Generally, government agencies or related engineering units will implement large-area or local leveling surveys on a regular basis or as needed to grasp the changes in topography and features. In fact, the first discovery of stratum subsidence in China was in the Taipei area in the 1950s. It was discovered by engineers when they checked the elevation of the embankment. From an iso-leveling point outside the stratum subsidence area to the stratum subsidence area by precision leveling, and set an appropriate amount of reference points in the stratum subsidence area, and compare the measurement results at different times to obtain the stratum subsidence; From the amount of stratum subsidence at each reference point, a line of equal subsidence can be drawn to find the center of subsidence. So far, precision leveling has been the most common and accurate method for investigating the status of the entire region's overall stratum subsidence. In addition, comparing the results of previous inspections in the past, we can also observe the changes in the range of stratum subsidence and its relationship with industrial distribution or land use. The limitation is that it is labor-intensive, time-consuming, difficult to automate, and measurement The time interval is large, and it is difficult to obtain continuous stratum subsidence data. At present, water conservancy units conduct traditional leveling surveys in areas where the coastal stratum subsidence is more serious, that is, they are limited to manpower time, so the surveys are carried out in rotation every 2 to 3 years on average. Explain that the sag rate of leveling is calculated as follows:

(1) The leveling pile elevation of the current period — the leveling pile elevation of the previous period = the sinking amount of the leveling pile in this period.

(2) Interpolate the "sagging trend surface of the current period" by using all the "sagging amount of leveling piles in this period".

(3) Average sinking trend surface = sinking trend surface of the current period/measurement period.

(4) Cumulative sinking trend surface = cumulative [sagging trend surface for each period]

(5) Use the average sinking trend surface to interpolate the average sinking rate contour.

(6) Use the cumulative sag trend surface to interpolate the cumulative sag amount contour.

(7) Calculate area with GIS technology using contour lines.