Because that the shape of the Earth is similar to a rotating ellipsoid, and the mass is not uniform, therefore, the vertical line and gravity line of all points on the surface of the earth are not the same. Therefore, a curved surface has to be defined first. For each point on this surface, the vertical line and gravity line shall be identical. This surface is called the standard level surface. Measuring the vertical distances to the standard level surface for two points, this is the relative height difference. The absolute height is based on the averaged sea level calculated using tide level observation records in the Keelung Tide Observation station and the astrometry movement period 18.6 years as basis. This sea level value is the base for height calculation. The heights for any points calculated starting from this base point are absolute height.

The measurement of level uses the surveyor's level to read the readings of two level gauges. The difference between these two readings is then the height difference between these two points, as shown in the following drawing.