Before a theodolite can read a single angle, it has to be put in the right place and made ready. That preparation is done afresh every time the instrument is set down, which is why these are called temporary adjustments.
This post covers all five, in the order they must be performed, and explains the practical points that make the difference in the field.
What Temporary Adjustments Are
Temporary adjustments are the adjustments which are required to be made at each setting of the instrument, before taking observations.
The word temporary distinguishes them from permanent adjustments, which concern the instrument’s internal axis relationships and are made only occasionally by a technician. Temporary adjustments are the surveyor’s routine — done at every station, every time.
The Five Adjustments
- Setting up
- Centering
- Levelling
- Focusing the eyepiece
- Focusing the objective
1. Setting Up
- Place the tripod over the required station.
- The legs should be spread so that they make an angle of about 60° with the horizontal.
- The shoes of the legs should be pushed into the ground by applying a force parallel to the legs.
- Take the instrument out of the box, lifting it from the base and not from the telescope, and screw it tightly onto the tripod head after removing the cap provided on the tripod head.
- Adjust the height of the tripod so that the telescope is at the eye level of the surveyor.
- For centering purposes, suspend a plumb bob from the hook beneath the inner spindle.
Three Details That Matter
Why 60 degrees? Legs spread too wide can slip outwards; legs too upright give a narrow base that topples easily. Around 60 degrees balances the two, giving a stable triangle without excessive outward thrust.
Why push parallel to the legs? Because force applied along a leg drives it straight into the ground without shifting the tripod head sideways. Pushing vertically down on a slanted leg would twist the whole setup and disturb the position you have just established.
Why lift from the base, never the telescope? The telescope is mounted on the horizontal axis and carries the delicate optics and cross-hairs. Lifting the instrument’s whole weight by it strains the axis bearings and can disturb the alignment of the line of collimation — introducing exactly the errors that permanent adjustment exists to remove.
2. Centering
Centering is done to place the vertical axis exactly over the station mark.
Approximate Centering with the Tripod Legs
Approximate centering is done with the help of the tripod legs, which are moved either radially or circumferentially.
| Leg Movement | Effect on Plumb Bob | Effect on Levelling |
|---|---|---|
| Radial (in or out along a radius) | Shifts in the direction of the leg movement | Not seriously affected |
| Circumferential (sideways, around) | Does not shift much | Is affected |
Why This Table Is So Useful
This is the most practically valuable piece of knowledge in the whole topic, because it tells you which movement to use for which problem.
The two movements are almost perfectly complementary. Radial movement changes position without spoiling level; circumferential movement changes level without spoiling position.
So a surveyor who needs to shift the plumb bob moves a leg radially, and one who needs to correct the level moves a leg circumferentially. Each adjustment can be made largely independently of the other, which is what makes setting up manageable rather than an endless chase between centering and levelling.
Approximate levelling is checked with reference to a small circular bubble provided on the tribrach.
Exact Centering with the Shifting Head
The exact centering is done by means of the shifting head or centering device:
- The shifting head is loosened.
- The upper plate of the shifting head is slid over the lower one until the plumb bob is exactly over the station mark.
- The screw-clamping ring is tightened after exact centering.
The shifting head allows the instrument to be translated horizontally by a small amount without touching the tripod legs at all — which means the levelling already achieved is preserved. It handles the final few millimetres that leg movement is too coarse to manage.
Centering in Windy Conditions
When there is a windy condition in the field, the plumb bob may start swinging. In that case, centering is done using an optical plummet.
A swinging plumb bob cannot indicate a fixed point — it merely sweeps back and forth. An optical plummet replaces the hanging bob with a small telescope built into the instrument, sighting vertically downward. Since nothing hangs free, there is nothing for the wind to catch.
3. Levelling
- Accurate levelling is done with the help of levelling screws or foot screws, with reference to the plate levels.
- Levelling is done to make the vertical axis of the instrument truly vertical, or to make the horizontal plate truly horizontal.
- The basic approach is to make the axis of the plate bubble horizontal, which happens when the plate bubble is in the centre.
The Chain of Reasoning
Notice how the three statements link together into a chain:
Bubble centred → axis of plate level horizontal → horizontal plate truly horizontal → vertical axis truly vertical
This is why a bubble tube is sufficient to level an instrument. You cannot see whether the vertical axis is vertical, but you can see whether a bubble sits in the middle of its tube — and the geometry of the instrument converts one into the other.
Number of Levelling Screws
Normally there are three levelling screws. However, some theodolites have four levelling screws, though these are not common.
The levelling procedure depends upon the number of levels provided on the upper plate.
Three screws is the standard because three points define a plane uniquely. A fourth screw is geometrically redundant and can even cause strain if over-tightened, since four points cannot all sit perfectly on one plane — which is why the four-screw arrangement never became common.
4. Focusing the Eyepiece
The eyepiece is focused to make the cross-hairs appear sharp and distinct.
This adjustment depends on the observer’s own eyesight, so it must be repeated whenever a different person uses the instrument. It has nothing to do with the target — only with seeing the cross-hairs clearly.
5. Focusing the Objective
The objective is focused to bring the image of the target into the plane of the cross-hairs.
This depends on the distance to the object, so it must be redone for every new sighting.
Why the Eyepiece Comes First
The order of these last two is not arbitrary.
The eyepiece must be focused first so that the cross-hairs are sharp. Only then can the objective be adjusted to bring the target’s image into exactly that same plane. Focus them in the wrong order and the image may sit slightly in front of or behind the cross-hairs — a condition called parallax, where the cross-hair appears to move across the target as the observer shifts their eye.
Parallax makes accurate pointing impossible, and correct focusing order is what prevents it.
The Five Adjustments Summarised
| Step | Purpose | Key Point |
|---|---|---|
| 1. Setting up | Place and secure the tripod and instrument | Legs at 60°, force parallel to legs, lift from the base |
| 2. Centering | Place the vertical axis exactly over the station mark | Radial leg movement shifts the bob; circumferential affects levelling; shifting head for exact centering |
| 3. Levelling | Make the vertical axis truly vertical | Achieved when the plate bubble is centred; normally three screws |
| 4. Focusing the eyepiece | Make the cross-hairs sharp | Depends on the observer’s eye |
| 5. Focusing the objective | Bring the target image onto the cross-hairs | Depends on the distance; removes parallax |
Quick Revision Notes
- Temporary adjustments are made at each setting, before taking observations.
- Five adjustments: setting up, centering, levelling, focusing the eyepiece, focusing the objective.
- Tripod legs spread at about 60° with the horizontal; force applied parallel to the legs.
- Lift the instrument from the base, not from the telescope.
- Tripod height set so the telescope is at the surveyor’s eye level.
- Centering places the vertical axis exactly over the station mark.
- Radial leg movement shifts the plumb bob without seriously affecting levelling.
- Circumferential leg movement does not shift the bob much but affects levelling.
- Approximate levelling is checked with the small circular bubble on the tribrach.
- Exact centering uses the shifting head, loosened and slid, then clamped.
- In windy conditions use an optical plummet instead of the plumb bob.
- Levelling makes the vertical axis truly vertical; achieved when the plate bubble is centred.
- Normally three levelling screws; some instruments have four, but these are uncommon.
Mistakes Students Commonly Make
- Swapping radial and circumferential effects. Radial shifts the bob; circumferential affects levelling.
- Listing four adjustments instead of five, usually by treating focusing as a single step. Eyepiece and objective focusing are separate.
- Saying centering places the instrument over the station. It places the vertical axis exactly over the station mark.
- Forgetting the 60° leg angle or the instruction to apply force parallel to the legs.
- Saying four levelling screws are standard. Three is normal, because three points define a plane.
- Confusing temporary with permanent adjustments. Temporary ones are done at every setting.
- Focusing the objective before the eyepiece, which leaves parallax.
Conclusion
Five steps, performed at every station before a single reading is taken. Set the tripod firmly with legs at about 60 degrees and lift the instrument by its base. Centre it so the vertical axis passes through the station mark, using radial leg movement for position and the shifting head for the final millimetres — or an optical plummet if the wind is up. Level it until the plate bubble sits centred, which makes the vertical axis truly vertical. Then focus the eyepiece for sharp cross-hairs and the objective for a sharp target, in that order, so no parallax remains.
Frequently Asked Questions
What are temporary adjustments?
The adjustments required to be made at each setting of the instrument before taking observations, as distinct from permanent adjustments which concern the instrument’s internal axis relationships.
What are the five temporary adjustments?
Setting up, centering, levelling, focusing the eyepiece and focusing the objective.
At what angle should tripod legs be spread?
About 60 degrees with the horizontal, with the shoes pushed into the ground by applying force parallel to the legs.
Why should the instrument be lifted from the base rather than the telescope?
Because the telescope is mounted on the horizontal axis and carries the optics and cross-hairs. Lifting the full weight by it strains the bearings and can disturb the line of collimation.
What is the purpose of centering?
To place the vertical axis of the instrument exactly over the station mark.
What is the difference between radial and circumferential leg movement?
Radial movement shifts the plumb bob in the direction of the leg movement without seriously affecting the levelling, while circumferential movement does not shift the plumb bob much but does affect the levelling.
How is exact centering achieved?
By loosening the shifting head and sliding its upper plate over the lower one until the plumb bob is exactly over the station mark, then tightening the screw-clamping ring.
What is used for centering in windy conditions?
An optical plummet, because a suspended plumb bob starts swinging in the wind and cannot indicate a fixed point.
What does levelling achieve?
It makes the vertical axis of the instrument truly vertical, or equivalently makes the horizontal plate truly horizontal. This is achieved when the plate bubble is centred.
How many levelling screws does a theodolite normally have?
Normally three. Some theodolites have four, but these are not common, since three points are sufficient to define a plane uniquely.
Why is the eyepiece focused before the objective?
So that the cross-hairs are sharp first, allowing the objective to then bring the target image into exactly that same plane. Focusing in the wrong order leaves parallax, where the cross-hair appears to move across the target as the eye shifts.