3　Horizontal Control Survey

3.1　General Provisions

3.1.1　The horizontal control network may be established using methods including satellite positioning survey，traverse survey and triangulation survey.

3.1.2　Horizontal control networks have four accuracy ratings:third order，fourth order，Class I，and Class II.

3.1.3　In the primary horizontal control network，the RMSE of the weakest point shall not exceed 5cm.

3.1.4　The primary horizontal control network shall be arranged in accordance with the following requirements:

1　The primary horizontal control network shall be planned comprehensively and cost-effectively according to the specific local conditions.

2　The primary horizontal control network shall be correlated with national or regional coordinate systems in accordance with Appendix A to this code.

3　Prior to surveys，data collection and reconnaissance survey shall be performed，and design optimization and accuracy estimation shall be made on drawings according to comprehensive analysis of the collected control survey data and topographic maps.

4　The accuracy and observation procedure for primary horizontal control network shall be determined reasonably based on the project scale，but it shall not be lower than the fourth order.

3.1.5　Horizontal control networks should be established based on the national horizontal coordinate system.To the extent that the deformation of projected length in the survey area is no more than 1/40000，the following horizontal coordinate systems may be used as appropriate:

1　Universal Gauss coordinate system with the meridian strips 3° wide；

2　Gauss conformal coordinate system with 3° wide meridian strips，the datum being the projection plane with compensation effect or the average elevation plane of survey area；or Gauss conformal coordinate system with any width meridian strips，the datum being the National Height datum 1985.

3　Independent coordinate systems may be used for small-sized survey areas or control networks with special accuracy requirements.

4　In areas where horizontal control networks are available，such coordinate systems may be used.

5　Within the station site，building coordinate system may be used.