Short Answer:

In surveying, meridians are reference lines used to measure directions or bearings. The main types of meridians used are true meridian, magnetic meridian, grid meridian, and arbitrary meridian. Each type serves a specific purpose depending on the method of surveying and the instruments used.

True meridian is based on the geographical north, magnetic meridian is aligned with the magnetic north shown by a compass, grid meridian relates to map coordinates, and arbitrary meridian is chosen for small or local surveys where high accuracy is not needed.

Detailed Explanation:

Types of meridians used in surveying

In surveying, accurate direction measurement is essential for preparing maps, establishing boundaries, and planning construction layouts. To determine directions correctly, a fixed reference line is needed. This reference is called a meridian. It is a north-south line used as the base for measuring horizontal angles or bearings of survey lines. Depending on the situation, different types of meridians can be used.

Each type of meridian helps surveyors establish directions according to the nature of the project and the tools available. Let’s understand each type:

1. True meridian

The true meridian is the geographical north-south line that passes through a point and aligns with the Earth’s rotational axis. It connects the true north pole and the true south pole.

• It is constant and does not change with time or place.
• Bearings measured from the true meridian are called true bearings.
• It is used in high-precision surveys and mapping.
• Instruments like transits and theodolites can help determine the true meridian using astronomical observations.

Since true meridian is based on the Earth’s axis, it provides the most reliable reference but is harder to establish in the field.

2. Magnetic meridian

The magnetic meridian is the direction shown by the magnetic needle of a compass, pointing towards the magnetic north.

• It changes slightly over time and from place to place due to magnetic variations in the Earth’s field.
• Bearings measured from this meridian are called magnetic bearings.
• Used in compass surveying where quick direction readings are needed.
• Magnetic declination (angle between true and magnetic meridian) must be considered for accuracy.

This is easy to determine in the field but less accurate over large areas due to variations.

3. Grid meridian

The grid meridian is used in map-based or coordinate system surveys. It is a line that runs parallel to the central meridian of a mapping grid (like UTM or state plane systems).

• Bearings measured from grid meridians are called grid bearings.
• They are used in large-scale map-based surveys and GIS applications.
• Grid meridians are fixed and do not vary with time like magnetic meridians.

Surveyors often use grid meridians when working with digital maps or satellite data.

4. Arbitrary meridian

An arbitrary meridian is any direction chosen by the surveyor to serve as a reference line, often used in small or private surveys.

• Bearings measured from this meridian are called arbitrary bearings.
• It has no relation to true, magnetic, or grid north.
• Useful when absolute direction is not required and only relative positions are needed.
• Common in quick layout surveys or construction site plans.

Though not scientifically accurate, arbitrary meridians are simple and sufficient for certain projects.

Conclusion:

The types of meridians used in surveying include true, magnetic, grid, and arbitrary meridians. Each type serves as a reference for measuring direction, depending on the tools used and the accuracy needed. True meridians offer the highest accuracy, magnetic meridians are used with compasses, grid meridians are suitable for map systems, and arbitrary meridians are used for local or simple surveys. Choosing the right meridian helps ensure correct direction and precise mapping in any civil engineering project.