1.14. Measurement of Bearings

Bearing is the direction of each survey line with respect to a reference meridian direction. Bearing of a line can be defined as the angle measured in clockwise direction from a given reference.

A bearing indicates the direction of a line compared to a defined reference point, known as the meridian. It helps surveyors determine the orientation of a path in relation to the true north. When we talk about the bearing, we specifically mention how we measure this direction in a clockwise fashion from the reference meridian.

Imagine using a GPS; when you want to navigate, it typically provides instructions based on directions from a fixed point (like north). Similarly, bearings help surveyors understand which way to go by giving a directional angle based on a north reference.

Meridian direction can be divided into two main directions; (a) True meridian, and (b) Magnetic meridian.

There are two types of meridian used in surveying. The true meridian is the geographical line that runs from the north to the south pole at a specific location. In contrast, the magnetic meridian is the direction indicated by a magnetic needle, which shows magnetic north. These two types of meridians may not always align, especially due to various magnetic influences in the area.

If you think of a map showing true north, it’s like pointing to the top of a globe directly to the North Pole. On the other hand, using a compass is like a treasure hunt where the needle points you a bit off from true north due to magnetic influences—kind of like the magnetic pull can mislead you slightly from the true path.

Bearings are measured in two ways: 1. Whole circle bearing (WCB) 2. Reduced bearing (RB) or Quadrantal bearing (QB).

Whole Circle Bearing (WCB) measures angles from a reference direction (like magnetic north) all the way around in a circular measure. It ranges from 0° to 360°. Reduced Bearing (RB), however, gives angles based on the closest cardinal direction, either north or south, and measures into the east or west quadrants. This simplifies calculations, especially in trigonometry.

Think about a round clock. The WCB is like telling the time from 12 o'clock (0° for north) all the way around to 6 o'clock (180°). Reduced Bearings are more like saying, 'It’s quarter past 3'—where you’ve defined it in reference to north or south.

The magnetic compasses used in surveying may be classified as: 1. Prismatic Compass 2. Surveyor's Compass 3. Trough Compass 4. Tubular Compass.

Surveys use different types of magnetic compasses for different needs. The Prismatic Compass is versatile and provides clear readings, while the Surveyor's Compass is not as commonly used. Trough and Tubular compasses serve as accessories to various instruments, focusing on indicating direction rather than detailed measurements.

Think of these compasses like different types of smartphones. The Prismatic Compass is like a smartphone with excellent camera features, perfect for versatile use. Meanwhile, the Trough or Tubular Compass is like a simple mobile device that just serves the primary function of making calls—very useful, yet limited in its capabilities.

Fore bearing and back bearing of a line AB are to be measured. Set the compass at station A, bisect station B.

When surveying, fore bearings are measured when you face a particular direction first (e.g., from A to B), while back bearings are measured the opposite way (from B to A). This helps verify that the measurements taken are accurate, as both bearings from two points should differ by a specific angle, typically 180°.

Imagine you're running a relay race where one runner passes the baton to the next. The fore bearing represents the first runner's direction towards the finish line, while the back bearing indicates where that runner came from. Just like in survey methods, checking these routes ensures that the baton was passed correctly without any mistakes.

The horizontal angle between magnetic meridian and true meridian is known as magnetic declination.

Magnetic declination occurs because the magnetic north pole does not perfectly align with the geographic north pole. Depending on where you are, this angle can affect how accurate a compass reading is when determining true bearings. Knowing the degree of declination allows surveyors to adjust their measurements.

Think about it like adjusting your view while navigating using Google Maps. The maps are accurate, but if there is interference (a detour, for instance), you must know how to navigate around that issue, similar to adjusting for magnetic declination when using a compass.

Deviation in reading caused by nearby magnetic objects is called local attraction.

In surveying, local attraction happens when nearby magnetic materials influence the compass needle, leading to incorrect readings. This can arise from things like power lines, pipelines, or even metallic objects on-site. Surveyors must be aware of these influences to ensure accuracy in their measurements.

Imagine being at a party with friends trying to find someone in a crowd. If your friend stands too close to a magnet (like wearing metallic clothing), it might distort your view of them. Similarly, magnetic items nearby can distort the compass reading, leading surveyors astray.

In a closed traverse, bearings of lines may be calculated if bearing of one of the line and the included horizontal angles between various lines are known.

To find the bearing of a line in a closed traverse, you can start with one known bearing and add the included angles between that line and the others. This method helps in calculating the geometrical relationships between different lines when surveying.

Think of it as piecing together a jigsaw puzzle. You have one corner piece (the known bearing), and as you fit in the angles that connect to other pieces (the included angles), it tells you the full picture of how the lines interact with each other.

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