Advantages and disadvantages of leveling, trigonometric leveling and GPS leveling.

Leveling is a method to measure the height difference between two points on the ground with a level and a leveling ruler. This method has strict theory and high measurement accuracy, but it has a large workload in the field and is suitable for working in flat areas. When the topography fluctuates greatly, trigonometric leveling can be used. This method is not affected by topography, and can directly measure the height difference between two points with large height difference. However, there are many sources of errors and the accuracy is not high. The national standard stipulates that it can replace the fourth-class leveling under certain conditions. There are also teams (such as Wuhan University) that have studied the substitution of second-class standards. GPS elevation is to convert geodetic height and normal height by using elevation anomaly. It is convenient and quick to obtain the height of the earth, but it is difficult to obtain the height anomaly. The quasi-geoid refinement being established in China needs to find the height anomaly and convert the geodetic height to the normal height. This method can not be affected by the terrain, save manpower and material resources, and its accuracy is poor at present.