In surveying, meridians are vital reference lines to measure directions and establish accurate land orientations. Meridians run from the north to the south poles, providing a stable framework for angular measurements and mapping. Meridians are fundamental in survey design, with the primary types being astronomic, magnetic, and assumed meridians. Each type offers distinct benefits and limitations, selected based on the project's scale and precision needs.

The astronomic meridian is aligned with the astronomic north, defined by Earth's axis and gravitational forces. It is determined through celestial observations, often using Polaris, a fixed star near the north celestial pole. This meridian's reliance on a stable celestial reference allows it to maintain consistent orientation over long periods, making it particularly suitable for extensive surveys where long-term reestablishment of direction is necessary. 

In contrast, the magnetic meridian is established using a compass needle's alignment with Earth's magnetic field. The magnetic meridian's ease of use makes it easier to establish than the astronomic meridian. However, the magnitude and direction of Earth’s magnetic field depends where the measurement is taken, and also changes over time. As a result, it is not suitable for large or permanent surveys, as shifts in the magnetic field can affect the accuracy of measurements when referenced later.

For smaller-scale projects, surveyors sometimes employ an assumed meridian, an arbitrary reference direction often approximating astronomic north. This approach simplifies surveys where high precision is less critical. However, if survey markers are lost, reestablishing an assumed meridian's direction can be challenging, as it needs an objective natural basis for realignment.

Grid meridians may be used in certain cases, especially in rectangular coordinate grid surveying systems. A grid meridian consists of a set of parallel meridians based on a central reference line, reducing calculation complexity over smaller survey areas.