The altitude of sea level is not uniform. What is the base of altitude?

The altitude of sea level is not uniform. What is the base of altitude?

The sea level of the Yellow Sea is used in China now, and the datum point is near Qingdao. From this, the elevation can be unified all over the country by using the leveling network of precise layout, measurement and calculation
In 1987, China stipulated that the mean sea level of China's elevation datum point at huanghaiping measured by Qingdao tidal station from January 1, 1952 to December 31, 1979 should be taken as the starting plane of national elevation. It is inferred that the elevation of national leveling origin on Guanxiang mountain in Qingdao is 72.260m. The elevation system established according to this starting plane is called 1985 National elevation datum, All mean the height from the mean sea level of the Yellow Sea