A bearing is measured from a meridian — but which meridian? Four different reference directions are used in surveying, and each produces a different bearing for the very same line.
This post covers all four, explains when each is used, and introduces the compass traverse.
Why More Than One Meridian Exists
Recall the definition: a meridian is any fixed direction from which the direction of a survey line is measured.
The word any is the key. Nothing in nature obliges us to use one particular direction as zero — we choose one for convenience. Different jobs make different choices convenient, and that is why four meridians exist rather than one.
1. True Meridian
The true meridian passing through a point is the line in which a plane passing through that point and the north and south poles intersects the surface of the earth.
In plainer terms: slice the earth with a plane that contains the point and both poles, and the line where that slice meets the ground is the true meridian.
Key Facts
- The direction of the true meridian at a point is invariable.
- It is established by astronomical observations.
- The angle between the true meridian and a survey line is the true bearing or azimuth.
- True meridians through various stations are not parallel, but converge to the poles.
The Great Advantage: It Never Changes
This is what makes the true meridian valuable. The north pole is where it is, permanently — so a true bearing recorded today will still be correct in fifty years.
Compare this with the magnetic meridian, which drifts continuously. For any survey of a permanent character — property boundaries, national mapping, anything intended to last — the true meridian is the sound choice precisely because it is invariable.
The Convergence Point
Note that true meridians are not parallel to one another. They all head for the same point at the pole, so they converge as you move north.
Over a small survey this is negligible, but over a large one it matters — two stations far apart have true meridians pointing in slightly different directions, so bearings measured from them are not strictly comparable. This is one reason the grid meridian was devised.
2. Magnetic Meridian
The magnetic meridian is the direction indicated by a freely suspended and properly balanced magnetic needle, unaffected by local attractive forces.
The angle between the magnetic meridian and a line is the magnetic bearing or simply the bearing of the line.
The Three Conditions in the Definition
Read the definition carefully, because it contains three requirements, and each one anticipates a problem covered later:
| Condition | What It Guards Against |
|---|---|
| Freely suspended | A needle that binds or sticks will not settle in the true magnetic direction |
| Properly balanced | An unbalanced needle dips or drags, giving a false reading |
| Unaffected by local attractive forces | Local attraction — iron objects nearby pulling the needle off the true magnetic meridian |
That third condition is the important one, and it becomes a whole topic of its own. A compass reading taken beside an iron railing is not a magnetic bearing at all, because the needle is not pointing at magnetic north.
Why the Magnetic Meridian Is Used Despite Its Faults
It moves, it can be disturbed by nearby iron, and it does not coincide with true north. So why use it?
Because it is immediately available anywhere. A compass gives you a reference direction in seconds, with no astronomical observation, no computation and no connection to any external framework. For ordinary work of limited extent, that convenience outweighs the drawbacks.
3. Grid Meridian
Sometimes, for surveys of a small area, the true meridian passing through the central station is taken as a reference meridian for the whole survey. This is called the grid meridian.
The bearings referred to it are called grid bearings.
The Problem It Solves
This follows directly from the convergence issue noted earlier. Since true meridians at different stations are not parallel, using each station’s own true meridian means every station has a slightly different reference direction — which complicates every calculation.
The grid meridian fixes this by a simple decision: pick one meridian and use it everywhere. All stations then share the same reference direction, so all bearings become directly comparable and the geometry becomes straightforward.
The cost is a small, known distortion away from the central station — acceptable for a small area, which is exactly the condition stated in the definition.
4. Arbitrary Meridian
Sometimes, for a survey of a small area, a convenient direction is assumed as a meridian, known as the arbitrary meridian. Often the direction from a station to some well-defined object is taken as the arbitrary meridian.
The angle between this meridian and a line is called the arbitrary bearing of the line.
When This Is the Sensible Choice
An arbitrary meridian sounds unscientific, but it is often the right answer.
If a survey is self-contained — the plan of a small site, not to be connected to any other survey — then the actual direction of north is irrelevant. All that matters is that every line is measured from the same reference. Choosing a church spire or a distant tower gives a reference that is sharp, permanent and instantly re-locatable, without requiring any instrument at all.
The limitation is equally clear: an arbitrary-bearing survey cannot be tied to any other survey until its meridian is related to true or magnetic north.
The Four Meridians Compared
| Meridian | Reference Direction | Bearing Called | Established By |
|---|---|---|---|
| True | Line through the point and both poles | True bearing or azimuth | Astronomical observations |
| Magnetic | Direction of a freely suspended magnetic needle | Magnetic bearing or simply bearing | Compass |
| Grid | True meridian of the central station, used throughout | Grid bearing | Chosen for the survey |
| Arbitrary | Any convenient assumed direction | Arbitrary bearing | Assumed, often towards a well-defined object |
How to Keep Them Straight
Sort them by how the direction is obtained:
- True — determined by nature, found by observing the stars.
- Magnetic — determined by the earth’s magnetism, found with a needle.
- Grid — determined by choice, but a principled one: the true meridian of the central station.
- Arbitrary — determined by convenience, with no claim to any physical meaning.
Only the first two exist independently of the surveyor. The last two are decisions.
Compass Traverse
A compass traverse is a survey in which the bearings of the survey lines are measured with a compass and the lengths of the lines are measured with a tape or chain.
The process of taking these measurements is known as compass surveying.
Why This Combination Works
A traverse is a connected series of lines running from station to station. For each line you record two things — how long it is and which way it points.
That pair is exactly what is needed to move from one station to the next. Knowing the length and bearing of line AB, you can compute where B lies relative to A. Repeat for BC, CD and so on, and the whole traverse unrolls from a single starting point.
This is why compass surveying escapes the limitations of chain surveying. A chain survey needs a network of triangles to fix anything; a compass traverse needs only a single chain of lines.
Quick Revision Notes
- Four meridians: true, magnetic, grid and arbitrary.
- True meridian: the line where a plane through the point and both poles cuts the earth’s surface. Direction is invariable, established by astronomical observations.
- True bearing is also called the azimuth.
- True meridians at different stations are not parallel — they converge to the poles.
- Magnetic meridian: direction shown by a freely suspended, properly balanced magnetic needle unaffected by local attractive forces.
- Grid meridian: the true meridian of the central station, adopted for the whole survey; gives grid bearings.
- Arbitrary meridian: a convenient assumed direction, often towards a well-defined object; gives arbitrary bearings.
- Compass traverse: bearings measured with a compass, lengths with a tape or chain.
Mistakes Students Commonly Make
- Saying true meridians are parallel. They converge to the poles.
- Confusing the grid and arbitrary meridians. The grid meridian is a true meridian — that of the central station — while the arbitrary one is simply assumed.
- Omitting any of the three conditions in the magnetic meridian definition: freely suspended, properly balanced, unaffected by local attraction.
- Forgetting that a true bearing is also called an azimuth.
- Saying the magnetic meridian is invariable. It is the true meridian that never changes.
- Thinking the grid meridian applies to large areas. It is used for surveys of a small area.
Conclusion
Four reference directions, chosen for four different reasons. The true meridian is fixed by the poles and never changes, which suits permanent work but requires astronomical observation. The magnetic meridian is instantly available from a needle, at the price of drifting over time and being vulnerable to nearby iron. The grid meridian solves the convergence problem by adopting one true meridian for an entire small survey. And the arbitrary meridian abandons any physical meaning in exchange for pure convenience — perfectly adequate for a self-contained plan. Combine any of these with taped lengths and you have a compass traverse.
Frequently Asked Questions
What are the four types of meridian?
True, magnetic, grid and arbitrary meridians.
What is the true meridian?
The line in which a plane passing through the given point and through the north and south poles intersects the surface of the earth.
How is the true meridian established?
By astronomical observations. Its direction at any point is invariable.
What is a true bearing also called?
The azimuth of the line.
Are true meridians at different stations parallel?
No. They converge towards the poles, which is why bearings from widely separated stations are not strictly comparable.
What is the magnetic meridian?
The direction indicated by a freely suspended and properly balanced magnetic needle that is unaffected by local attractive forces.
What is a grid meridian?
The true meridian passing through the central station, adopted as the reference meridian for a whole survey of a small area. Bearings referred to it are grid bearings.
What problem does the grid meridian solve?
It removes the difficulty caused by true meridians converging at different stations, since every station then shares one common reference direction.
What is an arbitrary meridian?
A convenient direction assumed as a meridian for a survey of a small area, often the direction from a station to some well-defined object.
What is a compass traverse?
A survey in which the bearings of the survey lines are measured with a compass and the lengths of the lines are measured with a tape or chain.