Understanding Meridians and Direction

Meridians are important lines used in geography and surveying to define direction. While geographic meridians (also known as true meridians) run from the North Pole to the South Pole along lines of longitude, other types of meridians are used depending on the reference direction.

The question asks about the specific type of meridians that are parallel to the direction a freely moving magnetized needle, like the one in a compass, takes. This direction is influenced by the Earth's magnetic field.

What are Magnetic Meridians?

A magnetic meridian is an imaginary line connecting the magnetic poles of the Earth. At any point on the Earth's surface, the magnetic meridian is the direction indicated by a freely suspended magnetic needle, unaffected by local magnetic disturbances. This direction is generally different from the true north (defined by geographic meridians).

The needle in a compass aligns itself with the local magnetic field lines. Therefore, the lines parallel to the direction shown by a compass needle are called magnetic meridians.

Comparing Different Types of Meridians

Let's look at the other options to understand why they are not the correct answer:

• Geodetic Meridians: These are the true or geographic meridians. They follow lines of constant longitude and converge at the geographic North and South Poles. They represent the direction towards the true north or south pole based on the Earth's rotation axis. A compass needle does not point exactly along a geodetic meridian (except in places where magnetic declination is zero).
• Grid Meridians: These are straight lines used in grid systems, such as the Universal Transverse Mercator (UTM) system. Grid meridians are typically parallel to a central meridian within a specific grid zone. They are a convenience for mapping and surveying over limited areas but do not align with either true north or magnetic north in most places.
• Dynamic Meridians: This term is not a standard type of meridian used in surveying or navigation to describe a directional reference based on a compass needle or geographic coordinates.

Based on these definitions, the meridians that follow the direction of a freely moving magnetized needle are specifically the magnetic meridians.

Revision Table: Types of Meridians

Additional Information: Magnetic Declination

The difference between the direction of the magnetic meridian and the geodetic (true) meridian at any point is called magnetic declination or magnetic variation. This angle varies depending on location and changes over time due to the Earth's constantly shifting magnetic field. When using a compass for navigation or surveying, it is often necessary to know the local magnetic declination to convert a magnetic bearing (direction relative to magnetic north) into a true bearing (direction relative to true north) or a grid bearing.

Magnetic declination is typically given as an angle east or west of true north. For example, a declination of 5° East means magnetic north is 5° east of true north at that location.