Theodolite work has a vocabulary of its own, and much of it sounds interchangeable until you look closely. Transiting and swinging are different motions. Face left and telescope normal are related but not identical. Lining in and balancing in differ by a single condition.
This post works through every definition, with the distinctions made explicit.
1. Centering
The process of setting up the instrument exactly over the station mark. A plumb bob suspended from a small hook attached to the underside of the inner spindle is used for precise centering.
Note where the hook is — beneath the inner spindle. That position is not arbitrary. The inner spindle defines the vertical axis of the instrument, so a plumb line hanging from it falls exactly along that axis. When the bob sits over the station mark, the vertical axis passes through the mark, which is precisely what centering means.
2. Horizontal Axis
Also called the trunnion axis or transverse axis. It is the axis about which the telescope can be rotated in a vertical plane.
3. Vertical Axis
The axis about which the telescope can be rotated in a horizontal plane. It is also known as the azimuth axis.
The Crossover Worth Fixing in Mind
| Axis | Plane of Rotation | Other Names |
|---|---|---|
| Horizontal axis | Vertical plane | Trunnion, transverse |
| Vertical axis | Horizontal plane | Azimuth |
An axis is named for its own orientation, not for the plane in which movement happens. Rotation always occurs perpendicular to the axis — think of a wheel spinning about a horizontal axle, with the wheel itself standing vertically.
4. Line of Sight
The imaginary line passing through the intersection of the cross-hairs of the diaphragm and the optical centre of the objective. It is also known as the line of collimation.
Two fixed points define it — the cross-hair intersection at the eye end and the optical centre of the objective lens at the far end. Because both are fixed within the telescope, the line of sight is a permanent property of the instrument, and the whole telescope simply points it wherever you aim.
5. Axis of Level Tube
- A line tangential to the longitudinal curve of the level tube at its centre.
- The axis of the plate level is horizontal when the bubble is centred.
The second statement is what makes a bubble tube useful at all. A bubble always rises to the highest point of the curved tube, so it sits at the centre only when the tangent there is horizontal. Centring the bubble is therefore a way of making a line horizontal by inspection — no measurement required.
6. Face Right
When the vertical circle of a theodolite is on the right hand side of the observer, the position is called face right and the observation made is called a face right observation.
7. Face Left
When the vertical circle is on the left hand side of the observer, the position is called face left and the observation is a face left observation.
Note: by taking the mean of both face readings, the collimation error is eliminated.
Why Averaging the Two Faces Works
This is one of the most valuable ideas in theodolite work, and the principle is simple.
Suppose the line of collimation is not exactly perpendicular to the horizontal axis — it is off by a small angle. On face left that error pushes every reading one way; flip to face right and the same physical error now pushes readings the opposite way, by exactly the same amount.
Take the mean and the two equal and opposite displacements cancel exactly. The error has not been measured or repaired — it has simply been arranged to cancel itself. This is the same logic that makes the fore-and-back-bearing check work in compass surveying: an observation taken twice in opposite senses reveals or removes what a single observation would hide.
8. Telescope Normal
The telescope is said to be normal or direct when its vertical circle is to the left hand side of the observer and the bubble is up.
9. Telescope Inverted
The telescope is said to be inverted when its vertical circle is to the right hand side of the observer and the bubble is down.
Normal and Inverted versus Face Left and Right
| Vertical Circle | Bubble | |
|---|---|---|
| Face left | Left | — |
| Face right | Right | — |
| Telescope normal (direct) | Left | Up |
| Telescope inverted | Right | Down |
The face definitions mention only the vertical circle. The normal and inverted definitions add a second condition — the position of the bubble. That extra condition is what distinguishes the pairs, and it is exactly the detail examiners test.
10. Transiting
Also called plunging or reversing. It is the operation of revolving the telescope by 180° in a vertical plane about its horizontal axis, thus making it point exactly in the opposite direction.
This is the defining capability of a transit theodolite, and the reason non-transit instruments became obsolete.
11. Swinging the Telescope
- Revolving the telescope in the horizontal plane, about its vertical axis, is called swinging the telescope, or simply swing.
- A right swing means clockwise rotation.
- A left swing means anticlockwise rotation.
By taking the mean of the left swing and right swing observations, the effects of error due to friction or backlash in the moving parts is eliminated.
Transiting versus Swinging
| Transiting | Swinging | |
|---|---|---|
| Plane of motion | Vertical | Horizontal |
| About which axis | Horizontal axis | Vertical axis |
| Amount | Exactly 180° | Any amount |
| Also called | Plunging, reversing | Swing |
| Eliminates | (Via changing face) three instrumental errors | Friction and backlash errors |
These two are the most commonly confused pair in the whole chapter. Transiting flips the telescope end over end; swinging turns it round like a lighthouse.
12. Changing Face
- The operation of bringing the telescope from the face left condition to the face right condition, and vice versa.
- The face is changed by plunging the telescope and swinging it by 180°.
Note What Changing Face Requires
Changing face is not a single motion. It combines both of the movements just defined — first transit the telescope, then swing it through 180 degrees.
Both are necessary. Transiting alone would point the telescope backwards; swinging 180 degrees afterwards brings it back to the original target, but now with the instrument in the opposite face.
The Three Errors Eliminated by Changing Face
- Error due to the line of collimation not being perpendicular to the horizontal axis.
- Error due to the horizontal axis not being perpendicular to the vertical axis.
- Error due to the line of collimation not being parallel to the axis of the altitude level.
This list is heavily examined, and there is a pattern that makes it easier to hold. All three are errors of perpendicularity or parallelism between the instrument’s axes. None concerns the ground, the target or the observer — they are purely faults in how the instrument itself is put together.
And that is exactly why changing face cures them. A geometric misalignment fixed within the instrument reverses its effect when the instrument is reversed, so the mean of two faces removes it.
13. Double Sighting
The process of measuring a horizontal angle or a vertical angle twice — once with the telescope in the normal condition and once with the telescope in the inverted condition.
Double sighting is changing face put to work. Rather than merely being able to change face, the observer deliberately measures on both so the mean can be taken and the three errors removed.
14. Lining In
The process of establishing intermediate points with the help of a theodolite on a given straight line whose ends are intervisible.
15. Balancing In
The process of establishing intermediate points with the help of a theodolite on a given straight line whose ends are not intervisible.
One Word Apart
Read the two definitions side by side and they are word-for-word identical except for a single “not”.
| Lining In | Balancing In | |
|---|---|---|
| Ends of the line | Intervisible | Not intervisible |
| Everything else | Identical — establishing intermediate points on a straight line with a theodolite | |
This is exactly the same distinction met earlier between direct ranging (ends intervisible) and indirect ranging (ends not intervisible). The problem is the same; only the instrument has changed from ranging rods to a theodolite.
The name balancing in hints at the method — the observer works between two intermediate points, adjusting each in turn until both settle onto the true line, balancing one against the other. That is the theodolite version of repeated alignment.
All the Definitions at a Glance
| Term | Meaning in Brief |
|---|---|
| Centering | Setting the instrument exactly over the station mark |
| Horizontal axis | Trunnion or transverse axis; telescope rotates in a vertical plane about it |
| Vertical axis | Azimuth axis; telescope rotates in a horizontal plane about it |
| Line of sight | Line through cross-hair intersection and optical centre of objective; the line of collimation |
| Axis of level tube | Tangent to the longitudinal curve of the tube at its centre |
| Face right / face left | Vertical circle on the observer’s right / left |
| Telescope normal | Vertical circle left, bubble up |
| Telescope inverted | Vertical circle right, bubble down |
| Transiting | Revolving the telescope 180° in a vertical plane; plunging or reversing |
| Swinging | Revolving the telescope in the horizontal plane; right = clockwise |
| Changing face | Plunging the telescope and swinging it 180° |
| Double sighting | Measuring an angle twice, once normal and once inverted |
| Lining in | Intermediate points on a line whose ends are intervisible |
| Balancing in | Intermediate points on a line whose ends are not intervisible |
The Three Error-Elimination Facts
Three separate statements about eliminating errors appear in this topic, and they are frequently mixed up:
| Take the Mean Of | Eliminates |
|---|---|
| Both face readings | Collimation error |
| Left swing and right swing | Friction and backlash in the moving parts |
| Changing face (three listed errors) | Collimation not perpendicular to horizontal axis; horizontal axis not perpendicular to vertical axis; collimation not parallel to altitude level axis |
The unifying idea behind all three is the same: make the observation twice in opposite senses, and any error that reverses with the instrument will cancel in the mean.
Quick Revision Notes
- Centering uses a plumb bob hung from a hook beneath the inner spindle.
- Horizontal axis = trunnion/transverse axis, gives vertical plane rotation.
- Vertical axis = azimuth axis, gives horizontal plane rotation.
- Line of sight = line of collimation, through the cross-hair intersection and the optical centre of the objective.
- Axis of plate level is horizontal when the bubble is centred.
- Face right = vertical circle on the right; face left = on the left.
- Mean of both faces eliminates collimation error.
- Telescope normal = circle left, bubble up; inverted = circle right, bubble down.
- Transiting (plunging, reversing) = 180° in a vertical plane about the horizontal axis.
- Swinging = rotation in the horizontal plane about the vertical axis; right swing is clockwise.
- Mean of left and right swing eliminates friction and backlash errors.
- Changing face = plunging + swinging 180°, and eliminates three axis-alignment errors.
- Double sighting = measuring an angle twice, once normal and once inverted.
- Lining in = ends intervisible; balancing in = ends not intervisible.
Mistakes Students Commonly Make
- Confusing transiting with swinging. Transiting is vertical; swinging is horizontal.
- Saying changing face is just plunging. It requires plunging and then swinging through 180°.
- Omitting the bubble condition from telescope normal and inverted. That condition is what separates them from face left and face right.
- Swapping lining in and balancing in. Remember: balancing in is the harder case, where the ends cannot see each other.
- Attributing friction and backlash elimination to changing face. That comes from the mean of left and right swing.
- Listing only two errors eliminated by changing face. There are three.
- Saying a right swing is anticlockwise. Right swing is clockwise.
- Confusing the axes with their planes of rotation.
Conclusion
The vocabulary of theodolite work is built on a handful of distinctions worth getting right. Transiting flips the telescope vertically; swinging turns it horizontally; changing face needs both. Face left and right depend only on which side the vertical circle sits, while normal and inverted add the bubble’s position. Lining in and balancing in differ solely by whether the two ends of the line can see one another. And running through nearly all of it is one principle — observe twice in opposite senses, take the mean, and errors built into the instrument cancel themselves out.
Frequently Asked Questions
What is centering in theodolite work?
The process of setting up the instrument exactly over the station mark, using a plumb bob suspended from a small hook attached to the underside of the inner spindle.
What is the difference between the horizontal and vertical axes?
The horizontal axis, also called the trunnion or transverse axis, is the axis about which the telescope rotates in a vertical plane. The vertical axis, also called the azimuth axis, is the axis about which it rotates in a horizontal plane.
What is the line of collimation?
The imaginary line passing through the intersection of the cross-hairs of the diaphragm and the optical centre of the objective. It is also called the line of sight.
What is face left and face right?
Face left is when the vertical circle is on the observer’s left hand side, and face right is when it is on the right. The observations made are called face left and face right observations.
What error is eliminated by taking the mean of both faces?
The collimation error.
What is transiting?
The operation of revolving the telescope by 180 degrees in a vertical plane about its horizontal axis, so that it points in exactly the opposite direction. It is also called plunging or reversing.
What is swinging the telescope?
Revolving the telescope in the horizontal plane about its vertical axis. A right swing is clockwise rotation and a left swing is anticlockwise.
What is eliminated by taking the mean of left and right swing observations?
The effects of error due to friction or backlash in the moving parts.
How is the face changed?
By plunging the telescope and then swinging it through 180 degrees.
Which errors are eliminated by changing face?
Error due to the line of collimation not being perpendicular to the horizontal axis, error due to the horizontal axis not being perpendicular to the vertical axis, and error due to the line of collimation not being parallel to the axis of the altitude level.
What is double sighting?
Measuring a horizontal or vertical angle twice, once with the telescope normal and once with it inverted.
What is the difference between lining in and balancing in?
Both establish intermediate points on a straight line using a theodolite. In lining in the ends of the line are intervisible, while in balancing in they are not.