Here is a fact that surprises students the first time they meet it. A single survey line, running in one fixed direction across the ground, can be correctly described by four different numerical bearings — all of them right at the same time.
Nothing about the line has changed. What differs is the meridian each bearing is measured from.
The Four Types
- True bearing
- Magnetic bearing
- Grid bearing
- Arbitrary bearing
The list corresponds exactly to the four meridians, and that is not a coincidence — each meridian produces its own bearing.
1. True Bearing (Azimuth)
The true bearing of a line is the horizontal angle between the true meridian and the survey line. It is also called the azimuth of the line.
Its Defining Quality
Because the true meridian is invariable, the true bearing of a fixed line is permanent. Record it today and it remains correct indefinitely.
This is why true bearings are used for work of a lasting character — national mapping, property boundaries, and any survey that must still make sense decades later. It is also why long-neglected surveys can be recovered: the reference direction has not moved.
2. Magnetic Bearing
The magnetic bearing of a line is the horizontal angle between the magnetic meridian and the survey line. When a bearing is quoted without qualification, the magnetic bearing is usually meant.
Its Defining Quality
The magnetic bearing is immediately available — a compass gives it in seconds, anywhere, with no computation.
But it carries two vulnerabilities. The magnetic meridian changes over time, so a magnetic bearing recorded years ago no longer describes the same direction. And the needle can be pulled aside by local attraction from nearby iron, giving a reading that is not a true magnetic bearing at all.
3. Grid Bearing
The grid bearing is the horizontal angle between the grid meridian and the survey line — the grid meridian being the true meridian of the central station, adopted for the whole survey.
Its Defining Quality
All grid bearings in a survey share one common reference direction. Since true meridians converge towards the poles, bearings measured from each station’s own true meridian are not strictly comparable with one another. Grid bearings remove that difficulty by referring everything to a single line.
The result is that geometry and computation become straightforward — angles between lines can be found by simple subtraction, without allowing for convergence.
4. Arbitrary Bearing
The arbitrary bearing is the horizontal angle between an assumed arbitrary meridian and the survey line.
Its Defining Quality
An arbitrary bearing makes no claim about north at all. Its only requirement is internal consistency — every line in the survey measured from the same assumed direction.
For a self-contained plan of a small site, that is entirely sufficient. The relative positions of everything will be correct, the shape of the plan will be right, and only the plan’s orientation on the page is arbitrary.
All Four Compared
| Bearing | Measured From | Changes With Time? | Typical Use |
|---|---|---|---|
| True bearing (azimuth) | True meridian | No — invariable | Permanent and precise surveys |
| Magnetic bearing | Magnetic meridian | Yes | Ordinary compass surveying |
| Grid bearing | Grid meridian (central station’s true meridian) | No | Small-area surveys needing one common reference |
| Arbitrary bearing | Assumed convenient direction | No | Self-contained plans of small sites |
The One Question That Sorts Them
All four definitions share the identical structure — the horizontal angle between [some] meridian and the survey line. Only the meridian changes.
So there is really just one thing to remember: name the meridian, and you have named the bearing. True meridian gives true bearing, magnetic meridian gives magnetic bearing, and so on.
Why the Same Line Gives Different Numbers
Picture a line running from station A towards a distant point. Now imagine four arrows all planted at A, each pointing a slightly different way:
- One towards the north pole (true meridian)
- One towards magnetic north (magnetic meridian)
- One parallel to the central station’s true meridian (grid meridian)
- One towards a church spire (arbitrary meridian)
The survey line has not moved. But the angle from each arrow to that line is different — so four different bearings are recorded, and every one is correct within its own system.
This is exactly why a bearing must always be stated together with its meridian. A bearing quoted alone is incomplete information.
The Relationship Between True and Magnetic
Of the four, the true and magnetic bearings are the pair most often needed together, because both are physically meaningful and both are used in practice.
The angle between the true meridian and the magnetic meridian is called the magnetic declination, and it is what allows one bearing to be converted into the other. That relationship is important enough to have its own topic later in this chapter.
Quick Revision Notes
- Four types of bearing: true, magnetic, grid and arbitrary — one for each meridian.
- Every bearing is the horizontal angle between a meridian and the survey line; only the meridian differs.
- True bearing is measured from the true meridian and is also called the azimuth. It does not change with time.
- Magnetic bearing is measured from the magnetic meridian, and is what is usually meant by “bearing” alone.
- Grid bearing is measured from the grid meridian — the true meridian of the central station.
- Arbitrary bearing is measured from an assumed convenient direction.
- The same line has four different bearings, all simultaneously correct.
- A bearing must always be quoted together with its meridian.
- The angle between true and magnetic meridians is the magnetic declination.
Mistakes Students Commonly Make
- Assuming a line has only one bearing. It has four, one per meridian.
- Using “azimuth” for any bearing. Azimuth means specifically the true bearing.
- Confusing grid and arbitrary bearings. The grid meridian is a real true meridian; the arbitrary one is simply assumed.
- Quoting a bearing without naming its meridian, which leaves the value meaningless.
- Saying true bearings change over time. It is the magnetic bearing that changes, because the magnetic meridian drifts.
- Forgetting that when “bearing” is used alone, the magnetic bearing is normally intended.
Conclusion
Four types of bearing, but only one definition — the horizontal angle from a meridian to the survey line. Swap the meridian and you swap the bearing, which is why a single unchanged line can carry four different numbers at once. True bearings, or azimuths, are permanent because the poles do not move. Magnetic bearings are instantly available but drift and can be disturbed. Grid bearings give a whole survey one shared reference. Arbitrary bearings give up on north entirely in exchange for convenience. Always state which meridian a bearing belongs to, or the number means nothing.
Frequently Asked Questions
What are the four types of bearings?
True bearing, magnetic bearing, grid bearing and arbitrary bearing.
What is a true bearing?
The horizontal angle between the true meridian and the survey line. It is also known as the azimuth of the line.
What is a magnetic bearing?
The horizontal angle between the magnetic meridian and the survey line. When the word bearing is used without qualification, the magnetic bearing is usually meant.
What is a grid bearing?
The horizontal angle between the grid meridian and the survey line, the grid meridian being the true meridian of the central station adopted for the whole survey.
What is an arbitrary bearing?
The horizontal angle between an assumed arbitrary meridian and the survey line.
Can one line have more than one bearing?
Yes. The same line has four different bearings, one for each meridian, and all of them are correct within their own reference system.
Why must a bearing always be stated with its meridian?
Because the numerical value depends entirely on which meridian it was measured from, so a bearing quoted alone is incomplete information.
Which bearing does not change with time?
The true bearing, because the true meridian is invariable. The magnetic bearing changes as the magnetic meridian drifts.
What is the angle between the true and magnetic meridians called?
The magnetic declination, which is what allows a magnetic bearing to be converted into a true bearing and vice versa.Here is a fact that surprises students the first time they meet it. A single survey line, running in one fixed direction across the ground, can be correctly described by four different numerical bearings — all of them right at the same time.
Nothing about the line has changed. What differs is the meridian each bearing is measured from.
The Four Types
- True bearing
- Magnetic bearing
- Grid bearing
- Arbitrary bearing
The list corresponds exactly to the four meridians, and that is not a coincidence — each meridian produces its own bearing.
1. True Bearing (Azimuth)
The true bearing of a line is the horizontal angle between the true meridian and the survey line. It is also called the azimuth of the line.
Its Defining Quality
Because the true meridian is invariable, the true bearing of a fixed line is permanent. Record it today and it remains correct indefinitely.
This is why true bearings are used for work of a lasting character — national mapping, property boundaries, and any survey that must still make sense decades later. It is also why long-neglected surveys can be recovered: the reference direction has not moved.
2. Magnetic Bearing
The magnetic bearing of a line is the horizontal angle between the magnetic meridian and the survey line. When a bearing is quoted without qualification, the magnetic bearing is usually meant.
Its Defining Quality
The magnetic bearing is immediately available — a compass gives it in seconds, anywhere, with no computation.
But it carries two vulnerabilities. The magnetic meridian changes over time, so a magnetic bearing recorded years ago no longer describes the same direction. And the needle can be pulled aside by local attraction from nearby iron, giving a reading that is not a true magnetic bearing at all.
3. Grid Bearing
The grid bearing is the horizontal angle between the grid meridian and the survey line — the grid meridian being the true meridian of the central station, adopted for the whole survey.
Its Defining Quality
All grid bearings in a survey share one common reference direction. Since true meridians converge towards the poles, bearings measured from each station’s own true meridian are not strictly comparable with one another. Grid bearings remove that difficulty by referring everything to a single line.
The result is that geometry and computation become straightforward — angles between lines can be found by simple subtraction, without allowing for convergence.
4. Arbitrary Bearing
The arbitrary bearing is the horizontal angle between an assumed arbitrary meridian and the survey line.
Its Defining Quality
An arbitrary bearing makes no claim about north at all. Its only requirement is internal consistency — every line in the survey measured from the same assumed direction.
For a self-contained plan of a small site, that is entirely sufficient. The relative positions of everything will be correct, the shape of the plan will be right, and only the plan’s orientation on the page is arbitrary.
All Four Compared
| Bearing | Measured From | Changes With Time? | Typical Use |
|---|---|---|---|
| True bearing (azimuth) | True meridian | No — invariable | Permanent and precise surveys |
| Magnetic bearing | Magnetic meridian | Yes | Ordinary compass surveying |
| Grid bearing | Grid meridian (central station’s true meridian) | No | Small-area surveys needing one common reference |
| Arbitrary bearing | Assumed convenient direction | No | Self-contained plans of small sites |
The One Question That Sorts Them
All four definitions share the identical structure — the horizontal angle between [some] meridian and the survey line. Only the meridian changes.
So there is really just one thing to remember: name the meridian, and you have named the bearing. True meridian gives true bearing, magnetic meridian gives magnetic bearing, and so on.
Why the Same Line Gives Different Numbers
Picture a line running from station A towards a distant point. Now imagine four arrows all planted at A, each pointing a slightly different way:
- One towards the north pole (true meridian)
- One towards magnetic north (magnetic meridian)
- One parallel to the central station’s true meridian (grid meridian)
- One towards a church spire (arbitrary meridian)
The survey line has not moved. But the angle from each arrow to that line is different — so four different bearings are recorded, and every one is correct within its own system.
This is exactly why a bearing must always be stated together with its meridian. A bearing quoted alone is incomplete information.
The Relationship Between True and Magnetic
Of the four, the true and magnetic bearings are the pair most often needed together, because both are physically meaningful and both are used in practice.
The angle between the true meridian and the magnetic meridian is called the magnetic declination, and it is what allows one bearing to be converted into the other. That relationship is important enough to have its own topic later in this chapter.
Quick Revision Notes
- Four types of bearing: true, magnetic, grid and arbitrary — one for each meridian.
- Every bearing is the horizontal angle between a meridian and the survey line; only the meridian differs.
- True bearing is measured from the true meridian and is also called the azimuth. It does not change with time.
- Magnetic bearing is measured from the magnetic meridian, and is what is usually meant by “bearing” alone.
- Grid bearing is measured from the grid meridian — the true meridian of the central station.
- Arbitrary bearing is measured from an assumed convenient direction.
- The same line has four different bearings, all simultaneously correct.
- A bearing must always be quoted together with its meridian.
- The angle between true and magnetic meridians is the magnetic declination.
Mistakes Students Commonly Make
- Assuming a line has only one bearing. It has four, one per meridian.
- Using “azimuth” for any bearing. Azimuth means specifically the true bearing.
- Confusing grid and arbitrary bearings. The grid meridian is a real true meridian; the arbitrary one is simply assumed.
- Quoting a bearing without naming its meridian, which leaves the value meaningless.
- Saying true bearings change over time. It is the magnetic bearing that changes, because the magnetic meridian drifts.
- Forgetting that when “bearing” is used alone, the magnetic bearing is normally intended.
Conclusion
Four types of bearing, but only one definition — the horizontal angle from a meridian to the survey line. Swap the meridian and you swap the bearing, which is why a single unchanged line can carry four different numbers at once. True bearings, or azimuths, are permanent because the poles do not move. Magnetic bearings are instantly available but drift and can be disturbed. Grid bearings give a whole survey one shared reference. Arbitrary bearings give up on north entirely in exchange for convenience. Always state which meridian a bearing belongs to, or the number means nothing.
Frequently Asked Questions
What are the four types of bearings?
True bearing, magnetic bearing, grid bearing and arbitrary bearing.
What is a true bearing?
The horizontal angle between the true meridian and the survey line. It is also known as the azimuth of the line.
What is a magnetic bearing?
The horizontal angle between the magnetic meridian and the survey line. When the word bearing is used without qualification, the magnetic bearing is usually meant.
What is a grid bearing?
The horizontal angle between the grid meridian and the survey line, the grid meridian being the true meridian of the central station adopted for the whole survey.
What is an arbitrary bearing?
The horizontal angle between an assumed arbitrary meridian and the survey line.
Can one line have more than one bearing?
Yes. The same line has four different bearings, one for each meridian, and all of them are correct within their own reference system.
Why must a bearing always be stated with its meridian?
Because the numerical value depends entirely on which meridian it was measured from, so a bearing quoted alone is incomplete information.
Which bearing does not change with time?
The true bearing, because the true meridian is invariable. The magnetic bearing changes as the magnetic meridian drifts.
What is the angle between the true and magnetic meridians called?
The magnetic declination, which is what allows a magnetic bearing to be converted into a true bearing and vice versa.