Tacheometry or tachemetry or telemetry is a branch of angular surveying wherein horizontal and vertical distances of things are obtained by optical means as opposed to the abnormal gradual means of measurement by tape or chain.
It is best adapted in obstacles because of the steep and damaged ground, deep ravines, stretches of water or swamp, which make chaining difficult or unattainable.
Tacheometry (from Greek, quick measure) is a system of rapid surveying, by which the positions each horizontal and vertical of factors on the earth surface relative to at least one another are determined without using a chain or tape or a separate levelling instrument.
Following are the objects of tachometric surveying as follows:
- Prepare contour maps or plans.
- Effectively used in hydrographic surveying.
- Location survey for roads, railways, reservoirs etc.
- It can be used to test more accurate measurements.
Principle of Tacheometry:
The principle of tachometry is to enable horizontal and vertical distances to be computed from readings on a stadia rod and thus eliminate the chaining operation.
The observations required for the absolute location of a point say P with reference to the instrument station 0,
(1) The bearing of the line OP.
(2) The angle of elevation or depression recorded on the vertical circle of the device.
(3) The readings of three diaphragm hairs on a stadia rod in P seen through a telescope; Top, middle and bottom. Middle child reading is known as axial child reading or central child reading.
Methods of Tacheometry:
Fixed hair method:
In this method, the distance between the two varieties is determined.
Readings are taken on staff to suit in line with all three studios.
A difference of readings is obtained between the top and bottom hair readings, known as staff intercepts.
It may be noted that staff intercept varies with the distance over which the employee is held.
Then the horizontal distance from the equipment to the staff station is calculated.
In modern instrument stadia, hairs are fixed as they give more accurate result as compared to movable hair.
This tacheometry method is almost similar to the stadia method besides stadia interval is variable.
An appropriate arrangement is made to separate the distance between the stadia hair to set them towards the two targets on the staff placed at the level under observation.
Thus in this case, the staff intercept, i.e., the gap between the 2 targets, stays fixed whereas the stadia interval, i.e., the distance between the stadia hair, is variable.
In the case of fixed hair method, inclined areas may also be taken.
As the ‘stadia method’ is probably the most extensively used process in the discipline of tachometric survey.
The Stadia method follows the principle that the ratio of perpendicular to the bottom of a triangle in the identical isosceles is constant.
Advantages of tacheometry:
- The method is more rapid, although less precise than chaining.
- Tachometers are more effective where river, valley, broken boundaries, hard slopes, and undulations are difficult to chase.
- For preparation of contour maps, wherein it is essential to measure horizontal and vertical distances.
- It is used for survey road, railways.
- It is also used for hydrographic surveying.
- Also, checks distances measured by tape, chain and dumpy levels.
- It is used where accuracy is just not required.
- This saves money and time.
Disadvantages of tacheometry:
- In a rough country, both horizontal and vertical measurements are tedious and chaining is inaccurate, hard, and slow.
- This method is usually the fastest and best for figure detection and topographic detailing.
Uses of tachometer:
- When obstacles like river, broken ground, water spill, tachometer give speed and accuracy of work.
- In a rough country where measuring horizontal and vertical distances is slow, difficult and inaccurate.
- This method is the fastest and best in filling details in topographic survey.
Tacheometry is a method of measuring both the horizontal distance and vertical elevation of a degree in the distance without the usage of sophisticated technology such as electronic distance measurement (EDM) or satellite transmission.