子午線

In surveying, meridians serve as reference lines for measuring directions. 

A meridian is a fixed line running from the north to the south poles, and the four meridian types used in surveying are astronomic, magnetic, assumed, and grid. 

An astronomic meridian is aligned with the astronomic north, determined by the Earth's axis and gravitational forces. 

Observations of stars, like Polaris, help in its alignment. It is particularly useful for extensive land surveys, as its direction remains constant, enabling reestablishment in later years.

On the other hand, the magnetic meridian is defined by a compass needle's orientation, affected by Earth's magnetic field. However, this meridian varies with location and time, reducing long-term accuracy.

For smaller surveys, an assumed meridian is employed. This approach involves selecting an arbitrary direction, typically approximating astronomic north. While using an assumed meridian can simplify survey tasks, it has limitations. Specifically, if survey markers are lost, reestablishing the assumed meridian's direction becomes challenging. 

Additionally, a grid meridian, comprising a series of parallel meridians based on a central reference line, may sometimes be used to reduce complexity in local surveys.