**Bearing of a line in Civil Engineering**

The bearing of a line is an important concept in Civil Engineering, as it is used to describe the direction or orientation of a line relative to a reference point or meridian. There are different types of meridians that can be used as reference points for measuring the bearing of a line. These include:

1. **True Meridian:** The true meridian is a line passing through the geographic North Pole and South Pole. It represents the true north-south direction on the Earth's surface.

2. **Magnetic Meridian:** The magnetic meridian is a line passing through the magnetic North Pole and South Pole. It represents the direction indicated by a magnetic compass needle, which is influenced by the Earth's magnetic field.

3. **Arbitrary Meridian:** The arbitrary meridian is an arbitrarily chosen reference line. It can be any line selected for convenience or specific purposes, such as a survey line or a reference line on a map.

4. **Grid Meridian:** The grid meridian is a line of reference on a map or a grid system. It is used to define the horizontal coordinates (easting and northing) of a point on the map.

**Explanation:**

- The bearing of a line is the horizontal angle it makes with a reference meridian. It is usually measured in degrees, clockwise from the reference meridian.

- The true bearing of a line is the angle it makes with the true meridian. It represents the true direction of the line relative to the Earth's geographic coordinates.

- The magnetic bearing of a line is the angle it makes with the magnetic meridian. It represents the direction indicated by a magnetic compass needle. However, it should be noted that the magnetic needle may not align exactly with the true meridian due to the Earth's magnetic field variations and local magnetic anomalies.

- The arbitrary bearing of a line is the angle it makes with an arbitrarily chosen reference meridian. This can be useful in surveying or mapping applications where a specific reference line is needed.

- The grid bearing of a line is the angle it makes with the grid meridian on a map or a grid system. It is commonly used in cartography and navigation to determine the direction between points on a map.

In summary, the bearing of a line is a measure of its direction relative to a reference meridian. The choice of the reference meridian depends on the specific application and the accuracy required.