‘Tacheometry’ refers to “quick measure” since the Greek root tach means speed. It is a revised instrument/machinery used for an advanced purpose. Fundamentally, tacheometry is a system of surveying dimensions, positions both vertically and horizontally, as well as points on the earth’s surface. All these necessary measurements are determined through tacheometry, with the use of a ‘tacheometer’.
Tacheometer: Important features
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- The nominal value of the multiplication constant should be 100, and its error shouldn’t be greater than 1 in 1000.
- The upper and lower stadia hairs should be equally spaced from the axial horizontal line.
- The additive constant should be 0 as the telescope should be anallactic.
- The telescope must have a strong magnification capability.
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What are the objects of the tachometer?
Following are the objects of a tacheometer:
- Preparation of contoured plans or maps.
- Location survey for railway, roads, reservoir, etc.
- Used in hydrographic surveys.
- Used for checking more precise measurements
Tacheometer: Where and why is it used?
Usually, people take the help of chains, levelling instruments, etc., for the specific measurements of landscapes. The problem arises when the irregularities increase. It is simple and convenient to measure plain lands/fields, but certain regions, with their complex construct, make it challenging for the measurements to be accurate with the help of those instruments.
And therefore, we make use of a tacheometer which essentially is a type of theodolite determining measurements electronically or electro-optically. It is a transit theodolite because its telescope can possibly revolve around the entire axis to complete a revolution.
Chains are cumbersome to work with. Some of the common obstructive places are:
River valleys
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Due to the water and land combined surface around the valleys, levelling instruments other than a tacheometer often fail to measure the uneven earth’s surface.
Undulations
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Fields, terrain, and landscape that are undulating have the appearance of a wave. The terrain is moderately sloping.
Steep slope
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A steep slope is one that is at an extreme inclination and is challenging to climb or measure.
Hilly regions
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Hilly regions are characterised by mountainous, rugged, and severely uneven surfaces that are particularly challenging to measure with chains and other less sophisticated land-measuring tools.
Tacheometer: Mechanism
There are two specific parts of instruments that are necessary to combine and design a full-fledged tacheometer. They are:
- Theodolite
- Stadia/levelling rod
Theodolite
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It comprises a mobile telescope installed to offer angular readouts while rotating around horizontal and vertical axes. These serve to connect the first point seen via the telescope to future sightings of additional points from the same theodolite position by indicating the orientation of the telescope.
Stadia/levelling rod
A graduated rod is used with an instrument that has stadia hairs to measure the distance between the point of observation and the location of the rod by observing the length of the rod covered by the space between the stadia hairs when they are fixed or when they are adjusted to cover a specific interval on the rod.
What are the methods of tachometer survey?
Tacheometric surveying is classified into two methods:
Stadia method
The tacheometry stadia technique is widely used to calculate both horizontal distance and vertical elevation in surveying.
By making only one observation from the instrument station, this method calculates the horizontal distance between the staff station and the instrument station, as well as the elevation of the staff station along the instrument’s line of sight.
There are two types of tacheometry surveys, which are classified as follows:
- Fixed hair method
- Movable hair method
Tangential method
The tangential tacheometric surveying method requires two observations from both the staff station and instrument station. This technique calculates the horizontal distance and the difference in elevation between the collimation line and the staff station.
The main advantage of this method is that it can be carried out using a standard transit theodolite. However, due to its slow working speed, this method is less commonly used compared to the stadia system.
Tacheometer: Uses
- Tachometers are used to prepare contour roads; a road contours a mountain that follows along its edge.
- They are used to survey roads and railways.
- Conducting hydrographic surveys
- Cross-checking already measured distances and
- Establishing a secondary control and confirmation about the measurements.
The usage of a good tacheometer is highly advantageous because it is one of the quickest ways for surveying. In uneven or challenging terrain, tacheometric surveying’s accuracy is generally excellent. Usually, it does not involve any time-consuming tasks that require chains and tapes. Therefore tacheometer surveying is way more efficient in terms of cost versus time.
What are the advantages of Tacheometric surveying?
Tacheometric surveying has several advantages including:
- Its speed of operation.
- This surveying method eliminates the need for tapes and chains, making it a faster process overall.
- The cost of tacheometric surveying is generally low compared to other surveying techniques.
- Despite its speed and low cost, tacheometric surveying can produce precise results, making it useful for tasks such as creating topographic maps, hydrological surveys, and cross-checking measurements obtained using other methods.
What are the disadvantages of a tacheometer?
Following are some of the drawbacks of a tacheometer:
- Instrumental mistakes can result from inaccurate markings on the stadia rod or faulty modifications in the instrument.
- Errors can be caused by manipulation and sighting due to lack of expertise or improper instrument centering and levelling.
- There are many natural sources of inaccuracies such as wind, unequal refraction, visibility, and asymmetrical expansion of instrument components.
FAQs
What method of tacheometry is most common and feasible to use?
The stadia method is the most widely used procedure.
Can the distances be directly read from the instrument?
Yes, the measurements and distances can be read directly from the instrument.
Any usual difficulty faced while using the tacheometer?
The staff is required to be placed perpendicular to the surface to be measured, which is slightly more complicated than holding the instrument vertically.
What are some common errors that can occur during a tacheometric survey?
Errors like manipulation and sighting error, instrumental error and other natural causes such as visibility, environmental factors and unequal refractions.
