Archive for category Surveying and Levelling
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.
Chain Surveying or Linear Surveying | Surveying and Levelling
It is the method of land surveying in which only linear measurements are made.
Chain surveying requires chain, tape and ranging rods.
Chain surveying is not suitable for large areas having many details.
(The term “details” means a natural or manmade features at or near the ground surface).
It consists of the following:
Hard details include buildings, roads, walls etc.
Soft details include river, vegetation, trees etc.
Overhead details include power and telephone lines.
Underground details include survey of water mains, sewer etc.
Principles of Chain Survey
A triangle is a simple figure which can be plotted from the lengths of three sides even if the angles are not known.
In chain survey, the area to be surveyed is divided into a framework consisting of triangles.
Local Attraction | Methods for Correcting the bearings affected by Local Attraction
The deflection of a magnetic needle from its true position due to the presence of magnetic influencing material such as iron ore, magnetic rock, underground pipeline, electric cables, iron pipes, electric poles in its vicinity is called “Local Attraction”.
Methods of Correcting the bearings
There are two methods of correcting the bearings affected by local attraction:
- Included angle Method
- Error Computation
Included angle Method
In this method, the included angles of the traverse are calculated first, then starting from the line which is unaffected by local attraction and using the included angles, the corrected bearings of the traverse are computed.
Error Computation Method
In this method, the direction and the amount of local attraction at each survey station is determined.
Then starting from the line which is unaffected by local attraction, the corrected bearing of the traverse are computed.
This method is more accurate than the included angle method.
Hence it is adopted by most of the surveyors.
Methods of Ranging in Chain Surveying | Guide to Surveying and Levelling
In measuring a survey line, the chain has to be laid out on the ground between the stations.
If the line is short, the chain could be put in alignment easily but if it is long or the end station is not clearly visible, then intermediate points has to be established in line with end points to know the directions of the line by ranging.
Types of Ranging
There are two types of ranging:
- Direct ranging
- Indirect ranging
Direct ranging is possible when the stations are intervisible.
Ranging is done by eye-judgement. Ranging rods are erected vertically beyond each end of survey line.
The surveyor stands 2m beyond the ranging rod while the assistant folds the ranging rod vertically in the intermediate stations.
The ranging rod is held roughly in line by the thumb and fore-finger.
The surveyor directs the assistant to move the rod to the left or right until the three ranging rods appear to be in a straight line.
To avoid errors due to the ranging rods not being vertical, the lower end of the rod are cited for alignment.
A practical Example for Calculation of the True Area of the field | Errors in Chain Surveying
In my previous article, we studied how to measure true distance considering the Errors occurring in Surveying.
Correction formulas to be kept in mind (for incorrect length of Chain)
True distance = L’/L*measured distance
True area = (L’/L)2*measured area
True Volume = (L’/L)3 * measured volume
Where, L’ = incorrect length of chain
L = correct length of chain
The chain was tested before starting the surveying and was found to be 20m. At the end of surveying, it was tested again and was found to be 20.12m. The area of the plan of the field drawn to a scale 1cm = 6m was 50.4sqm. Find the true area of the field in sqcm.
Corrections due to incorrect length of chain, Errors in Chain Surveying, Guide to Chain Surveying, Measure the true area in Chain Surveying, Measure True Volume in Chain Surveying, Meaure True distance in Chain Surveying, True Area of the field
Chaining on Sloping Ground | Surveying and Levelling
There are two methods for determining horizontal distance on sloping ground.
- Direct Method
- Indirect Method
Direct Method of Chain Surveying
This method is also known as “Stepping Method”.
The horizontal distances are directly measured by the process of stepping.
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.
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.
Horizontal distance PQ = S1 + S2 + S3 + S4 + S5
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