Methods of Levelling | Guide to Surveying and Levelling
In this article, we will discuss two important methods of Levelling. We will also study these Methods with the help of Numerical Examples in our successive articles.
There are two Methods of Levelling:
Height of Collimation Method
Rise and Fall Method
Height of Collimation Method
This method is simple and easy.
Reduction of levels is easy.
Visualization 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 levellings.
The horizontal distances are directly measured by the process of stepping.
Procedure
A path of chain or tape is stretched out from ‘P’.
The path length of chain or tape depends on the steepness of the ground.
The follower holds the zero end of the chain at ‘P’ and directs the leader at P1 to be in the line of PQ and stretch the chain or tape above the ground in horizontal line.
Direct Method | Chain Surveying
The leader then transfers the point ‘P1’ to P2 on the ground by means of plumb bob or dropping a pebble or an arrow,
Now the followers take the new position ‘P2’ and directs the leader to move forward and stretch the tape or chain in a line of PQ.
Now the followers take the new position ‘P2’ and directs the leader to move forward and stretch the tape or chain in a line of PQ and the new position is P3.
Again the leader transfers the point P3 to P4 on the ground as done earlier.
This process is repeated till the point Q is reached.
Calculation of Areas in Surveying | Simpson’s Rule
In one of my previous articles, I discussed Midpoint Ordinate Rule and Average Ordinate Rule in detail with an example and listed out various important methods used for the calculation of areas in Surveying. In this article, we will deal with the next important method (rule) i.e. Simpson’s Rule along with a numerical example used for the calculation of areas in the field of Surveying.
Here are the five important rules (Methods) used for the calculation of areas in Surveying:
It states that, sum of first and last ordinates has to be done. Add twice the sum of remaining odd ordinates and four times the sum of remaining even ordinates. Multiply to this total sum by 1/3rd of the common distance between the ordinates which gives the required area.
Where O1, O2, O3, …. On are the lengths of the ordinates
d = common distance
n = number of divisions
Note:
This rule is applicable only if ordinates are odd, i.e. even number of divisions.
If the number of ordinates are even, the area of last division maybe calculated separated and added to the result obtained by applying Simpson’s rule to two remaining ordinates.
Importance of Contouring in the field of Surveying
Contouring is an imaginary line on the ground obtained by joining points having same elevation.
Characteristics of Contours
Contour lines are closed, however they may be close on the map itself or outside the map depending upon the topography.
The spacing between contour lines depends upon the slope of the ground.
In steep slopes, the spacing is small, for gentle slopes the spacing is large.
If the contour lines are equally spaced, they indicate uniform slope.
Contour Analysis
If the contour lines are parallel, straight or equally placed, they represent plane surface.
In a series of contour lines on the plan or map indicates either a hill or depression.
In case of the hill, the values of the elevation go on increasing towards the centre whereas in case of depression, the values go on decreasing towards the centre.
Different methods for the calculation of Areas in the field of Surveying
In this article, we will list out different methods to calculate the areas in Surveying and also study each of the method in depth… We will also explain each method with a suitable example for your better understanding…
Here are the five important rules (Methods) used for the calculation of areas in Surveying:
We will now move on with our discussion on the first rule “Midpoint ordinate rule” with a suitable example.
Midpoint-ordinate rule
The rule states that if the sum of all the ordinates taken at midpoints of each division multiplied by the length of the base line having the ordinates (9 divided by number of equal parts).
Midpoint ordinate rule | Method for calculating area in Surveying
In this, base line AB is divided into equal parts and the ordinates are measured in the midpoints of each division.
Area = ([O1 +O2 + O3 + …..+ On]*L)/n
L = length of baseline
n = number of equal parts, the baseline is divided
d = common distance between the ordinates
Example of the area calculation by midpoint ordinate rule
The following perpendicular offsets were taken at 10m interval from a survey line to an irregular boundary line. The ordinates are measured at midpoint of the division are 10, 13, 17, 16, 19, 21, 20 and 18m. Calculate the are enclosed by the midpoint ordinate rule.
