Levelling

Question 1

What is levelling?

Answer 1

Process of determining elevations (heights) of points of interest above or below a reference datum or
differences in elevations.

Question 2

What are the various heighting methods?

Answer 2

1. Differential levelling
   obtaining of height difference between points by measuring their
   vertical distance from a horizontal line of sight.
2. Trigonometric heighting
   determination of elevation
   differences of lower accuracy
   than spirit levelling. (useful
   where difficult for differential
   levelling to be undertaken)
3. Barometric heighting
   reading air pressure differences from which elevation differences are computed.
4. Gravimetry
   measuring the gravitational potential variation between different points it is possible to
   correlate this to differences in
   heights.
5. Satellite positioning
   drawback is the determination of the separation between the geoid an ellipsoid in areas of interest.

Question 3

Difference between a level line and a horizontal line

Answer 3

Level line
line that is normal to the direction of gravity as shown by a plumbline at any point.
- curved - shape of the earth
- all points are the same height.

Horizontal line
tangential to the level line at a
particular point.
- perpendicular to the direction of gravity.

Question 4

Differenciate between datum, reduced level and benchmarks

Answer 4

Datum:
level surface to which elevations of points may be referenced. eg. Mean Sea Level (MSL).

Reduced level:
elevation (above or below) of a point in relation to the Datum.

Benchmarks
permanent monument or
feature on stable rock for which elevation is known.

Question 5

What is a level surface?

Answer 5

A (curved) surface orthogonal to the plumb line everywhere.

Question 6

Examples of levelling instruments

Answer 6

1. Level, tripod, staff and stape
2. Change plate

Question 7

Types of levels

Answer 7

1. Spirit level
   consists of a sealed tube with liquid and a bubble inside. When the bubble is centered between marked lines, the level is horizontal.
   -carpentry, masonry
2. Automatic level
   optical telescope with an automatic compensator mechanism.
   - construction, surveying
3. Laser level
   emits a laser beam to create a level line or plane, either visible or detectable by a laser receiver.
   - grading land
4. Digital level
   uses electronic sensors and a digital readout to measure height differences.
   - geodetic levelling

Question 8

Levelling procedure

Answer 8

1. A horizontal line of sight is established using:
   - Spirit level tube
   - Swinging pendulum
2. A graduated staff is read through the telescope of the level.
3. The elevation of points can be established by first reading the staff on a bench mark.
4. The staff is then moved to the desired point, the level is turned and the staff is read again.

The reading at the benchmark is called the backsight (BS)
The reading taken after turning the instrument and moving the staff is the foresight (FS)

5. To continue levelling, the staff is kept on the point at A and
   the instrument moved to the midpoint between A and the
   next point, B.

A is called the change point (CP) or turning point (TP).
The staff at A is carefully turned toward the instrument and a
BS reading taken.
Then the staff is moved to B and a FS reading is made.

The procedure is repeated as many times as needed.

The levelling should always end on a BM as a check!

Two note reduction methods for calculating elevations from the BS
and FS observations exist.

a. Height of Collimation method
HC = Elev + BS
Elev = HC – FS

b. Rise and Fall method
Rise (or Fall) = BS – FS
Elev = Previous Elev + Rise (or Fall)

Question 9

Factors that affect levelling accuracy

Answer 9

1. Reading of staff.
2. Bubble not being central.
3. Instrument (level) being out of adjustment. Ensuring that backsights and foresights are equal in length lessens effects of maladjustment.
4. Differential settlement of the tripod.
5. Tilting and settlement of the staff.
6. Sensitivity of the bubble or compensator.

Question 10

Sources of levelling errors

Answer 10

1. EQUIPMENT ERRORS
   (a) Collimation error
   (b) Compensator not working
   (c) Parallax
   (d) Defective staff
   (e) Defective Tripod
2. FIELD/ON-SIGHT ERRORS
   (a) Staff reading error.
   (b) Unstable change point.
   (c) Non‐vertical staff.
   (d) Booking error
   (e) Instrument not level
   (f) Handling the instrument and tripod
3. EFFECT OF CURVATURE AND REFRACTION
   (a) Earth curvature.
   (b) Vertical collimation error in the instrument.
   (c) Temperature relation expansion in the staff.

Question 11

How to reduce levelling errors

Answer 11

1. Levelling should start and finish at known Bench Marks.
2. Where possible, all sight lengths should be below 50m.
3. The staff must be held vertically.
4. BS and FS must be kept equal for each instrument position.
5. Rise and Fall method should be used when heighting controls.
6. HCM should be used when setting out.
7. For Automatic levels,staff readings should be booked immediately they
   are observed.
8. When taking levels across a wide gap such as a river or a ravine
   (where it is impracticable to make backsights equal to foresights),
   reciprocal levelling provides a solution. This technique corrects for
   effects of curvature and refraction.

Question 12

Applications of levelling

Answer 12

1. Establishing vertical control.
2. To establish heights of points during constructions – setting out levels.
3. For contouring purposes.
4. For road crosssection, longitudinal sections/profiles or
   volumes of Earthwork in civil engineering works.
5. For provision of levels of inclined surface during construction.