Methods of Tachometric Survey

 Methods of Tachometric Survey:

Various methods of tacheometry survey are based on the principle that the horizontal distance between an instrument Station “A” and a staff station “B” depending on the angle subtended at point “A” by a known distance at point “B” and the vertical angle from point “B” to point “A” respectively.

This principle is used in different methods in different ways. Mainly there are two methods of tachometry survey.

(1)Stadia system, and

(2) Tangential system.

1. Stadia System of Tacheometry;

In the stadia system, the horizontal distance to the staff Station from the instrument station and the elevation of the staff station concerning the line of sight of the instrument is obtained with only one observation from the instrument Station.

In the stadia method, there are mainly two systems of surveying.

(1) Fixed hair method and,

(2) Movable hair method.

i. Fixed Hair Method:

In the fixed hair method of tacheometric surveying, the instrument employed for taking observations consist of a telescope fitted with two additional horizontal cross hairs one above and the other below the central hair.

These are placed equidistant from the central hair and are called stadia hairs.

When a staff is viewed through the telescope, the stadia hairs are seen to intercept a certain length of the staff and this varies directly with the distance between the instrument and the stations.

As the distance between the stadia hair is fixed, this method is called the “fixed hair method.”

ii. Movable Hair Method:

In the movable Hair method of tacheometric surveying, the instrument used for taking observations consist of a telescope fitted with stadia hairs which can be moved and fixed at any distance from the central hair (within the limits of the diaphragm).

The staff used with this instrument consists of two targets (marks) at a fixed distance apart (say 3.4 mm).

The Stadia interval which is variable for the different positions of the staff is measured, and the horizontal distance from the instrument station to the staff the station is computed.

Note: Out of the two methods mentioned above of tacheometric surveying, the “fixed hair method “is widely employed.

2. Tangential System of Tacheometric Surveying:

In this system of tacheometric surveying, two observations will be necessary from the instrument station to the staff station to determine the horizontal distance and the difference in the elevation between the line of collimation and the staff station.

The only advantage of this method is that this survey can be conducted with ordinary transit theodolite.

As the ordinary transit theodolite is cheaper than the intricate and more refined tacheometer, so, the survey will be more economical.

So, as far the reduction of field notes, distances and elevations are concerned there is not much difference between these two systems.

But this system is considered inferior to the stadia system due to the following reasons and is very seldom used nowadays.

This involves the measurement of two vertical angles, and the instrument may get disturbed between the two observations.

The speed is reduced due to more number of observations and the changes in the atmospheric conditions will affect the readings considerably.

The staff used in this method is similar to the one employed in the movable hair method or stadia surveying. The distance between the targets or vanes maybe 3-4 m.