Tacheometric surveying is a type of angular surveying which involves the measurement of both horizontal and vertical distances by using optical instruments. Tacheometric surveying eliminates the use of traditional chain and tape, instead, tacheometer and stadia rods are used.
The use of a tachometer makes surveying quick and more accurate. The conventional surveying method does linear measurement of the field which makes it a very slow and exhausting process.
Tacheometric surveying could easily be conducted in broken and hilly areas, swamps, areas covered with stretches of water, etc. where traditional chaining is very tedious and inaccurate.
What is the Principle of tacheometric surveying?
Tacheometric surveying is based on the principle of an isosceles triangle, which means there is a constant ratio of the distance of the base from the apex and the length of the base.
What is the Purpose of tacheometric surveying?
A Tacheometric survey is generally conducted for the preparation of contour plans and topographic maps. Reconnaissance survey for highways and railways etc. Establishment of secondary control points.
What are the Instruments used in tacheometric setup?
In a tacheometric set up the following tools and equipment are used:
1. Tacheometer
Tacheometer is a transit theodolite fitted with stadia hairs in the diaphragm. It is the most important part of the tacheometric survey setup.
The telescope provided in a tacheometer consists of two horizontal hairs called stadia hairs along with regular crosshairs. These hairs are equidistant from each other and togetherly termed as stadia lines or stadia webs.
2. Stadia Rods
Tacheometric survey of distances more than 100m cannot be conducted just by using staff there arises the need for Stadia rod. These rods are generally 3-5m in length with the smallest subdivision of 5mm on its scale.
3. Anallactic lens
Lens placed between the object-glass and the eyepiece of the telescope to remove the additive constant are called anallactic lenses. This simplifies the expression for the distance between the instrument station and staff position. This lens is provided in an external focusing telescope only.
Different ways of conducting a tacheometric survey
1. Stadia tacheometry
Stadia tacheometry is the most commonly used method to determine horizontal distance and vertical elevation. The horizontal distance from the instrument station to the staff station and the elevation of the staff station in the line of sight of the instrument is derived with just one observation from the instrument station.
Stadia tacheometer is further divided into two methods:
a. Fixed hair method
In the fixed hair method, stadia hairs are placed at fixed intervals and the intercept on the stadia rod varies based on the horizontal distance between the instrument station and the staff.
When the staff rod intercept is more than the length of the staff, then only half an intercept is read, which must be equal to the difference between the central stadia hair reading and the lower or upper stadia hair reading.
b. Movable hair method: The staff interval in this method is kept at equal intervals by changing the distance between the stadia hairs. Targets on the staff are marked at a known interval and the stadia hairs are required to move to bisect the upper target at the upper hair and the lower target at the lower hair. This method is not commonly preferred because it is inconvenient to measure the stadia interval with accuracy.
2. Tangential method: In the tangential method, two observations are made necessary to the staff station from the instrument station to evaluate the horizontal distance and the difference in the elevation between the staff station and the line of collimation. This survey can be carried out using ordinary transit theodolite which is way cheaper than a tacheometer, which reduces the surveying cost too.
Field notes, distances and elevations deduced don’t differ much in both tacheometric survey methods. But measuring two vertical angles disturbs the setup of the instrument hence this method is not preferred nowadays. The requirement for noting the number of observations makes this a slow process.
Steps to conduct tacheometric surveying
By following the below-mentioned steps, tacheometric surveying can be performed:
Step 1: Tacheometer is set up at the desired point and levelled in accordance with altitude level.
Step 2: Tune the vertical vernier to zero and measure the length between the top of the peg to the centre of the object-glass using a stadia rod.
Step 3: Align the instrument at the station point with respect to the true meridian or magnetic meridian.
Step 4: Read the staff kept on a nearby benchmark with the theodolite instrument and make note of the vertical angle, the bearings and the readings of the three hairs. In case the benchmark is missing we can move ahead with fly levelling.
Step 5: Representative points are marked with reference to staff reading, bearing and vertical angles.
Step 6: Sight the second station and observe the bearing, the vertical angle and the staff reading of three wires.
Step 7: The instrument should be moved on to the second station and once again the alignment and levelling are done.
Step 8: Sight the third station and observe the bearing, the vertical angle and the staff reading of three wires.
Step 9: Repeat a similar process.
Advantages of tacheometric surveying
Tacheometric Surveying has a few advantages that are:
1. In tacheometric surveying, the backsight and foresight distances are balanced conveniently if the level is equipped with stadia hairs.
2. While levelling and cross-sectioning, stadia is an easy way of finding distances from level to points on which rod readings are taken.
3. The tacheometric surveying method is more rapid than any other method.
4. While surveying with low relative accuracy, where only horizontal angles and distances are required, the tacheometric surveying method gives rapid results.
5. While doing topographic surveys-where both the relative location of points in a horizontal plane and the elevation of these points are desired, tacheometric surveys are most useful.