Tacheometry

In Plane Surveying the various methods employed to do the surveying are Chain Surveying, Compass Surveying, Theodolite Surveying, Plane Tabling and Tacheometry or the combination of these. Taccheometry is the method is very fast and simple as compared to the other methods.

Tacheomtry is the one of the indirect method of linear measurements in the plane surveying. Using the Tacheometry we can determine the horizontal and vertical distance by taking some angular measurements with the help of Tacheometer. 

The various instruments used for the Tacheometric Surveying are:

  1. Tacheometer
  2. Metric Staff or Target Staff
  3. Plumb bob
  4. Pegs

1) Tacheometer:  Tacheometer is an instrument provided with a telescope to site the target(Metric staff) in which a stadia diaphragm is provided corresponding to which we get the readings from the metric staff.

The tacheometer generally use the anallactic lens because it makes the calculations easy. If you use the anallactic lens then the multiplying constant of the instrument becomes 100 and the additive constant becomes 0. So one of the constant is eliminated from the upcoming equation and it makes it easy to calculate.

Stadia diaphragm is the plane where the image of the target(Metric Staff) is formed or we can say that we focus the telescope to get the image in this plane. To read the readings of the staff there are three cross hairs provided in this plane corresponding to which we have to read the readings on the metric staff.

So we get three readings corresponding to each cross hair. The difference between the extreme readings is known as the staff intercept.

2) Metric Staff or Target staff :

Metric staff is a rectangular solid mass graduated with the readings, either made of timber or it is made of metal. These days metallic staffs are used. It can be 3m to 5m in length. The least count of a surveying metric staff is 5mm. 

3) Plumb bob : Plumb bob is a solid metallic mass attached with a string. The solid mass is generally a steel mass in conical shape. It is used for the centering of the tachometer.

4) Pegs: Pegs are the generally wooden structures, with a rectangular/square(40mm*40mm) cross section and a length minimum of 30cm. One end is made sharp to insert into the ground. It is used to locate the station/base point on the ground. 

Theory

If D is the horizontal distance between any two points P and Q and V is the vertical distance then we can get the values by the following formulae:

a) When the line of sight is horizontal, D = Ks +C   

b) When the line of sight is inclined and staff is vertical,

D=K.s.Cos2­­(vertical angle) + C.cos(vertical angle)
V= K.s.(sin2(vertical angle))/2 + C.Sin(vertical angle)
K= Multiplying constant
D= Horizontal distance between P and Q.
C= Additive constant
V = Vertical distance between P and Q.
s= Staff intercept

Vertical angle = It is the angle measured in degrees which the line of sight of the instrument makes with the horizontal.

Procedure of tacheometry

Suppose we have to find the horizontal and vertical distance between two points P and Q on the ground. The following procedure is adopted  to get them:

  1. One Surveyor will set the tachometer at P, do all the temporary adjustment of the instrument.
  2. One Surveyor will hold the Metric staff at the point Q.
  3. Sight the metric staff from P to Q, note the staff intercept and also the vertical angle of the line of sight of the instrument.
  4. After getting the required readings i.e. staff intercept and the vertical angle, put them in the formulae for getting the distances. 

Note: For anallactic telescope,  K=100 and C= 0

So the calculations are easy.

If we do not know the values of the constant for any instrument then we have to calculate them by using some field methods. 

Conslusion

After reading this article you must know the use of Tacheometry, various instruments used, and the formulae and the procedure for getting the final distances between two points.  I will put more on this topic. If you need further help on this topic, please left the comments.


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