Methods of working out levels on a job site

Methods of Levelling: Height of Collimation MethodRise and Fall Method 

Collimation Method: It consist of finding the elevation of the plane of collimation ( H.I.) for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.

Rise and Fall Method: It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.

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Collimation Method;

It consist of finding the elevation of the plane of collimation ( H.I.) for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.

  1. Elevation of plane of collimation for the first set of the level determined by adding back side to R.L. of B.M.
  2. The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.]
  3. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P. From the second position of the level to the R.L. C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check
    Sum of B.S. – sum of F.S. = last R.L. – First R.L.
  • This method is simple and easy.
  • Reduction of levels is easy.
  • Visualisation is not necessary regarding the nature of the ground.
  • There is no check for intermediate sight readings
  • This method is generally used where more number of readings can be taken with less number of change points for constructional work and profile levelling.​

2.Rise and Fall Method;

It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.
Arithmetic check
Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L.
This method is complicated and is not easy to carry out. Reduction of levels takes more time.
Visualization is necessary regarding the nature of the ground. Complete check is there for all readings.
This method is preferable for check levelling where number of change points are more.


Basic definitions

Bench Mark and Reference Datum
In order to calculate the heights of points a datum is required, i.e. a reference level. This is usually the mean sea level. For this purpose, the use of Bench Marks is necessary, and these are classified as follows:Bench Mark (BM) – a point with known height above mean sea level (or other reference datum). These are permanent points (e.g. unchanged by weather conditions) and are provided by the Department of Lands and Surveys.

Reduced Level
The height of any target point is referred to as Reduced Level (RL), because it is reduced to a known datum.

Back-sight (BS)
First staff reading taken immediately after setting up the instrument.

Foresight (FS)
last staff reading taken before moving the instrument to another location.

Intermediate sight (IS)
All readings taken between a BS and a FS.

Common sources of errors in levelling
1. Instrument not correctly levelled.
2. Telescope not correctly focused.
3. The wrong cross-hair reading recorded (e.g. top instead of middle).
4. Staff incorrectly read or not held vertical.
5. Staff incorrectly booked.

All the above are blunders cannot be corrected unless the work is repeated!


Picture

​Levelling operations

Now consider Figure 2. The level is set up as shown, and using the staff at points A and B, height readings are recorded. This is just the height read through the telescope horizontal line of sight (known as line of collimation). If no reduced level is known only the difference in height can be found between A and B, not their absolute levels.

Staff Reading at A is 1.135m
Staff Reading at B is 1.875m

If we know that RLA = +120.000m (above datum), then RLB = 120.00 – 0.740 = +119.260m i.e. a fall
from A. If RLB was known we would calculate a rise in level. Hence, the following can be defined:

Rise – staff reading is less than previous reading.
Fall – staff reading is greater than previous reading.


Experiment (Rise & Fall method):

Aim:
To determine the required level of given points by Rise and fall method

Apparatus Required:
Automatic level, Tripod and Levelling staff

Picture

Procedure:

  1. Set up the leveling instrument at Level position 1.
  2. Hold the staff on the Datum (RL+50 m) and take a reading. This will be a
    backsight, because it is the first staff reading after the leveling instrument has
    been set up.
  3. Move the staff to A and take a reading. This will be an intermediate sight.
  4. Move the staff to B and take a reading. This also will be an intermediate sight.
  5. Move the staff to C and take a reading. This will be another intermediate sight.
  6. Move the staff to D and take a reading. This will be a foresight; because after
    this reading the level will be moved. (A changeplate should be placed on the
    ground to maintain the same level.)
  7. The distance between the stations should be measured and recorded in the
    field book (see Table 1)
  8. Set up the level at Level position 2 and leave the staff at D on the changeplate.
    Turn the staff so that it faces the level and take a reading. This will be a
    backsight.
  9. Move the staff to E and take a reading. This will be an intermediate sight.
  10. Move the staff to F and take a reading. This will be a foresight; because after
    taking this reading the level will be moved.
  11. Now move the level to Leveling position 3 and leave the staff at F on the
    changeplate.

Now repeat the steps describe 8 to 10 until you finished at point J.


Picture

​Field procedures for leveling
For vertical control the level survey should start and close on points of known height (same point or different). If the survey starts and closes on the same point (e.g. a TBM), as below, this is termed as a closed level survey.

​RL(TBM) = +430.000m Instrument stations: 1, 2, 3.
Target points: TBM, A, B, C, D, E, F.



Picture

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