Due to the influence of tide and topography,the accuracy and reliability of synchronized hydrometry are greatly limited in broad tidal river reach.In order to obtain higher precision to make the data of synchronous observation and time series good comparability,it is necessary to establish high-precision plane datum,elevation datum and velocity datum.Theory and practice show that it is an effective way which can carry out three dimensional depth measurement based on the RTK technology and flow measurement based on the velocity reference of GNSS RTK through establishment of a wide range of high precision GNSS control network.