PROCEDURE TO CONSIDER IN CHAIN SURVEY

PROCEDURE TO CONSIDER IN CHAIN SURVEY

Chain survey is of two types

1.                              Open traverse

2.                              Closed traverse

Before starting the survey

1.                              Make a sketch of the area to be surveyed and locate all the prominent objects in it.

2.                              Divide the area included in the sketch into convenient number of triangles and

mark vertices on the ground by wooden pegs

3.                              Determine the magnetic north with the help of magnetic compass

While dividing the area into triangles, make sure that the vertices of the triangles or station points are free from obstacles and are easily accessible.

And that

1.                              The number of stations are as small as possible

2.                              The triangles are equilateral or near equilateral

3.                              The size of triangles are as large as possible and

4.                              The size of triangle as far as possible run along the bounding lines of the area and

close to the objects so that the offset are not too far away.

After these preliminary steps the chain can be started from any of several points selected as stations. The chain is stretched along the line.

Before the chain is pulled forward offsets to all the objects falling within the range of the first chain length are taken with the help of the tape measure and an optical square.

All offsets must be vertical to the chain line. The distances of the offsets from the chain line and from the starting point to the point where the offsets line meets the chain line are measured and recorded in a field book an example of which is given later in this section.

The data so recorded are then plotted on a map of suitable scale.

The preparation of the map completes the chain survey

In chain survey at least five persons are needed

·                     Two persons handle the chain

·                     Two other measure the offsets and

·                     One person prepare the field book

The front man holds one of the handle of the chain the chain and walks from the starting point to the end point.

He carries ten arrows with him. When he walks approximately a chain length the rear man instruct him to stop by shouting ‘chain’.

The rear man then instructs the front man to be on the straight line by aligning him with the objects or the ranging rod fixed at the end of the line.

Kinks if any on the line are removed and the chain is straightened.

The first (front) man then fixes an arrow at the end of the chain.

The men disputed to take offsets finish their work and the field man records the data in the field book.

After this is done the front man pulls the chain forward.

The whole process is repeated until the front man reaches the end of the line.

The rear man goes on collecting the arrows fixed at the end of each length.

The number of arrows in the hands of rear man gives the length of the line measured

The part then moves to the next point and repeat the process.

Let us take an example of an area to be surveyed as given in figure 1

We start from point A and move towards point B. The chain is stretched on line AB with one end of the chain being at point A

The chain is straightened and aligned. An offset aa’ is measured. The distance between Aa and aa’ are recorded in the field book as shown in figure2 .

Similarly distance Ab Ac and bb are also measures. It may be restated here that all offsets distances are measured along straight lines drawn as perpendicular from the objects to the chain line.

For figure 1 there will be five field book records, one each for lines AB, BC, CD, DE and EA in the same way was given to for AB.

The data given in the field book are reduced to scale and the map of the area is drawn

Figure 1           Procedure for Chain Survey

                      

The field book in which all the measurements are recorded has three columns

The central column is unclosed and is intended to record the measurements along the chain line

Starting with the bottom of column, the starting point is noted and then the distances of all points at which offsets are taken are recorded upwards.

The side columns are utilized to record the length of the offsets of the side of the chain line are recorded in the right hand and column and those on the left side in the left hand column.

Figure              2            The field book

Some problems in chain surveying

In chain surveying we measure only the horizontal distances. This is easy to do if the ground is not sloppy. if the ground is sloppy we can use one of the following formula to convert to actual distance

These formulas are

1.                     X = L cos A

Where              X = Horizontal distance

                                    L = Actual length along the slop

                                    A = Slop in degree

2.

           

            Where              h = the height 

L = actual length along the slope

Those who can not work with the above formula ca use that is commonly called measurement in steps

Figure              3.           Measurement instep along sloppy

The method is illustrated in figure 2  

The ranging rod is fixed at point B

With a help of tape the horizontal distance Ab is measured and recorded. The same procedure is done between B and C, and c and D.

If there are obstacles in the way of measurements the following procedures is suggested.

Lets suppose that we have to measure the distance between two objects located in two opposite of the river or on two sides of a pond

If the letter is the case we can measure three perpendicular distances as shown in figure 3

In this figure

            PQ = XY

In case the objects lie on two sides of the river, the method illustrated in figure 3 b can be used.

In case the point B is fixed on the other side of the river

Both A and B fall on the same chain line

AO is drawn perpendicular to AB from A and produced to Q making AO = AQ.

QP is drawn parallel to AB. It is also perpendicular to AQ at Q.

Point P lies in straight line POB

Angle AOB being opposite, are equal and hence AB = PQ

ERRORS IN CHAIN SURVEYING

Most of errors in chain surveying result from the carelessness of the surveyors in handling equipment from the defects in instruments

If instruments are defective errors will be repetitive and will grow cumulatively as work proceeds.

Some of common causes are mentioned below

1.                              Length of a chain is not correct

2.                              Chain is not aligned and stretched

3.                              Chain or tape is not horizontal

4.                              Ranging poles or arrows are misplaced

5.                              Field man did not record the data correctly.

Advantages of chain survey

Chain survey has the advantage of being relatively simple, inexpensive and moderately accurately.

Chain survey can be used for the following purposes

1.                              To show property lines

2.                              To survey small area

3.                              To survey flat area

4.         When time factor is not crucial chain survey is therefore not crucial. Chain survey is a slow process it is therefore not suited for quick surveys unless the area to be surveyed is very small.