Abstract: In view of the problem that the measurement results of the automated monitoring system in the subway protection area are easily affected by temperature, humidity, air pressure, atmospheric refraction, train vibration and other factors, and it is extremely difficult to conduct quantitative analysis on a certain factor, therefore, this paper proposes an improved polar coordinate secondary datum real time difference method, builds the automated monitoring system based on the new method and carries out accuracy evaluation, Case analysis shows that the new method can effectively weaken the impact of external environmental factors on the measurement results, accurately obtain the three-dimensional coordinates of monitoring points, further improve the monitoring accuracy of the automated monitoring system, and more accurately feedback the impact of external construction on the rail transit structure.

Key words: polar coordinates, secondary datum, real time difference, distance correction, azimuth correction, height difference correction