4　Vertical Control Survey

4.1　General Provisions

4.1.1　Vertical control survey networks have four levels of accuracy:second，third，fourth and fifth orders.Leveling survey method should be used for vertical control networks of various accuracies.However，EDM-trigonometric leveling technique may be used for networks of fourth and fifth orders，and GPS technique may be used for fifth order.

4.1.2　In a vertical control leveling network or route，the elevation difference RMSE of the two stations that are the farthest apart shall not exceed 3cm.

4.1.3　The primary vertical control network shall have an accuracy not lower than the third order，and shall be arranged in closed loops.Densification network should be arranged as connecting route or junction point network.

4.1.3　The vertical control survey should be based on the National Vertical Datum 1985.Where regional height system or independent height system is used，such system should be connected with the national height system.

4.1.5　At least three permanent benchmarks shall be established in nuclear power plant site.

4.1.6　The starting point of primary network shall have a minimum reference accuracy of third order.Where several height systems are available near the survey area，a survey connection may be established to these systems as appropriate at a survey accuracy not lower than the lowest accuracy of these systems.Prior to referencing or connecting surveys，adjacent elevation points in various systems shall be checked for elevation differences.See Appendix H for transformation between the zero points of main elevation reference systems in China.