Méridiens

Surveyors use four meridian types as reference lines for measuring directions. Astronomic meridians align with Earth's axis using celestial observations. Magnetic meridians follow a compass needle's orientation. Assumed meridians use an arbitrary direction approximating north. Grid meridians consist of parallel lines based on a central reference, reducing complexity in local surveys.

An astronomic meridian is aligned with Earth's axis and determined through celestial observations like Polaris. Its direction remains constant over long periods, enabling reestablishment in later years. This stability makes it particularly useful for extensive land surveys where long-term accuracy and consistent orientation are critical requirements for azimuths and bearings measurements.

A magnetic meridian's orientation depends on Earth's magnetic field, which varies by location and changes over time. This variability reduces long-term accuracy, making it unsuitable for large or permanent surveys. When the magnetic field shifts, measurements referenced later may become inaccurate, limiting its reliability for extensive projects.

An assumed meridian is employed for smaller-scale surveys where high precision is less critical. Surveyors select an arbitrary direction typically approximating astronomic north, simplifying survey tasks. However, if survey markers are lost, reestablishing the assumed meridian's direction becomes challenging without an objective natural reference basis.

A grid meridian comprises a series of parallel meridians based on a central reference line, designed specifically for rectangular coordinate grid surveying systems. Unlike astronomic or magnetic meridians, grid meridians reduce calculation complexity over smaller survey areas by providing a simplified geometric framework rather than relying on celestial or magnetic references.

Celestial observations, particularly of Polaris, a fixed star near the north celestial pole, determine an astronomic meridian's alignment. These observations establish the meridian's connection to Earth's axis and gravitational forces. This celestial reference provides a stable, unchanging basis for direction measurement, ensuring consistent orientation across extended time periods.

Meridian selection depends on project scale and precision requirements. Astronomic meridians suit extensive surveys requiring long-term stability. Magnetic meridians offer ease of establishment but limited accuracy for permanent projects. Assumed meridians simplify smaller surveys where precision is less critical. Grid meridians reduce complexity in local rectangular coordinate systems for traverse angle computations.