Correct option is B
Meridian is defined as a fixed line of reference that can be any line in the survey or an imaginary line. Meridians used in surveying can be
1. Arbitrary Meridian or Assumed Meridian
A meridian that is arbitrarily chosen is called an arbitrary or assumed meridian. This meridian cannot be re-established if the original points are lost.
The first line of a traverse is taken as an arbitrary meridian in a general situation. It helps to determine the relative direction of the traverse lines.
2. True Meridian/Astronomic Meridian
If the meridian is the north-south line passing through the geographical poles of the earth, then it is called a True Meridian/Astronomic Meridian. This meridian is used for astronomical observations.
The directions measured with respect to the true meridian do not change as the true meridian is a fixed direction.
3. Grid Meridian
If a meridian lies parallel to the central true meridian for a specified area that is covered by a plane coordinate system, it is called a grid meridian.
For example, in order to survey a large area of a state, the true meridian of the central location of the country is taken as the reference meridian. This reference meridian is called as grid meridian.
4. Magnetic Meridian
If the meridian reference line lies parallel to the magnetic lines of force (magnetic poles) of the earth, i.e. passing through the magnetic north and magnetic south of the earth is called as as magnetic meridian.
A magnetic meridian can be identified using a freely-suspended magnetic needle that is influenced by the earth's magnetic needle. Hence, for each location, magnetic poles created changes. Hence a magnetic meridian is not a fixed direction.
All the magnetic meridians will converge to a central point, called the magnetic pole. Used for rough surveys and to check surveys.
The magnetic meridian and true meridian won't coincide with each other in place. The horizontal angle between these two meridians is known as magnetic declination.
