Abstract: There are nine repeated relative gravity observation networks in northern China. whose Measurements of these networks are carried out independently and they have different control points and data processing methods. Gravity value changes far away from the control points are always imprecise and contain systematic error. In this paper, we use absolute gravity points and The Large North China Gravity Network as control gravity network, and calculate combined adjustment of the nine repeated relative gravity networks. The precision of the repeated relative gravity observation networks increased by 0~4×10^-8 m/s² and the nine repeated relative gravity observation networks can be united as a whole gravity network. Systematic error of some relative gravity observation networks can reach 60×10^-8 m/s². We also calculated reliability of correlation coefficients of relative gravity observation networks and made systematic error correction. After the systematic error correction, it is expected that repeated gravity data can better response to earthquake preparation. At last, we analysed the changes of gravity field of 2010 in northern China.

Key words: Gravity network, Combined adjustment, Gravity datum, Systematic error