Absolute elevation, why choose the Yellow Sea plane outside Qingdao as the zero point. What are the criteria and dimensions for countries to choose? or ...

Absolute height refers to the height measurement relative to a fixed reference plane. In geography, this reference plane is usually the global or regional average sea level. In China's surveying system, the absolute elevation is usually based on the Yellow Sea plane outside Qingdao, also known as "Yellow Sea elevation".

The reason why the Yellow Sea plane outside Qingdao is chosen as the zero point is mainly due to historical and geographical factors. Qingdao is a coastal city with relatively stable sea level, which is very representative in the coastal areas of China. In addition, other different elevation datum have been adopted in the history of our country, but since 1985, the Yellow Sea plane outside Qingdao has been adopted as the elevation datum in our country.

Different countries and regions may adopt different absolute elevation datum, and the selection criteria may be different due to geographical location, historical evolution, survey technology and other factors. For example, the United States takes the sea level in the middle of the Atlantic as the benchmark, and Britain takes the sea level at the mouth of the Thames in London as the benchmark.