As the name suggests, the so-called excess of sea-surface elevation is the height. But the sea ebb and flow, Big Wave Xiaolangdi stop, there is a moment when calm, and the monthly high tide and ebb of the daily sea surface height is also significantly different. Therefore, it was thought only to a firm with average sea-surface elevation as the starting surface. Defined as the higher elevation is the average sea-surface height. This is usually referred to the absolute height or elevation.
         How to draw a firm does the average sea surface? In this regard, mapping experts have long thought through set up in the coastal tide station approach. Select a country or region has a very convenient location, off the coast of the sea open, flat seabed, geological structure is relatively stable, representative and regular features such as half-day wave of Hong Kong to establish a long-term use of tide stations, according to long-term tide information to determine an average sea-surface as a zero-elevation surface. And then use the precision measurement of joint measurement standards to land a good standard of pre-starting point, the determination of the altitude of this point as a nation, or the date of elevation throughout the region. Before the founding of New China, China's lack of co-ordination of various departments to establish the altitude of a lot of tide stations, elevation system a bit confusing. This gave China's economic construction and national defense issues and problems brought about by many. After the founding of New China to establish a unified height system in China, established in 1957 in Qingdao of China tide stations basic tide stations, and to the station from 1950 to 1956, the tidal information as the calculation of the average sea surface elevation system, starting in China datum. Other tide tide stations results can only be compared as a reference and research use of sea surface topography. According to the economic and scientific and technological development needs of the late 80's through the review, and taking into account a longer period of time tide information system and height of the original date of elevation to a new recognition of the difference between old and new base elevation is only 29 millimeters.