What is the definition of sea level?

Definition: sea level is the average height of the ocean, as follows: sea level is the average height of the ocean. Refers to the water level that the ocean can maintain at a certain moment assuming that there is no sea surface fluctuation caused by tides, waves, swells or other disturbance factors. Its height is obtained by long-term observation with artificial water gauge and tide gauge. According to the observation time, it can be divided into daily average, monthly average, annual average and multi-year average sea level. The daily average sea level not only changes with weather conditions, but also has seasonal, semi-annual, one-year and multi-year periodic changes. The difference between the highest and lowest monthly mean sea level in a year (annual range): 60-70cm in Bohai Sea, 35-50cm in Yellow Sea, 30-35cm in East China Sea and 20-40cm in South China Sea. The annual average sea level difference can reach about 10 cm. 1956, China stipulated that the multi-year average sea level observed by Qingdao tide gauge station was the national unified elevation starting surface, which was called Qingdao average sea level or Yellow Sea datum. The altitude indicated on China's topographic map is calculated from this sea level.