accuracy requirements of field measurement are different. The requirements for sight distance difference and cumulative value of sight distance difference are different, and the requirements for the difference between red and black surface readings and the difference between two height differences are also different.

Fourthly, the sight distance requirements of instruments and leveling staff are different.

Five, indoor calculation accuracy requirements are different.

Generally speaking, the accuracy requirements are different. Third-and fourth-class leveling has its own independent measurement method, that is, the order of reading ruler is different.