Surveying is one of the oldest sciences on Earth. Since the 17th century, scientists began to invent the first high-precision measuring instruments, including theodolite. What is a theodolite? What is it for? Why are theodolites used to this day? Let's try to deal with these issues.
Theodolite Information
Theodolite is a high-precision geodetic instrument designed to measure angles (horizontal and vertical) in a topographic survey. Theodolite has a U-shape and is located on a special stand.
Parts of theodolite and principles of their work
The design of each goniometer provides the following main parts:
- A telescope with an increase in a certain magnification, mounted on the tribrach columns. An observer looks at her.
- The vertical and horizontal (limb) circle on which the count is made.
- Microscope (scale or bar) to take readings from the circles.
- A rotary ruler with strokes applied on it, rigidly fastened with limbs (alidade).
- Induction and fixing screws, allowing smoothly adjusting and fixing the position of the device.
- Centering, or optical plummet, which allows you to determine the exact position of the device above a point of terrain.
- Tripod for mounting theodolite.
Classification
According to the principle of operation, theodolites are divided into optical, laser, digital and photographic.
What is an optical theodolite? This is the most accurate and reliable goniometer, which does not require batteries when working and is the most unpretentious in operation. Counts are made on a goniometer scale. There is no internal memory, so a field observation diary is usually kept.
What is a laser theodolite? This is a goniometer device, the basis of which is the use of laser beams, used as accurate pointers. The measuring instrument and telescope are a single unit. Measurements are made in an automated mode and are displayed.
What is a digital theodolite? The basis of this device is barcode discs, which came to replace ordinary circles. Measurements are performed automatically. Typically, a memory device is provided in the design that stores all measurement data. Theodolites with LCD display and batteries can be used at low temperatures.
Phototheodolites are an independent class of goniometric devices. The goniometer is structurally connected to a photo or movie camera. They are used to determine the coordinates or trajectories of objects.
By design, theodolites are divided into simple and repetitive. In simple instruments, the alidade rotates independently of the limb. In repeat, they can rotate independently or jointly.
By accuracy, goniometers are divided into three types:
- High precision (error not more than 1 second).
- Accurate (2 to 10 seconds).
- Technical (from 0.25 to 0.5 minutes).
Preparation for work
For measurements, a reference point is selected above which the instrument is installed with a high degree of accuracy. In the case of underground surveying, which takes place during surveying, theodolite is installed under it.
The telescope should be located at eye level of the observer. Theodolite is placed above the point on the eye by moving the tripod, and then precisely using an optical or filament plummet. Three screws of a horizontal platform help to establish a horizontal position of the device above a point. So that the vertical axis of the device coincides with the plumb line, a bubble of a cylindrical level is brought to the central position.
Then the device is fixed, the correctness of its installation is checked: the theodolite is rotated around the axes and the position of the bubbles of round and cylindrical levels is observed. One division on the level scale is the maximum allowable. The grid of threads, the strokes of the limbs and the scale microscope should be clearly visible in the telescope.
Angle measurement
Measurement of horizontal angles by theodolite is as follows. Alidad is retracted to the left by about 30-40 degrees and is fixed. With the help of a guide screw, the crosshairs of the net of threads are aimed at the point of sight. Theodolite readings. Next, the fastening screw loosens and points to another point, and readings are taken. To increase the accuracy of measurements, it is necessary to re-shoot. The telescope is transferred through the zenith, readings are taken. In office conditions, the average measurement result is calculated. The error of the result during repeated surveys should not be more than double the accuracy of the device. Vertical angles are measured similarly, but using a vertical circle.
Fields of application
Theodolite is used in geodesy, topography, during construction work and in other areas where high measurement accuracy is required. Theodolites are needed:
- When constructing a geodetic network by the method of triangulation, polygonometry.
- When compiling topographic maps and plans.
- During general construction work (fixing a vertical or horizontal position of the structure).
Theodolite is one of the most important geodetic instruments, characterized by high measurement accuracy and multifunctionality. It can be used to measure vertical and horizontal angles. It is indispensable for work where you need to clearly determine the position of the plumb line.