In this article, we will discuss
What is Tacheometric Survey | Principle of Tacheometric surveying | Tacheometric Surveying Methods and Techniques | The Advantages of Tachometer-Based Surveying | Applications of Tacheometric Survey
What is Tacheometric Survey
Tacheometric surveying is a type of angular surveying in which a tachometer measures the distance between two horizontal and vertical points. So, the time-consuming process of measuring horizontal distance was done away with chain surveying.
Chain or tape surveying isn't as good as tacheometric surveying. It's faster and easier, especially on rough terrain like broken ground, deep ravines, swamps, long stretches of water, etc. This method of surveying has an accuracy of between 1/1000 and 1/10000.
Principle of Tacheometric Surveying
The idea behind tacheometric surveying is based on the distance from the base to the top of an isosceles triangle is always the same as the length of the ground.
The formula for calculating horizontal distance is:
H= K*s* cos ² θ
For the same tachometer, the formula for the vertical distance is:
V= (K*s*sin2θ)/2 = Htanθ
s= staff intercept = Top Reading – Bottom Reading
K= Multiplying Constant (generally taken as 100)
θ = Vertical angle on the theodolite
So, after figuring out the vertical distance, you can figure out the reduced level of the instrument station (R.L.), the height of the instrument (H.I.), the central wire reading (R), and the R.L. of any point under observation as follows:
R.L of the Point = R.L of the Instrument Station + H. I ± V-R
Tacheometry Survey History
Tacheometry Survey has been around since the 1600s, but a French engineer named General J.A. Chombart de Lauwe brought it back to life in 1901.
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Tacheometric Surveying Procedure
1. Set up the instrument above the given station, and ensure it is level by looking at the altitude level.
2. Set the vernier on the vertical circle to zero. Then, when the altitude level is in the middle of its run, use a measuring tape to get an accurate height for the instrument (the vertical distance from the top of the peg to the center of the objective).
3. Another way to find out how high something is is to put the stadia rod in front of the telescope and read through the object's glass.
4. Follow the steps below to point the instrument in the right direction:
- You can use either the magnetic or the actual meridian as the reference meridian.
- Set one of the verniers to zero when the reference meridian is magnetic, then turn the telescope around its vertical axis while loosening the bottom clamp until the compass needle points north.
- To orient, the instrument with reference to the true meridian, the correct bearing of a reference point or another station of the traverse with respect to the first station must be known.
- Set the vernier to read this bearing, and then turn the telescope around its outer axis until the station or reference object is in half.
5. While holding the staff on the benchmark, measure the bearing, the vertical angle, and the top, bottom, and axial hairs (the line of sight may be horizontal or inclined).
6. If there isn't a local benchmark, fly level from the nearest accessible Bench Mark (B.M. ), and a temporary B.M. may be built close to the area.
7. Using the bearings, vertical angles, and staff readings, the instrument station can find all representative locations under its control (to the top, bottom, and axial hairs). These points of view are called "side shots."
8. When all the representative spots from the first station have been found, look at the second station. Write down the angle and the readings on the staff for the top, bottom, and axial hairs.
6. If there isn't a local benchmark, fly level from the nearest accessible Bench Mark (B.M. ), and a temporary B.M. may be built close to the area.
7. Using the bearings, vertical angles, and staff readings, the instrument station can find all representative locations under its control (to the top, bottom, and axial hairs). These points of view are called "side shots."
8. When all the representative spots from the first station have been found, look at the second station. Write down the angle and the readings on the staff for the top, bottom, and axial hairs.
9. Change the instrument to the second station. Set up, center, and level the instrument like before, then measure its height.
10. To get to the first station, take a backsight. Also, pay attention to the bearings, the vertical angle, and how the staff reads the top, bottom, and axial hairs.
11. Since each station is seen twice, two distances and heights are calculated for each station. These must be within legal limits, or the operator must be redone.
Tacheometric Surveying Methods and Techniques
The horizontal distance between an instrument station "A" and a staff station "B" is determined by the angle subtended at point "A" by a known distance at point "B" and the vertical angle from point "B" to point "A."
Tacheometric surveying can be further divided into two types of techniques:
1. Stadia Method of Tacheometry
The stadia method of tacheometry is a common way to figure out horizontal distance and vertical height.
With this method, you only need one observation from the instrument station to figure out the horizontal distance between the staff station and the instrument station and the height of the staff station along the line of sight of the instrument.
This method of surveying can be further classified into the following two types:
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i. Fixed Hair Method
In this type of surveying, a telescope with two extra crosshairs, one above and one below the center crosshair, is used to make observations.
So, stadia hairs are all the same distance from the center hair.
If you look through the instrument's telescope, you can see that the stadia hairs stop at a certain length of the staff.
This is the tachymetric surveying method that is used most often.
ii. Movable Hair Method
The movable hair method can move the crosshairs on the telescope attached to the instrument. This is different from the stadia hair method.
You can also use the center hair to keep the cross-hairs from moving.
This way, the stadia interval can be changed for different staff positions. Then, the horizontal distance is worked out. The tool is used with two targets at fixed distances.
2. Tangential Method of Tacheometric Surveying
Tangential tachymetric surveying requires two observations between the staff station and the instrument station. This is how you determine the horizontal distance and height difference between the collimation line and the staff station.
The most significant benefit of this method is that it can be done with an average transit theodolite. Because the work goes so slowly, this method is used less often than the stadium system.
Errors in Tacheometric Surveying and How to Avoid Them
The following are reasons why the tacheometric survey often gets things wrong.
1. Instrumental Errors
Instrumental errors can result from inaccurate graduations on the stadia rod or harmful permanent modifications to the instrument.
2. Manipulation and Sighting Error
The surveyor's skill and speed determine how much error is caused by manipulation and sight. These are caused by lousy centering and leveling of the instruments and wrong readings from the stadia.
3. Natural Causes
Wind, uneven expansion of instrument parts, visibility, and different refraction are all natural causes of errors. This last one is the most important.
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The Advantages of Tachometer-Based Surveying
The following are some of the advantages of tacheometric surveying:
- A survey like this is done in a short amount of time.
- Tapes and chains are no longer needed for the process. Because of this, it takes less time.
- It is a cheap way to measure land.
- This method is accurate enough to be used, among other things, to make topographical maps, do hydrological surveys, and check measurements from other methods.
Challenges of a Tacheometric Survey
Tacheometric Survey faces particular challenges, and they are discussed below.
- It takes a lot of practice, which is the key to being a good surveyor.
- It's not very good at figuring out how far away something is on the horizontal plane.
- It only works well when the staff can see each other simultaneously.
- When using a tripod, the geometry angle is not given. Because of this, the staff's height at station B must be known ahead of time.
- It's hard to get a good measurement of the required vertical angle.
- It is hard to measure the horizontal distance from the staff to the object because the height of the team at station B is unknown or hard to figure out.
- It can't be used if trees, cliffs, or buildings are in the way.
- It can only be used with a 90° single-level instrument, so arc angles can't be measured.
- To be effective, large amplitudes of change are required.
- It is not suitable for use in high-wind areas or on slick ground.
- It cannot be used in areas where the staff is insufficient, or the equipment is weather-damaged.
- It cannot be used to survey over long distances, but it is instrumental in field areas where quick and easy survey procedures are required.
Applications of Tacheometric Survey
Tacheometric surveys can be used in different areas. They include
- Topographic survey- It detects and delineates ground features such as clearings, roads, and vegetation.
- Hydrology determines runoff, erosion, infiltration, and groundwater recharge.
- Tracing of airfields- It detects obstacles that may endanger traffic.
- Environmental assessment- It can be used to determine changes in land use that would result in altered water resources, such as surface runoff, infiltration and recharge, watershed hydrology characteristics, and changes in groundwater levels