Tacheometry is a division of angular surveys where the horizontal and vertical distances are obtained through optical means, as opposed to the normal chain and tape operation. It is achieved with the help of two different styles of instruments-theodolite transit and Stadia string. On the other side, other traditional survey techniques, such as chain surveying or traverse surveying, allow that the surveyor takes a linear field calculation by a tape or chain. Such methods are generally sluggish and often tiresome.

Different Methods of Tacheometric Measurements

• Stadia System.
• Tangential system.

Stadia System of Tacheometric Surveying

In the stadia method, the horizontal gap from the instrument station to the staff station and the staff station height referring to the instrument’s line of sight is achieved with just one measurement from the Field device.

• Fixed hair method and,
• Movable hair method/Subtense Method.

Fixed hair method: In this process, the angle at the instrument at A subtended by a known short distance along workers held at B is rendered with the aid of a stadium diaphragm having stadium wires at a set or constant distance apart. Therefore, the intercept of staff which implies the discrepancy in the readings relating to the top and bottom stadium wires will depend on the distance of the stadium/level workers from the tacheometer.

Whether the intercept of workers is greater than the staff range, the only half intercept would be read. Tachometry is the most popular form and the same ‘stadia form’ usually applies to this method.

Movable hair method: This system is exactly the same as the stadia approach except that the length of the stadia is variable. A reasonable provision is created to adjust the gap between the Hair of the stadia in order to match it against the two goals on the workers held at the observation level.

Thus, in this situation, the intercept of workers, i.e., the gap between the two goals is held constant while the period of stadia, i.e. the gap between the hair of stadia, is variable. Inclined sights can also be taken as in the case of fixed hair form.

Tangential System of Tacheometric Surveying

In this tachymetric survey method, two measurements from the instrument station to the staff station would be required to evaluate the horizontal gap and the disparity in elevation between the collimation line and the staff station.

The main drawback of this approach is that this test will be performed using ordinary transit theodolite. Because the ordinary transit theodolite is cheaper than the complicated and more sophisticated tachometer, the test would be more economical.

Thus, as far as the reduction of field observations, distances and elevations are concerned, the disparity between these two methods is not important. But for the following purposes, this system is deemed inferior to the stadia method and is quite seldom used today.

This includes calculating two vertical angles, so the device can be interrupted during the two measurements. Due to further tests, the pace is decreased and the variations in ambient conditions can greatly influence the readings. The workers used in this process is identical to the one utilized in the stadia surveying system of mobile hair. The distance can be 3-4 m between the goals or vanes.

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.

Even though the Tacheometric surveying method is not very accurate owing to the above-mentioned errors and the method has few advantages. This method is employed in location surveys of lines of communications and precautions have to be taken for the effective use of this method.

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