Tacheometry Advantages and Disadvantages | Tacheometer Features | Instruments used in Tacheometry | Errors in Tacheometric Surveying | Limitations of Tacheometry

Tacheometry Advantages and Disadvantages: -

Advantages:

1. High Accuracy: Tacheometers are known for their high accuracy in measuring distances, angles, and elevations.

2. No Requirement for Line of Sight: Tacheometers do not require a line of sight between the instrument and the object being measured. This makes them suitable for use in areas where obstacles obstruct the view.

3. Quick and Efficient: Tacheometers are fast and efficient in measuring distances, angles, and elevations. They can measure multiple points in a short period, saving time and effort.

4. Multiple Applications: Tacheometers are versatile instruments that find use in various applications such as civil engineering, construction, mining, and topography.

5. Ability to Measure Slopes: Tacheometers can measure the slope of the ground, making them ideal for contouring and mapping.

Disadvantages:

1. Expensive: Tacheometers can be quite expensive, making them unaffordable for small-scale projects.

2. Require Skilled Operators: Tacheometers require skilled operators who have undergone training in handling the instrument. The accuracy of the measurements depends on the skill level of the operator.

3. Limited Range: Tacheometers have a limited range of measurement, and objects that are outside this range cannot be measured.

4. Require good weather conditions: Tacheometers require good weather conditions to give accurate measurements. Fog, rain, and snow can affect the accuracy of readings.

5. Fragility: Tacheometers are fragile instruments that require special handling and transportation. They can easily break if mishandled, causing costly repairs and downtime.

Tacheometer Features: -

• The multiplying constant should have a nominal value of 100 and the error contained in this value should not exceed 1 in 1000.
• The axial horizontal line should be equidistant from the upper and lower stadia hairs.
• The telescope should be anallactic which means the additive constant should be zero.
• The telescope should be powerful magnification property.

Instruments used in Tacheometry: -

(i) A tacheometer
(ii) A levelling or stadia rod.

(i) Tacheometry:

A tacheometer 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. The types of stadia diagram commonly used are shown in fig. 

Stadia Diagram

In general, the telescopes used in stadia surveying are of three kinds: 

1.The simple external-focusing telescope which is also known as the stadia theodolite

2.The external-focusing anallatic telescope (Porro’s telescope). This is usually known as the tacheometer.
3.The internal-focusing telescope

(ii) Stadia Rod:
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. Some common patterns of stadia rods are shown in fig. 

Stadia Rod

The term tacheometer is restricted to a transit theodolite provided with an anallatic telescope.

Errors in Tacheometric Surveying: -

The sources of errors in stadia measurements are as follows:

1. Instrumental Errors.

2. Personal Errors.

3. Natural Errors.

1. Instrumental Errors:

(i) Imperfect adjustment of the tacheometer:

This error can be elimi­nated by carefully adjusting the instrument, particularly the altitude bubble.

(ii) Incorrect divisions on the stadia rod:

In ordinary work, this error is negligible but for precise work, the error can be minimised by using the standardised rod and applying corrections for incorrect length to the observed stadia intervals.

(iii) Incorrect value of the multiplying constant (f/t):

This is the most serious source of error. The value of the multiplying constant should be tested before commencing the work by comparing the stadia distances with mea­sured distances during the hours which correspond to those of field-observations.

2. Personal Errors:

(i) Inaccurate centering and levelling of the instrument.

(ii) Non-vertical by of the staff or rod. It may be eliminated by using a plumb-line or a small circular spirit level with the staff.

(iii) Inaccurate Focussing.

(iv) Reading with wrong hair.

The personal errors can be eliminated by applying habitual checks.

3. Natural Errors:

(i) High wind:

The work should be suspended in high wind.

(ii) Unequal refractions:

This error is prominent during bright sunshine and mid-day hours of hot summer days. The work can be suspended under such circumstances.

(iii) Unequal expansion:

The instrument should be protected by an umbrella during hot sun.

(iv) Bad visibility:

It is caused by glaring of strong light coming from the wrong direction.

Degree of Accuracy: -

The error in a single horizontal distance should not exceed 1 in 500, and in a single vertical distance 0.1 m.
Average error in distance varies from the 1 m 600 to 1 in 850.

Error of closure in elevation varies from 0.08 √km to 0.25 √km where km = distance in km. error of closure in a stadia traverse should not exceed 0.055 √P metres, where P = perimeter of the traverse in metres.

Limitations of Tacheometry: -

1. Limited Range: The range of a tacheometer is limited to a few hundred meters, which may not be suitable for larger surveying projects.

2. Weather Conditions: Tacheometer readings may be affected by adverse weather conditions, such as fog, rain, snow, or dust, which can limit the accuracy of the measurements.

3. Line of Sight: The tacheometer must have a clear line of sight to the target. Obstacles such as trees, buildings, or terrain can obstruct the line of sight and prevent accurate measurements.

4. Environmental Interference: Electromagnetic interference, such as radio waves or nearby power lines, can interfere with the accuracy of tacheometer readings.

5. Skill Level Required: Tacheometers require a skilled operator who is familiar with the equipment and the principles of surveying. Inexperienced operators can result in inaccurate measurements and errors in the data collected. 

6. Cost: Tacheometers can be expensive, and the cost of maintenance and repairs can also add up over time. This may not be feasible for small-scale projects or those on a limited budget.

Que. What is Tacheometer?

Ans - transit theodolite fitted with special stadia diaphragm is known as tacheometer.

Que. Tacheometer is used for

• 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.

Que. Instrument used in tacheometric surveying

Ans - (i) A tacheometer

         (ii) A levelling or stadia rod.

Que. Tacheometer and Theodolite difference

Ans - theodolite measures angles while a tachometer measures speeds.

Que. What is a tacheometer called?

Tacheometry is also known as Telemetry and Tachemetey.

Read More -

What is Tacheometer | Types of Tacheometer | Principle of Tacheometric Surveying | Methods of Tacheometer | Tacheometer Uses | Tacheometer Procedure.