There are five ways to record direction as you run a traverse, and they differ in one crucial respect — how often you rely on the magnetic needle.
That single question determines the accuracy of each method, because a compass needle is far less precise than a theodolite. This post covers all five, in ascending order of accuracy.
The Five Methods
- Loose needle method of bearings
- Fast needle method of bearings
- Method of included angles
- Method of direct angles
- Method of deflection angles
1. Loose Needle Method
In the loose needle method, the direction of the magnetic meridian is established at each traverse station, and the direction of the line is measured with reference to that magnetic meridian. In other words, the magnetic bearing of each line is measured at each station.
- A theodolite fitted with a magnetic compass is used for measuring the magnetic bearings of the traverse lines.
- The loose needle method is also known as the free needle method.
Why It Is the Least Accurate
The phrase to notice is at each station. The needle is consulted every single time.
That means every line’s direction carries the full inaccuracy of a magnetic compass — and worse, it is exposed to local attraction at every station. Any iron near any station corrupts that line’s bearing.
The instrument is a theodolite, capable of reading to 20 seconds, but the method throws that capability away by referring everything back to a needle.
2. Fast Needle Method
In the fast needle method, the magnetic meridian is established only at the starting station and the magnetic bearing of the first line is measured. The magnetic bearings of all other lines are determined indirectly, from the magnetic bearing of the first line and the included angles.
The Accuracy Argument
The magnetic bearing of the first line has the accuracy of the compass, but the difference of bearings of two adjacent lines has the accuracy of the theodolite.
This is the heart of the method, and it repays careful reading.
Only one measurement in the entire traverse depends on the needle — the bearing of the first line. Every subsequent direction is obtained from included angles measured with the theodolite, which are accurate to 20 seconds rather than to the half degree of a compass.
The compass error therefore appears once, as a constant rotation of the whole traverse. The shape of the traverse — determined entirely by theodolite angles — is far more accurate than any compass could give.
The fast needle method is more accurate than the loose needle method, and is generally preferred in the field.
Loose versus Fast — the Key Contrast
| Loose Needle | Fast Needle | |
|---|---|---|
| Meridian established | At every station | At the starting station only |
| Bearings measured | Every line, magnetically | First line only, then computed |
| Accuracy of directions | Compass accuracy throughout | Theodolite accuracy for all differences |
| Exposure to local attraction | Every station | One station |
| Also called | Free needle method | — |
3. Method of Included Angles
Traversing by the method of included angles is the most commonly used method.
- The magnetic bearing of any one line — generally the initial line — is measured in the field.
- All the included angles are also measured.
- Bearings of all other lines are determined from the bearing of the initial line and the included angles.
- This method is more accurate than the fast needle method.
What an Included Angle Is
An included angle is one of the two angles formed at a station by the two traverse lines meeting there.
Interior or Exterior — Which You Get
| Traverse Run | Clockwise Angles Are |
|---|---|
| Counterclockwise | Interior angles |
| Clockwise | Exterior angles |
Note: all angles are measured clockwise. This is done because in a theodolite the graduations increase in the clockwise direction.
Why This Rule Exists
This is a practical convention with a mechanical reason behind it.
A theodolite’s horizontal circle is graduated clockwise from 0° to 360° — the same convention as whole circle bearings. Measuring clockwise therefore means readings increase as you turn, and the angle is simply the second reading minus the first.
Measure anticlockwise and every angle would come out negative, requiring a 360° correction each time — exactly the nuisance described in the included angles topic. Always turning clockwise avoids it entirely.
The consequence is that the direction you run the traverse decides whether you get interior or exterior angles, and that in turn decides which sign to use in the angle sum check.
4. Method of Direct Angles
- Similar to the method of included angles.
- However, in this method, direct angles or the angles to the right are measured.
- The method is generally used in an open traverse.
“Angles to the right” means always turning clockwise from the back station to the forward station — consistent with the theodolite’s graduation direction.
5. Method of Deflection Angles
Deflection angles are the angles which a line makes with the prolongation of the receding line.
Where It Is Used
The method is mainly used for open traverses conducted for the survey of roads, railways, canals, pipelines, sewers and similar works, where the traverse lines make small deflection angles.
Why Deflection Angles Suit Route Surveys
The phrase small deflection angles explains everything.
A road or railway runs broadly straight. At each station it changes direction only slightly — perhaps 5 or 10 degrees. An included angle at such a station would be around 170 or 175 degrees, an awkward number that mostly consists of “nearly straight ahead”.
A deflection angle records only the change — the departure from straight. So instead of writing 175°, you write 5°, which is the information that actually matters.
Deflection angles must be labelled left or right, since a line can deflect either way from the prolongation.
Note that this connects directly to highway engineering: the deflection angle is exactly the quantity used in setting out horizontal curves, where the curve’s length and radius follow from it.
All Five Compared
| Method | What Is Measured | Accuracy / Use |
|---|---|---|
| Loose needle | Magnetic bearing at every station | Least accurate; also called free needle |
| Fast needle | Magnetic bearing of the first line only, plus included angles | More accurate than loose needle; generally preferred |
| Included angles | Bearing of one line plus all included angles | Most commonly used; more accurate than fast needle |
| Direct angles | Angles to the right | Generally used in open traverse |
| Deflection angles | Angle from the prolongation of the receding line | Open traverses for roads, railways, canals, pipelines, sewers |
The Accuracy Ranking
Loose needle < Fast needle < Included angles
Both comparative statements are given explicitly in the syllabus and are frequently examined. The progression follows a single logic — the less you depend on the magnetic needle, the more accurate the traverse.
- Loose needle: needle used at every station.
- Fast needle: needle used once, at the start.
- Included angles: needle used once, and the angular work is organised so that all directions derive from theodolite measurements throughout.
Quick Revision Notes
- Five methods: loose needle, fast needle, included angles, direct angles, deflection angles.
- Loose needle establishes the magnetic meridian at every station; also called the free needle method.
- Fast needle establishes the meridian only at the starting station.
- In fast needle, the first line has compass accuracy but differences of adjacent bearings have theodolite accuracy.
- Fast needle is more accurate than loose needle and is generally preferred in the field.
- Method of included angles is the most commonly used and is more accurate than fast needle.
- An included angle is one of the two angles formed at a station by the two traverse lines meeting there.
- Clockwise angles are interior if the traverse runs counterclockwise, and exterior if it runs clockwise.
- All angles are measured clockwise, because theodolite graduations increase clockwise.
- Direct angles are angles to the right, generally used in an open traverse.
- Deflection angles are the angles a line makes with the prolongation of the receding line, used for roads, railways, canals, pipelines and sewers.
Mistakes Students Commonly Make
- Swapping loose and fast needle. Loose uses the needle at every station; fast uses it once.
- Assuming “fast” refers to speed of work. It refers to the needle being held fast, that is, not re-established at each station.
- Reversing the interior and exterior rule. Counterclockwise traverse gives interior angles when measured clockwise.
- Forgetting why angles are measured clockwise — because theodolite graduations increase clockwise.
- Defining a deflection angle from the forward line. It is measured from the prolongation of the receding line.
- Omitting the alternative name free needle method for the loose needle method.
- Getting the accuracy order wrong. It is loose < fast < included angles.
Conclusion
Five methods, ranked by one question — how often do you trust the needle? The loose needle method consults it at every station and is accordingly the least accurate. The fast needle method uses it once, so the shape of the traverse is set by theodolite angles rather than magnetic bearings. The method of included angles refines that further and is the one most commonly used. Direct angles and deflection angles then serve open traverses, with deflection angles particularly suited to roads and railways where each station brings only a small change of direction.
Frequently Asked Questions
What are the five methods of angular measurement in traversing?
The loose needle method, fast needle method, method of included angles, method of direct angles, and method of deflection angles.
What is the loose needle method?
A method in which the direction of the magnetic meridian is established at each traverse station and the magnetic bearing of each line is measured there. It is also known as the free needle method.
What is the fast needle method?
A method in which the magnetic meridian is established only at the starting station and the magnetic bearing of the first line is measured. All other bearings are determined indirectly from that bearing and the included angles.
Why is the fast needle method more accurate?
Because only the first line depends on the compass. The difference of bearings between adjacent lines has the accuracy of the theodolite, so the shape of the traverse is far more accurate than a compass alone could give.
Which method is most commonly used?
The method of included angles, which is also more accurate than the fast needle method.
What is an included angle?
One of the two angles formed at a station by the two traverse lines meeting there.
When are clockwise angles interior and when exterior?
They are interior angles if the traverse is run counterclockwise, and exterior angles if the traverse is run clockwise.
Why are all angles measured clockwise?
Because the graduations on a theodolite increase in the clockwise direction, so readings increase as the instrument is turned and no negative angles arise.
What is a deflection angle?
The angle which a line makes with the prolongation of the receding line, recording only the change in direction rather than the full angle between lines.
Where is the method of deflection angles used?
Mainly for open traverses in the survey of roads, railways, canals, pipelines and sewers, where the traverse lines make only small deflection angles.