A tacheometer is the instrument used in the tachymetric surveying and is a transit theodolite fitted with special stadia diaphragm.
The telescope of the tacheometer contains two horizontal hairs which are called stadia hairs and specially termed as stadia lines and are equidistant from the central cross-hairs.
The telescope fitted with a tachometer must fulfill additional requirements like accuracy, speed, and more refined vertical circles. The power of magnification of the telescope of the tacheometer is higher.
In the tacheometer, the lens system is of better quality and the object-glass is of greater diameter and the power of magnification should not be less than 20 to 25.
For the obtaining of a bright image, the effective aperture should not be less than 3.5 cm to 4.5 cm in diameter and the instrument of the multiplying constant is generally kept as 100.
For close sights, this is used and by rack and pinion for the movement of the eyepiece in the vertical plane, the top and bottom hairs are read and arrangements may be provided. On the vertical circle, it should be possible to take direct readings up to an angle of 5 degrees.
Major Features of Tacheometer:
The nominal value of the multiplying constant should be 100 and the error contained in this should not exceed 1 in 1000 in this value.
From the upper and lower stadia hairs, the axial horizontal line should be equidistant.
The additive constant should be zero and the telescope should be analectic.
The magnification property of the telescope should be powerful.
Methods of Tacheometer Survey:
Based on the principle there are various methods of tacheometer survey such as;
Stadia System of Tacheometer:
With only one observation from the instrument, 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 in the stadia system.
There are mainly two systems of surveying in-stadia system such as;
Fixed Hair Method
Movable Hair Method
Fixed Hair Method:
In the fixed hair method of the tacheometer survey, the instrument is fitted for taking observations to consist of a telescope with two additional horizontal cross hairs one below the central hair and other above the central hair.
From the central hair, these are placed equidistant and are known as stadia hairs.
The stadia hairs are seen to intercept a certain length of the staff when staff is viewed through the telescope and with the distance between the instrument and the stations, this varies directly.
This method is called the fixed hair method because the distance between the stadia hair is fixed.
Movable Hair Method:
In this method, for taking observations, the instrument used is consists of a telescope fitted with stadia hairs and can be moved and fixed at any distance from the central hair.
At a fixed distance the staff used with this instrument consists of two targets and with stadia interval, the different positions of the staff are measured and from the instrument station to the staff station horizontal distance computed.
Tangential system of Tacheometer:
In this system of tacheometer survey, to determine the horizontal distance and the difference in the elevation between the line of collimation and the staff station, two observations will be necessary from the staff station to the instrument station.
With the ordinary transit theodolite, this method can be conducted and due to the following reasons, this system is considered inferior to the stadia system used nowadays.
Procedure for Tacheometer Surveying:
There are the following steps in the procedure of tacheometer surveying as given below;
Over the station set up the instrument and with reference to the altitude level accurately level the instrument.
To zero set the vernier of the vertical circle and with a measuring tape, accurately measure the height of the instrument with the altitude level at the center of its run.
By keeping the stadia rod first in front of the telescope and reading through the object-glass, the height may be found.
On the benchmark hold the staff and take the bearing, read the vertical angle and the top, bottom, and axial hair readings.
By taking the bearings, the vertical angles, and the staff readings all the representative points under the command of the instrument station are located and these observations are known as side shots.
Then take a foresight at the second station and note down the bearings and to the top, bottom, and the axial hairs vertical angle and the staff readings correspond.
Then to the second station shift the instrument and center and level the instrument and measure the height of the instrument.
To the first station take a backsight and to the top, bottom and axial hairs observe the bearings, vertical angle, and the staff reading. For the obtaining distances and elevations of the stations, each station is sighted twice two Values.
Errors and Precautions in Tacheometer:
Errors Due to Instruments:
Due to manipulation and sighting or due to natural causes, imperfect permanent adjustments in the instrument, and incorrect graduations on the stadia rod, the errors may be instrumental errors.
By actual observation, the constants of the instrument given by the manufacturer should be verified in the field when the instrument is in perfect permanent adjustment.
To the observed readings, the graduations on the rod should be carefully examined.
Errors Due to Manipulation and Sighting:
These errors depend on the efficiency and skill of the surveyor and due to inaccurate leveling and centering of the instrument these errors may occur and by these incorrect stadia readings are taken.
There should be no Parallax to view the stadia rod clearly and care should be taken to see that the axial hair is not mistaken for stadia hair while taking stadia hair readings.
By seeing whether the mean of the stadia hair readings is equal to axial hair reading, the accuracy of the stadia hair readings may be checked.
Errors Due to Natural Causes:
Due to wind, unequal expansion of the instrument parts, unequal refraction, and visibility the natural errors may be caused.
Taking readings in mid-day should be avoided and these errors occur when unequal refraction of rays of light passes through different densities of the air layers.
From the ground, the lines of sight should not be within a meter.