Levelling needs only two pieces of equipment — something to give a horizontal line of sight, and something to measure against it. Those are the level and the staff.
This post covers both, along with the telescope’s optics and the sensitivity of the bubble tube that makes the whole thing work.
The Two Instruments
- A level
- A levelling staff
The Level and Its Four Parts
The purpose of a level is to provide a horizontal line of sight. It consists of four parts:
| Part | Function |
|---|---|
| Telescope | To provide the line of sight |
| Level tube | To make the line of sight horizontal |
| Levelling head | To bring the bubble into its centre of run |
| Tripod | To support the instrument |
Reading the List as a Sequence
The four parts describe a chain of purpose. The tripod holds everything steady; the levelling head adjusts the instrument until the bubble centres; the level tube confirms by that centred bubble that the sight is horizontal; and the telescope then delivers that horizontal sight to the staff.
Note the phrase centre of run — the middle position of the bubble in its tube. As established in the theodolite chapter, the axis of a level tube is horizontal precisely when the bubble sits centred, because a bubble always rises to the highest point of a curved tube.
The Four Chief Types of Level
- Dumpy level
- Y level (also called the wye level)
- Reversible level
- Tilting level
1. Dumpy Level
The name originated from the fact that this level was formerly equipped with an inverting eyepiece, and was hence shorter than a Y level of the same magnifying power — hence “dumpy”, meaning short and stout.
Advantages of the dumpy level over the Y level:
- Simpler construction with fewer movable parts
- Fewer adjustments to be made
- Longer life of the adjustments
Why These Three Advantages Are Really One
Read them together and they form a chain of cause and effect: fewer moving parts → fewer adjustments needed → those adjustments hold longer.
Every movable joint in an instrument is a place where wear can occur and alignment can drift. Reducing the number of them makes the instrument inherently more stable, which is why the dumpy level became the standard for ordinary work.
2. Y Level
The essential difference between the dumpy level and the Y level is that in the dumpy the telescope is fixed to the spindle, while in the Y level the telescope is carried in two vertical “Y” supports.
The Y level has an advantage over the dumpy in that the adjustments can be tested with greater rapidity and ease.
The Trade-off
Here is a genuine engineering compromise, and both sides are defensible.
The telescope in a Y level rests in open supports and can be lifted out and turned end for end. That makes checking its adjustment very quick — reverse it, and any error reveals itself immediately.
But the same removability means the telescope is not rigidly fixed. More movable parts, more chance of drift, more adjustments needing attention. The dumpy sacrifices easy testing for a construction that needs testing less often.
| Dumpy Level | Y Level | |
|---|---|---|
| Telescope | Fixed to the spindle | Carried in two vertical Y supports |
| Movable parts | Fewer | More |
| Adjustments needed | Fewer, and they last longer | More |
| Testing adjustments | Slower | Quicker and easier |
3. Reversible Level
The reversible level combines the features of both the dumpy level and the Y level — an attempt to gain the dumpy’s rigidity together with the Y level’s ease of testing.
4. Tilting Level
In a tilting level, the line of sight and the vertical axis need not be exactly perpendicular to each other. It helps in quick levelling, and is mainly designed for precise levelling work.
Why Relaxing a Condition Improves Accuracy
This seems contradictory at first — how can an instrument be more precise by being less strictly aligned?
The answer is that a tilting level separates two jobs that other levels combine. In a dumpy level, you must make the vertical axis truly vertical before the line of sight will be horizontal, so the whole instrument has to be levelled carefully for every sight.
A tilting level lets you level the instrument only approximately, then tilt the telescope alone until its own bubble is centred, immediately before each reading. The line of sight is made horizontal directly, at the moment it is used, rather than indirectly via the vertical axis.
That is why it is both quick and suited to precise work — the horizontality of each individual sight is guaranteed independently.
The Levelling Staff
There are three forms of self-reading staff:
- Solid
- Folding
- Telescopic
Graduations and Length
| Property | Value |
|---|---|
| Smallest division | 0.01 ft or 5 mm |
| Fine graduations on some staffs | Up to 2 mm |
| General length | 10 ft or 3 m |
The term self-reading means the observer reads the graduation directly through the telescope, rather than a staffman moving a target to the line of sight. Folding and telescopic staffs exist purely for transport — a 3 m rigid staff is awkward to carry, so it is made to fold or collapse.
The Telescope
A telescope consists of three parts:
- Objective
- Eyepiece
- Diaphragm
Objective
It is a compound lens called an achromatic lens, consisting of a double convex lens made of crown glass and a convexo-concave lens made of flint glass. With this compound lens both spherical and chromatic aberrations are eliminated.
Why Two Different Glasses
The key is that crown glass and flint glass bend light by different amounts for different colours. Pairing them so that one lens’s colour spread is cancelled by the other’s produces an image free of colour fringing — which is exactly what “achromatic” means.
Eyepiece
| Type | Detail |
|---|---|
| Ramsden’s eyepiece | The most commonly used. Composed of plano-convex lenses of equal focal length, with a spacing of (2/3)F |
| Huygens’ eyepiece | Not commonly used |
Diaphragm
Consists of the cross hairs. In a stadia theodolite, the stadia wires are also included — the two extra horizontal hairs used for tacheometric distance measurement.
Optical Defects of a Single Lens
Aberration is the deviation of the rays of light.
Spherical Aberration
The rays from a given point are not all collected exactly at one point.
Chromatic Aberration
The violet ray is refracted most and the red least. Due to this defect a blurred and coloured image is formed.
The Two Compared
| Spherical Aberration | Chromatic Aberration | |
|---|---|---|
| Cause | The shape of the lens surface | Different colours refracting differently |
| Effect | Rays from one point not collected at one point | Blurred and coloured image |
| Depends on colour? | No | Yes — violet most, red least |
The elimination of aberrations is only one of the requirements in the design of a telescope. Other possible defects in a simple lens are coma, astigmatism, curvature and distortion.
Parallax
If the image formed by the objective is not in the same plane as the cross hairs, any movement of the eye is likely to cause an apparent movement of the image with reference to the cross hairs. This is called parallax.
Parallax makes accurate reading impossible, because the staff graduation apparently cut by the cross hair changes depending on where the observer’s eye happens to be. It is eliminated by the temporary adjustments — focusing the eyepiece first, then the objective.
Temporary Adjustments of a Level
- Setting up the level
- Levelling up
- Elimination of parallax
These parallel the theodolite’s temporary adjustments, with centering absent — a level need not sit over any particular point, since it measures heights rather than angles from a station.
Sensitivity of the Bubble Tube
Sensitivity is the angular value of one division of the bubble tube.
- Generally the linear value of one division is kept as 2 mm.
- A tube is more sensitive if the bubble moves by more divisions for a given change in the angle.
Five Ways to Increase Sensitiveness
- Increasing the internal radius of the tube
- Increasing the diameter of the tube
- Increasing the length of the bubble
- Decreasing the roughness of the walls
- Decreasing the viscosity of the liquid
Why Each One Works
These five sort into two groups, which makes them far easier to remember.
Geometry (items 1–3): a bubble tube is a very shallow arc. Increasing the radius makes it flatter still, so a small tilt moves the bubble much further along the tube. Greater diameter and bubble length similarly increase the movement for a given tilt.
Friction (items 4–5): the bubble must actually move when the tube tilts. Rough walls and a viscous liquid both resist that movement, so the bubble lags behind the true position. Smoother walls and thinner liquid let it respond freely.
Note that the radius is the dominant factor — it is the geometry of the arc that principally determines sensitivity, which is why it heads the list.
Quick Revision Notes
- A level has four parts: telescope, level tube, levelling head, tripod.
- Four chief types: dumpy, Y, reversible, tilting.
- Dumpy: telescope fixed to the spindle; advantages are simpler construction with fewer movable parts, fewer adjustments, longer life of adjustments.
- Y level: telescope carried in two vertical Y supports; adjustments can be tested with greater rapidity and ease.
- Reversible level combines features of both.
- Tilting level: line of sight and vertical axis need not be exactly perpendicular; allows quick levelling; designed for precise work.
- Three forms of self-reading staff: solid, folding, telescopic. Smallest division 0.01 ft or 5 mm, some to 2 mm; general length 10 ft or 3 m.
- Telescope parts: objective, eyepiece, diaphragm.
- The objective is an achromatic lens — double convex crown glass plus convexo-concave flint glass.
- Ramsden’s eyepiece is most common, plano-convex lenses of equal focal length spaced (2/3)F; Huygens’ is not common.
- Spherical aberration: rays from a point not collected at one point. Chromatic aberration: violet refracted most, red least, giving a blurred coloured image.
- Other lens defects: coma, astigmatism, curvature, distortion.
- Sensitivity = angular value of one division; linear value generally 2 mm.
- Sensitiveness increased by increasing internal radius, diameter, bubble length, and decreasing wall roughness and liquid viscosity.
Mistakes Students Commonly Make
- Reversing the dumpy and Y level distinction. In the dumpy the telescope is fixed; in the Y level it sits in Y supports.
- Saying the tilting level is less accurate because its axis need not be perpendicular. It is designed for precise levelling, because each sight is made horizontal directly.
- Swapping the glasses. Double convex is crown; convexo-concave is flint.
- Confusing the aberrations. Chromatic involves colour; spherical does not.
- Saying red is refracted most. Violet is refracted most, red least.
- Giving the Ramsden spacing as F or (1/2)F. It is (2/3)F.
- Saying sensitivity increases with increased viscosity. It increases when viscosity is decreased.
- Including centering among a level’s temporary adjustments. A level needs only setting up, levelling up and elimination of parallax.
Conclusion
A level’s whole job is to hand you a horizontal line of sight, and its four parts deliver exactly that — tripod for stability, levelling head to centre the bubble, level tube to prove the sight is horizontal, telescope to carry it to the staff. The dumpy trades easy testing for rigidity, the Y level does the reverse, and the tilting level sidesteps the problem entirely by making each individual sight horizontal at the moment of use. Behind it all sits the bubble tube, whose sensitivity depends chiefly on the radius of its arc and on how freely the bubble can move.
Frequently Asked Questions
What are the four parts of a level?
A telescope to provide the line of sight, a level tube to make that line horizontal, a levelling head to bring the bubble to its centre of run, and a tripod to support the instrument.
What are the four chief types of level?
The dumpy level, the Y level, the reversible level and the tilting level.
What is the essential difference between a dumpy and a Y level?
In the dumpy level the telescope is fixed to the spindle, while in the Y level the telescope is carried in two vertical Y supports.
What are the advantages of the dumpy level?
Simpler construction with fewer movable parts, fewer adjustments to be made, and longer life of the adjustments.
What is the advantage of the Y level?
Its adjustments can be tested with greater rapidity and ease, because the telescope can be lifted from its supports and reversed.
What is special about a tilting level?
The line of sight and the vertical axis need not be exactly perpendicular to each other, which allows quick levelling. It is mainly designed for precise levelling work.
What are the three forms of self-reading staff?
Solid, folding and telescopic. The smallest division is 0.01 ft or 5 mm, with some staffs graduated to 2 mm, and the general length is 10 ft or 3 m.
What is the objective made of?
It is a compound achromatic lens consisting of a double convex lens of crown glass and a convexo-concave lens of flint glass.
What is the difference between spherical and chromatic aberration?
In spherical aberration the rays from a given point are not all collected exactly at one point, a defect of lens shape. In chromatic aberration the violet ray is refracted most and the red least, producing a blurred and coloured image.
What is parallax in a telescope?
The apparent movement of the image with reference to the cross hairs when the eye moves, caused by the image formed by the objective not lying in the same plane as the cross hairs.
What is sensitivity of a bubble tube?
The angular value of one division of the bubble tube. The linear value of one division is generally kept as 2 mm.
How can the sensitiveness of a bubble tube be increased?
By increasing the internal radius of the tube, increasing its diameter, increasing the length of the bubble, decreasing the roughness of the walls, and decreasing the viscosity of the liquid.