This is done by measuring the compression of the soil layers. In this study only data showing absolute subsidence has been used. Absolute subsidence is the variation in ground elevation against a fixed reference level, such as mean sea level. It requires a geodetic survey to measure the changes in elevation. This is the most common way in the Bangkok area to obtain information on land subsidence.
Absolute subsidence is measured using surface reference points or benchmarks.
The surface reference point settles down at the same magnitude as the ground surface. The surface reference point is actually embedded at a depth of one metre but the top of the point, a brass ball point, is at the ground surface. The surface leveling needs to be carried out every year.