What is Tacheometric Survey? Tacheometric Survey Instruments and Uses

Tacheometric Survey Definition

Tacheometry Survey is a branch of surveying that deals with various measures such as vertical and horizontal distances that are measured indirectly using just instrumental observation.

The vertical angle contained between two well-defined places on the staff and the known vertical distance between them are used to calculate the horizontal distance.

Alternatively, readings of the staff indicated by two permanent stadia wires in the telescope’s diaphragm (reticle). The height difference is calculated by subtracting the angle of depression or elevation of a given point on the staff from the horizontal distance previously acquired.

The azimuth angle is calculated in the same way as before. Thus, one observer at the instrument determines all of the measurements required to find a point both vertically and horizontally with reference to the place where the tacheometer is centered, with no help other than a man holding the level staff.

It is most suited when faced with challenges such as steep and broken land, deep ravines, stretches of water, or marshes.

Tacheometry is primarily employed in the preparation of contour plans and traversing, but it is also appropriate for hydrographic surveys, road and railway location surveys, and so on. It is also used for minor surveys when altitudes are not specified.

Tacheometry Survey Principles

The idea of tacheometry is based on the property of isosceles triangles, which states that the ratio of the distance of the base from the apex to the length of the base is always constant.

In other words, the isosceles triangle has the same ratio of base to height above ground level to that of base to height below ground level. The trick used is to use this property while finding more than one vertical line on the map.

Tacheometry Survey History

Tacheometry Survey in terms of its development dates back to the 16th Century but has been revived by a French engineer General J.A. Chombart de Lauwe in 1901.

Initially, he discovered that surveyors used about 6 different laws to calculate heights. In 1926, Chombart de Lauwe created the ‘universal method’ of tacheometry i.e., VL, RVP/LP and SP methods were combined to give a single formula for all-terrain types and was published.

He also published in 1927, ‘Le Traverse a Plan et Liaut’ which became standard tacheometry text book for reference.

Tacheometric Survey Instruments

Tacheometric Surveying instruments include:

Tacheometer

The tacheometer is used primarily for determining the horizontal distance between two objects.

Compass

The compass, when attached to a wooden frame, is placed in a fixed position and then circles continue to be traced on the map as the instrument is moved over the ground.

Level

The level, placed on a firm base, is used to ensure that it remains level while being moved from one point to another by means of a rope or chain.

Stadia Rod

A simple measuring device consisting of two straight rods with a level at the tip.

Transit Circle

A transit circle, which is used to find the distance between two objects, consists of a small stone bearing the hour circle and a vertical cross.

Anallatic lens

Anallatic lens, also known as a retro refractive lens, is also used to find the distance between two objects. It consists of a lens with a small hole at the bottom and is placed on the ground in front of an object.

Tacheometry Survey Uses

Tacheometry survey can be utilized in several ways:

1. Forecasting the length of the path to be traveled in steep terrain or by a vessel in shallow water, or detecting drainage lines.
2. Plotting wireless radio towers on high ground to avoid interference from neighboring stations and provide good reception from them.
3. Monitoring changes in land use along with subsequent effects on hydrological conditions, such as soil erosion and sedimentation of reservoirs, affecting transportation routes and riverside properties.
4. Mapping surface movements of glaciers and counting their surface protuberances in areas where maps are not available or are unreliable.
5. Plotting various targets by the military in rough terrain, including artillery and rocket positions, ammunition dumps, and telecommunication center
6. Surveying and mapping mountain areas by mapping their natural features.
7. Map compilation in places where maps may not be available or are unreliable such as the deserts of Morocco and Saudi Arabia

8. It can be used as the establishment of secondary control points.

9. Tacheometry is a reconnaissance survey used on highways, roads, and railroads.

Tacheometry Surveying Methods

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” correspondingly.

This theory is applied in many manners in various ways.

Tacheometry surveys are primarily conducted in two ways.

Stadia System

The horizontal distance to the staff Station from the instrument station and the elevation of the staff station in relation to the line of sight of the instrument are acquired in the stadia system with just one observation from the instrument Station.

In the stadia method, there are mainly two systems of surveying.

• fixed hair method- In this method, the hair at the staff station is adjusted to the length of object.
• movable hair method – The hair of the staff station is leathered over a fixed point and then moved to the required elevation at which second cut tree or old nail is fixed.

Tangential System

In this method most modern surveying equipment is used to measure horizontal distance between two points. A ladder is used to measure the height of the staff at a point.

Then using a tangent line, the horizontal distance between two staffs is computed by measuring the angle formed by the tangent line and given distance; hence vertical angle can be ascertained at a given station.

Tacheometric Survey Procedure

The standard procedure of carrying out Tacheometric Survey is:

1. Using the survey equipment, a sight line is established between stations A and B.
2. The angle between the sight line and the base of the stadia rod is measured at station A.
3. Then at station B, a cut object such as a nail or tree limb is placed in front of the instrument with its vertical height corresponding to that of the object at station A. The distance from staff to object and from staff to cut object is measured using a tape or ruler.
4. The angle between the sight line and the cut object is determined using a protractor.
5. The angle at station A is then compared to the angle of tangent as measured at station B.
6. The horizontal distance between points A and B can then be calculated by adding the elevation difference between stations A and B to the horizontal difference between stations A and B.
7. The distance between stations A and B can be determined by measuring the angle formed by the sight line and tangent line.
8. The distance between points A and B can also be determined if a fixed object such as a nail, tree limb or staff is erected at station A and the horizontal distance from the staff at station B is measured.
9. The horizontal distance from point B to the object at station A is then measured and compared to its vertical height at point A. The derived value is the distance between points A and B.
10. If the object is moved until it coincides with the sightline and its height is compared to that of its previous position, the horizontal distance between points A and B can be calculated by adding the curvature of line measured at station B.

Tacheometry Survey Advantages

The advantages are as follows:

1. It can be used in rough terrain where other methods may not be possible
2. It is economical and practical compared to other methods.
3. It can be difficult to find someone who can use the theodolite, the telescope, or other equipment effectively at remote locations, however, tacheometry is easier to learn and use than theodolites or electronic distance meters.
4. It is less expensive than using a satellite positioning system for remote locations and for areas with few surveys marks
5. It is important to know the ground underfoot. No one has a sense of direction when they are lost and have a good sense of direction is essential on any expedition
6. It can be used to find the exact position of an object which is not visible from above or from the sides
7. The surveyor can also be in a comfortable spot where he/she does not have to move around for long periods of time increasing productivity
8. It can be used to survey in areas where there is no power and hence no way to charge the equipment
9. It can be used to monitor changes in land use which subsequently affect water resources. For example, it has been shown that as rain forest gets cleared, the amount of rainfall decreases substantially.
10. It can be used to find extreme altitudes of mountains and hence spot for possible mineral or water resources like glaciers where it may be expected to flow down in the form of rivers.
11. It can be used to survey water courses and hence help in navigation.

Tacheometric Survey Challenges

Tacheometric Survey faces certain challenges and they are discussed below.

1. It requires a fair amount of practice and practice is the key to good surveying
2. It is not very accurate in the measurement of horizontal distance
3. It only works effectively when the lines of sight between the staff are parallel
4. The geometry angle is not given when using a tripod, due to which it is necessary that the height of staff at station B should be known in advance
5. The required vertical angle is difficult to measure accurately.
6. The horizontal distance from staff to the object is difficult to measure because of the height of the staff at station B is not known or is difficult to calculate
7. It cannot be used if there are obstacles in the way like trees, cliffs, and buildings
8. It can only be used with a single level instrument with a range of 90°, i.e., no arc angles can be measured
9. It requires large amplitudes of change in order to be effective.
10. It is not suitable for areas with high winds or for use on slippery ground
11. It cannot be used in areas where the staffs are inadequate or where the equipment is damaged by weather
12. It cannot be used to survey over longer distances but it is very useful in field areas that require quick and easy survey procedures.

Tacheometric Survey Applications

Tacheometric surveys can be used in different areas. They include:

1. Topographic survey- It is used for detection and delineation of features on the ground such as clearings, roads, and vegetation.
2. Hydrology- Its applications include use in determining runoff, erosion, infiltration and groundwater recharge
3. Tracing of airfields- It is used for detection of obstacles that may cause danger to traffic
4. Environmental assessment- It can be used to determine changes in land use which would lead to altered water resources, like surface run off, infiltration and recharge, characteristics of the watershed hydrology and changes in ground water levels.

Tacheometric Survey FAQs

What is stadia method?

a surveying method for determining distances and elevation differences using a telescopic instrument with two horizontal lines via which the markings on a graded rod are also noted

What is the difference between theodolite and tacheometer?

Theodolites are a type of device which allow you to take measurements from a point in space. Tacheometers are another way to do that by using two rods (staffs) instead of one.

How many systems are in Tacheometric measurement?

Tacheometric measurement systems include the following: The many tacheometric survey systems can be classed as follows: The Stadia System (a) Fixed Hair Method of Stadia Method (b) Movable Hair Method of Subtense Method. The tangential system is a type of mathematical system.

Are there any areas in the world where Tacheometric Survey is not available?

Tacheometric survey can be performed anywhere in the world by using two rod methods or by using electronic distance measuring equipment.

What are the errors in Tacheometric surveying?

The inaccuracies might be due to manipulation and sighting, or they could be caused by natural reasons. Instrumental mistakes may occur as a result of faulty permanent modifications in the instrument or inaccurate graduations on the stadia rod.

 Describe the conditions under which the Tacheometric surveying is advantageous?

The following are the advantages of tacheometric surveying over the direct technique of measuring horizontal distances and elevation differences:

The surveying process moves at a breakneck pace. In normal conditions, surveying accuracy is pretty acceptable, and in tough terrain, it is even superior. The cost of surveying is less.

What is the basic problem of using Tacheometric survey?

The basic problem is that tacheometry survey works with relative values, as opposed to absolute measurements.

The spatial relationships of surveyed areas must be known in order for the distance determined by tacheometry to correspond with what would be expected where they are simply measured in the same way but with the use of a different geometrical device.

What are the advantages of tacheometric survey over other methods?

1. It is economical and practical compared to other methods.
2. Theodolites and electronic distance meters are complicated and expensive to use.
3. It is difficult to find someone who can use the theodolite, the telescope, or other equipment effectively at remote locations, however tacheometry is easier to learn and use than theodolites or electronic distance meters.
4. It is less expensive than using a satellite positioning system for remote locations and for areas with few surveys marks

What are the Tacheometric constants?

There are four fundamental constants of tacheometric surveying:

The Stadia System (a) Fixed Hair Method of Stadia Method (b) Movable Hair Method of Subtense Method. The tangential system is a type of mathematical system.

Which type of tacheometer should one use?

A tacheometer with a permanently fixed stadia rod, tangent to the horizon from a level instrument with an arc angle range from 0° to 90° is the most commonly used for survey work.

What is Subtense method in Tacheometric surveying?

When the terrain is undulating, the subtense tachometry approach is used. Tacheometric surveying: Tacheometry is a subset of angular surveying in which horizontal and vertical distances are measured optically rather than using chain and tape.

Which operations are completely eliminated in Tacheometric surveying?

Tacheometry is a subfield of angular surveying in which the horizontal distance from the instrument station to the staff, and hence the vertical distance of a point, is calculated by instrumental observation. As a result, changing is fully eliminated. When compared to chaining, this method is faster but less precise.

Where are the methods of computation and implementation of tacheometry?

Tacheometry computations are performed by trigonometrical (or isochronal) formulae. The implementation is done using a tacheometer.

How do you use a Stadia rod?

In order to use a stadia rod, one must first determine the distance from the instrument towards the point. Stadia rods are used with a surveying instrument that has three cross hairs (this is called a stadia rod).

When sighting on an object at a known distance, an observer adjusts the instrument so that two of the cross hairs bisect each other at that object’s location.

How do you measure horizontal distances?

Stadia rods are used to determine horizontal distances. Two stadia rods are placed parallel with one another, and their centers are both distant from the instrument station.

The rod at the instrument station is called the stadia rod and the two other rods (each of which must be at a different point) are called the tangents.

A third stadia rod, parallel to one of the tangents, can be placed at a known distance from the instrument station. After sighting on a point, the distance from the tangents to the instrument station and the unknown distance to the other stadia rod are obtained.

What is mean grade in Tacheometric surveying?

Mean grade is a mathematical method used to measure horizontal distances.

It is a mathematical conversion between square rods and tacheometric rods. It is a geometric conversion that uses a knowledge of the shapes of tacheometric triangles in order to provide an accurate calculation of distance.