Objectives Deformation monitoring for concrete dams, earth and rock dams and other water conservation projects is highly demanding, usually requiring a monitoring accuracy of 1 mm (≤ 3 mm). However, the complexity of the engineering monitoring environment can compromise the precision of the current deformation monitoring techniques, such as global navigation satellite system (GNSS), robotic total stations (RTS), etc. It is acknowledged that these techniques may not meet ideal specifications in certain senses, due to the occasion or long distance.

Methods To address the above problems, we propose a GNSS/RTS tightly coupled deformation monitoring method that utilizes the distance and angle observations. The distance, angle, and other measurements recorded by the RTS are used as baseline constraints to supplement the GNSS observation matrix strength and improve the fixing rate of the GNSS carrier phase ambiguity and positioning precision.

Results The results show that RTS observations can improve the position dilution of precision values of GNSS satellites and significantly optimize the geometric distribution configuration of observations in complex environments. Compared with single GNSS positioning, the average elevation accuracy is improved by 18.0%,48.9% and 61.3% after combining distance, angle, and distance+angle observations, respectively. The RTS angle observations are more effective in improving the deformation monitoring accuracy. In addition, different types of RTS observations in tightly coupled positioning can improve the GNSS fixing rate of ambiguities, and the carrier phase residuals tend to follow an ideal normal distribution with the combination of angle observations.

Conclusions Angle observations play a more critical role in enhancing the accuracy of GNSS/RTS tightly coupled deformation monitoring.