Direct and Indirect Ranging

 Measurement of distances with Chain and Tape on ground with Direct and Indirect Ranging

 

Principle of Chain Surveying:          

The principle of chain surveying is derived from principle of triangulation. The area to be surveyed is divided into a number of triangles and the sides of the triangle are measured in the field. To get good results in plotting, the framework should consist of triangles which are as nearly equilateral as possible. Preferably all the sides of a triangle should be nearly equal having each angle nearly 60^o to ensure minimum distortion due to errors in measurements of sides and plotting.

 Theory:     

The process of establishing intermediate point on a straight line between two end points is known as ranging. Ranging must be done before a survey line is chained. It may be necessary to establish a number of intermediate points prior to chaining when chain line is much longer. Ranging may be done by direct observation by the naked eye or by line ranger or by theodolite. Generally, ranging is done by naked eye with the help of three ranging rods.

Ranging is of two kinds:

1.   Direct Ranging

2.   Indirect or Reciprocal, Ranging

1.   Direct Ranging

When intermediate ranging rods are fixed on a straight line by direct observation from end stations, the process is known as direct ranging. Direct ranging is possible when the end stations are intervisible.

Direct ranging can be done by two methods:

1.   By Eye

2.   By Line Ranger 

Procedure:

A)    Ranging by Eye:

1.   Consider two points A and B at the end of survey line which is spaced at a distance greater than one tape length.

2.   For ranging by eye judgment, ranging rods are fixed at station A and station B.

3.   The surveyor stands half a meter distance back of ranging road at A in line with AB.

4.   The assistant then moves the ranging road under guidance of the surveyor in such a way that the ranging rod held by assistant is in line with AB at point ‘C’ between A and B.

5.   Similarly other intermediate points are located by eye judgment. In this way, ranging by eye finally brings the intermediate points in a straight line with respect to station A and B.

       B)    Ranging by Line Ranger:

 

For more accurate ranging, instruments like line rangers are used. Line ranger consists of either two plane mirrors or two right angled isosceles prisms, silvered to reflect the incident rays. A handle is provided to hold the ranger in hand which also helps in transferring correct location of intermediate station on ground.

1.   Fix the ranging rods at P and Q at fixed distance.

2.   Surveyor holds the line ranger approximately very near to the line between two ranging rods already fixed at P and Q so as to obtain the intermediate points R in line PQ , as shown in figure.

3.   The surveyor moves with line ranger across line PQ then the upper prism receives rays from Q and lower prism receives rays from P. the images of these ranging rods at P and Q can be seen through the face GH of the line ranger. The images of these two ranging rods may not be coinsiding indicating that the line ranger is not in line with PQ

4.   Then surveyor with the line ranger moves forward  and backward in perpendicular direction to the line PQ till the images of two ranging rods at P and Q are in a straight line or coinciding with each other as shown in figure.

5.   When images of two ranging rods P and Q coincided, that instant a small pebble is dropped from the lower end of handle on ground, giving the exact point ‘R’ in a line with PQ.

6.   In this way the intermediate point ‘R’ is located in a line with PQ, such that three points P, R and Q are in a straight line.

2. Indirect or Reciprocal Ranging:

Indirect ranging is used when the end stations are not intervisible due to high ground or a hill or if the ennds are too long. Insuch cases, intermediate points can be fixed on the survey line by a process known as reciprocal ranging.

Procedure: 

1.   Let A and B be the two stations with a rising ground or hill.

2.   Let two chainmen with ranging rods take up positions at M1 and N1, such that, chainmen at M1 can see both rods at N1 and B and the chainmen at N1 can see the ranging rods at M1 and A.

3.   The chainman at N1 directs the chainmen at M1 to shift the ranging rod at M2 in line with A.

4.   Then chainman at M2 directs the chainmen at N1 to shift the ranging rod to N2 in line with B.

5.   The positions of ranging rods will be changed until finally they are both in line with A & B exactly on line AB.

Conclusion:        

1. Direct ranging is possible when the end stations are intervisible.

2.  Direct ranging can be done by Eye ranging or by Line Ranger.

3.  Most of the ranging operation in surveying is done by eye ranging.

4. But for more accurate ranging, instruments like line rangers are used.

5. Indirect ranging is used when the end stations are not intervisible due to high ground or a hill or if the ends are too long.