TACHEOMETER AND ITS  FEATURES – ALL YOU NEED TO KNOW

Tacheometry is a branch of surveying in which the horizontal and vertical distances are determined by angular observations with a tacheometer, the chaining operation being altogether eliminated Tacheometry is not as accurate as is changing, but it is far more rapid in rough and difficult country where levelling is tedious and chaining is both inaccurate and slow.

Thus it is best suited when obstacles such as steep and broken ground, deep ravines, stretches of water or swamps are met with Tacheometry is mainly used while preparing contour plans and traversing and is also suitable for hydrographic surveys, location surveys of roads, railways, etc. It is also sometimes employed for small surveys in which elevations are not determined.

A tacheometer is a transit theodolite with a unique stadia diaphragm. It is the primary tool used in tacheometric surveying.

The tacheometer is an instrument that enables one to measure horizontal distances by using a telescope with a magnifying lens.

Tacheometer’s Key Features

A tacheometer must fundamentally include the following features:

1. The nominal value of the multiplication constant shall be 100, and the inaccuracy in this value should not be greater than 1 in 1000.
2. The axial horizontal line should be equidistant between the upper and lower stadia hairs.
3. The telescope should be anallactic, which implies the additive constant should be zero.
4. The telescope’s magnification should be powerful.

Instruments used in Tacheometry:

The main instruments used in tacheometry are:

(i) A tacheometer, and

(ii) A levelling or stadia rod.

• A taceometer in general sense is a transit theodolite having a telescope fitted with a stadia diagram, i.e. a telescope equipped with two horizontal hairs called stadia hairs in addition to the usual central hair. The additional hairs are equipped from the central one and are also known as stadia lines.
• An ordinary levelling staff can be used if the sights are short but in tacheometry since the sights are usually of much greater length, therefore, an ordinary levelling staff cannot serve the purpose. But a specially designed graduated rod known as stadia rod is used. The stadia rod is transport, it may be folding or telescopic. It is 3 to 4m long and 5cm to 15 cm wide. The graduations are bold and clear with a least count usually less than the least count of an ordinary levelling staff, the stadia rods should be as light as possible.

Methods of Tacheometer Survey

There are numerous techniques of tacheometer survey based on the idea, such as the

• Stadia System
• Tangential System.

Stadia System

In the stadia system, the horizontal distance to the staff station from the instrument station and the elevation of the staff station in relation to the instrument’s line of sight are obtained with only one observation from the instrument.

There are primarily two systems for surveying in-stadia systems, which are as follows:

• Method of Fixed Hair
• Method of Movable Hair

Method Of Fixed Hair

The apparatus for taking observations in the fixed hair method of the tacheometer survey consists of a telescope with two additional horizontal crosshairs, one below the center hair and one above the central hair.

These are known as stadia hairs because they are placed equidistantly from the central hair.

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

Because the spacing between the stadia hair is fixed, this method is known as the fixed hair method.

Movable Hair Method

In this method, the device used for taking observations is a telescope furnished with stadia hairs that can be moved and set at any distance from the central hair.

The staff employed with this instrument consists of two targets at a set distance, and at stadia intervals, the different positions of the staff are measured, and the horizontal distance from the instrument station to the staff station is computed.

Tangential System.

To establish the horizontal distance and difference in elevation between the line of collimation and the staff station in this tacheometer survey system, two observations from the staff station to the instrument station will be required.

This approach can be performed using a standard transit theodolite, but it is regarded inferior to the stadia system used today for the following reasons.

PROCEDURE FOR TACHEOMETER SURVEYING:

There are the following steps in the procedure of tacheometer surveying as given below;

1. Over the station set up the instrument and with reference to the altitude level accurately level the instrument.
2. 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.
3. By keeping the stadia rod first in front of the telescope and reading through the object-glass, the height may be found.
4. On the benchmark hold the staff and take the bearing, read the vertical angle and the top, bottom, and axial hair readings.
5. 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.
6. 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.
7. Then to the second station shift the instrument and center and level the instrument and measure the height of the instrument.
8. 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 Tacheometric Surveying:

• The defects may be technical faults attributable to deception and seeing, or natural causes. Imperfect permanent changes in the instrument and incorrect graduations on the stadia rod can trigger the instrumental errors.
• Before starting the research, when the instrument is in full permanent adjustment, the constants of the instrument provided by the manufacturer should be confirmed by actual observation in the area.
• It is particularly relevant for critical surveys where the most significant parameter is precision (and not the time). The graduations on the rod should be closely checked and necessary adjustments should be made to the measured measurements if some difference is found.
• The errors arising from manipulation and spotting rely on the surveyor’s productivity and abilities. Which are induced by inadequate instrument centering and lowering, and erroneous readings of the Stadia?
• There should be no Parallax to clearly see the stadia pin. When taking stadia hair readings, caution can be taken to see if the axial hair is not confused for stadia hair. The precision of stadia hair readings can be tested by seeing if the mean of stadia hair readings is equivalent to axial hair reading.
• Wind, unequal expansion of the instrument components, and visibility and unequal refraction can cause errors due to natural causes. The most critical of these is the second.
• This is attributed to the differential refraction of light rays by moving through various pressure layers of soil. Therefore, in order to prevent this mistake, midday readings will be stopped to the degree practicable.
• The maximum allowable gap in the distance is 1 in 500 to 1 in 900 and in elevations 0.08 to 0.10 m. Always, the sightlines need not be below a meter from the level. The closing error in a traverse of the tacheometry does not surpass 0. LIFE, where p is the meter diameter of the traverse.