The Global Positioning System (GPS) has been developed since 1970. Not only has it completed the launch of satellites in early 1995, it has officially provided a round-the-clock 3D positioning service to all corners of the world. Its software and hardware are also increasing with the increase of users. The functions are becoming more powerful, and prices are falling. In view of the limitation that leveling is easily affected by weather and terrain, is time-consuming, and is not easy to be automated, some domestic experts and scholars have invested in GPS for the research of stratum subsidence monitoring and early warning. After several previous studies, although the elevation measured by GPS is the geometric ellipsoidal height (Ellipsoidal Height), it is still different from the gravity-related orthometric height (Orthometric Height) obtained by leveling. Geomatic Undulation is required if required. It takes a lot of effort to get the exact value of the earth's ups and downs. Fortunately, for the change in the amount of sag, the change trend of the GPS ellipsoid height difference at the same level point is equal to the positive height difference change trend of the leveling measurement. Therefore, it is worthy of application and promotion in terms of the advantages of saving manpower, time and money compared with leveling.