Weisbach Method of correlation :

Weisbach Method of correlation :

The weisbach method of correlation is usually preferred when only one shaft is available. 

The 

method consists of suspending two plumb wises which form a plumb plane and azimuth of the 

same is determined at the surface and taken U/G at the shaft inset and thereafter transferred to 

the U/G base or reference line.

Two plane wires A & B made of anticorrosive crucible steel 0.1 cm diameter having 

breaking strength of 150” g and to avoid king on a small drums fitted with brake or ratchet to hold 

them in any desired position are lowered down the shaft tin washers are passed down by the 

wires from the surface & they are free of obstruction. The smaller plummets are then replaced by 

symmetrically shaped finned plumb bobs made of lead. When the wires hang vertically each 

plumb bobs immersed in a barred of water to cut down oscillation due to (i) simple pendulum 

motion, (ii) Irregular mass in strata, (iii) air current, (iv) dropping water. The longer the wire is left in 

suspension it will reduce the oscillation and will eventually come to rest. It may be necessary to 

raise the plumb bob from time to time to allow for the stretch of the wires.

From the surface traverse, station R and R1 fixed as 

surface base RR1 whose azimuth is very carefully determined. 

The theodolite is set at T1 at surface as closed to the near 

wire B as focusing will allow and almost in the with the plumb 

plane AB produce and connecting to the surface base. The 

small angle BT1A should be only few minutes of arc. The 

triangle ABT1 is known as weisbach triangle.

To measure the small angle BT1A1 the theodolite is 

setup at T1 and all the temporary adjustments are made. The 

telescope is directed to wire B and the angle BT1R is 

observed. In the usual manner, taking at least three round of 

readings on each face of the instrument the mean value is the 

correct value of the angle, In a smaller manner the value of 

angle AT1R is obtained and difference between the two 

angles thus obtained is the correct value of the angle BT1A.

Continuous Azimuth Method :

Let it is required to make a closed theodolite traverse of the lines joining the section points A B C 

D E.

The instrument is setup as station A and usual temporary 

adjustments are made for centering leveling & removing parallel.

The body is clamped and the vernier clamp is released the 

instrument is turned in a clockwise direction to sight the fore station 

and the vernier is clamped & the station point is accurately bisect by 

the vernier tangent. Now record the bearing of line AB.

If the bearing of the cone AB is already know be fore hand, the 

vernier reading B to be set according before taking any sight from 

station B.

The instrument is next setup at B. the reading on the vernier is 

the fore bearing of AB. Body clamp B and station is bisected using 

the body clamp is tightened in this position.

The telescope is now transited, vernier circulated and station C is sighted. The vernier 

clamp is tightened & final bisection of the station point being obtained by means of vernier tangent

screw. The instrument now records the bearing of BC directly.

Thus when similar operations are replaced at each station the forward azimuth of each 

draft is automatically recorded by the instrument.

To check accuracy of work after the traverse the instrument should be set up at station A 

and the bearing of the first draft AB should be taken whether is give the correct known bearing.

Advantages :

1. Bearing are obtained directly.

2. Field work is expendious.

3. Calculation are simplified.

4. Booking on simple.

5. Checking for magnetic bearing is possible at any iron free station.

Disadvantages :

1. Any accidental shifting of the instrument at any stage affect the reading recorded.

2. double transisting at each station gives rise to error if the collimation adjustment is not 

perfect.

3. The amount of error being twice the error is collimation.

4. If a closing angular error is discovered these is no sure way to ascertain whether it is 

cumulative.

Double foresight method :

In this method the bearing of initial line and the horizontal 

clockwise angle between adjacent lines are measured. The 

Azimuth of all other lines are reduced by calculate.

Let it is required to make a closed traverse of the line joining 

station A B C D E by this method.

The instrument is set up at A and usual temporary 

adjustments are made. The A vernier is set at zero & the instrument 

is directed towards the meridian as shown by the compass 

provided with body clamp and tangent. The body is clamped and 

the vernier is released. The telescope is now directed towards 

station B and final bisection of the station is made by the vernier 

tangent screw. The bearing of the line AB recorded.

The instrument is set up at B. The temporary adjustment having been made the telescope 

is sighted to station A. Both plates being clamped, the two vernier are read and reading recorded. 

The usual practice is to set the A vernier to zero, through this is not essential.

Advantages :

1. any error in the line of collimation does not seriously affect the result.

2. Any number of observation of each angle by the method of repetation.

Disadvantages :

The only draw back to this method for regular and extensive work is that it adds to the 

burned of office computation, which however is often warranted by the higher degree of accuracy 

attainable.

Precise Magnetic correlation :

Principle : In this method the magnetic bearing of surface & U/G base line are determined and 

then a traverse is carried out from each to a single plum wire suspended in the shaft. The 

magnetic azimuth of each line is determine and the difference applied to the grid bearing of the 

surface base will give the grid azimuth of the U/G base. The traverse will give the co-ordinate of 

the U/G.

Procedure : The method used the tabular compass attached to the theodolite to determine the 

magnetic bearing of the lines. Two theodolites will be used. One on the surface and the other U/G. 

The location of the surface base line and the U/G base line have got to be chosen with great care.

If it is all possible the surface base line should be vertically above the U/G base. The station 

where the observation are to be taken must free from local disturbances. On the surface it is fairly 

easy to see whether there are disturbing influences very difficult to find around which is free from 

local attraction due to pipe lines, power cables and use of steel below ground.

The time of observation should be chosen carefully because of the vibration of the 

magnetic needle.

The two theodolites are set up on the stations and magnetic observation must be taken 

simultaneously on the surface & U/G the observation are replaced at fixed interval graphs are 

plotted of the observation. The following day the position of the two theodolites is reversed and the 

observation repeated.

Advantages:

1. The method entails little of no disorganization of the colliery routine.

2. If there are several base lines in different levels these can all be oriented with little trouble.

3. If sufficient care is taken it gives fairly good result.

Disadvantages:

1. A minor irregularity in the magnetic needle itself after the result serious.

2. The results of the correlation are vitiated if the work is carried out magnetically disturbed 

day.

3. Besides the irregular variation, the secure & diurnal variation have direct influence over 

magnetic readings.

4. There is possibility of change in the instrument contact during the period of transference 

form the surface to the U/G base.

5. The stations where the magnetic observations are to be made must be free from local 

attraction.