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What is a Theodolite and How Does it Work? Download a PDF with Detailed Diagrams and Descriptions



How to Use a Theodolite: Parts and Functions Explained in PDF Format




A theodolite is a surveying instrument that measures horizontal and vertical angles to determine the relative positions of points. It is used for various purposes, such as mapping, construction, engineering, geodesy, astronomy and meteorology. It consists of several parts, each of which has a specific function and use. In this article, we will explain the parts and functions of a theodolite in PDF format, which you can download for free at the end of this article.




Theodolite Parts And Functions Pdf Freel



Parts of a Theodolite




A theodolite consists of the following main parts:


  • Telescope: This is the optical device that allows the surveyor to view the target object. It can rotate about two perpendicular axes: the horizontal axis and the vertical axis. The horizontal axis is also called the trunnion axis or the altitude axis, and the vertical axis is also called the azimuth axis or the horizontal circle axis. The telescope has a cross-hair or a reticle in its focal plane, which is used to align the target object with the center of the field of view. The telescope also has a focusing knob that adjusts the clarity of the image.



  • Vertical circle: This is a graduated circle that measures the vertical angle or zenith angle of the target object. It is attached to the telescope and moves with it. The vertical circle can be read from two opposite verniers: the A vernier and the B vernier. The A vernier reads clockwise from 0 to 360, and the B vernier reads counterclockwise from 0 to 360. The difference between the two readings is called the index error, which can be corrected by adding or subtracting it from the mean reading.



  • Index frame: This is a metal frame that supports the vertical circle and its verniers. It also has a clamp and a tangent screw that are used to lock and fine-tune the position of the telescope in the vertical plane.



  • The standards: These are two metal supports that hold the telescope and allow it to rotate about the vertical axis. They are usually shaped like an A or a U, and are connected by an upper plate and a lower plate.



  • The upper plate: This is also called the vernier plate or the alidade. It is fixed to the standards and carries two opposite verniers: the C vernier and the D vernier. These verniers are used to read the horizontal angle or azimuth angle of the target object from a graduated circle called the horizontal circle, which is fixed to the lower plate. The upper plate also has a clamp and a tangent screw that are used to lock and fine-tune the position of the telescope in the horizontal plane.



  • The lower plate: This is also called the scale plate or the limb. It is fixed to a leveling head that connects it to a tripod. It carries a graduated circle called the horizontal circle, which measures the horizontal angle or azimuth angle of the target object. The horizontal circle can be read from two opposite verniers: the C vernier and the D vernier, which are fixed to the upper plate. The lower plate also has a compass that indicates the magnetic north direction.



The leveling head: This is a device that connects the lower plate to a tripod. It has three leveling screws that are used to adjust and level


How to Use a Theodolite




A theodolite is a precision instrument used for measuring angles both horizontally and vertically. Theodolites are primarily used for surveying applications and have been adapted for special purposes in areas such as metrology and rocket launch technology. To use a theodolite, you need to follow these steps:


  • Mark the point where you want to set up the theodolite. You can use a surveyor's nail or stake to mark the point accurately.



  • Set up the tripod over the marked point. Adjust the height of the tripod so that the theodolite will be at your eye level. Make sure the tripod is stable and level on the ground.



  • Mount the theodolite on the tripod. Secure it with screws and knobs. Make sure the instrument is centered over the marked point using the optical plummet or plumb bob.



  • Level the theodolite using the bubble level or plate level. Adjust the leveling screws until the bubble is centered in the vial.



  • Focus the telescope using the focusing knob. Adjust the eyepiece until you see a clear image of the cross-hair or reticle.



  • Sight the target object using the telescope. Align the cross-hair with the center of the object. You can use the sighting device or alidade on top of the telescope to help you locate the object.



  • Clamp and fine-tune the position of the telescope using the clamps and tangent screws on both axes. Make sure you do not disturb the leveling of the instrument.



  • Read and record the horizontal and vertical angles from the graduated circles and verniers. You can use magnifiers to read them more accurately. You can also correct for any index error by taking two readings from opposite verniers and taking their mean.



  • Repeat steps 6 to 8 for any other target objects you want to measure angles from.



Functions of a Theodolite




A theodolite can perform various functions depending on its type and purpose. Some of the common functions of a theodolite are:


  • Measuring horizontal and vertical angles between two or more points.



  • Determining directions and bearings of lines.



  • Setting out angles and lines on the ground.



  • Leveling and transferring heights.



  • Prolonging or ranging lines.



  • Measuring distances indirectly using trigonometry.



  • Observing celestial bodies and phenomena.



Types of Theodolites




Theodolites can be classified into different types based on their design, features and applications. Some of the common types of theodolites are:


  • Repeating Theodolite: This type of theodolite measures angles by repeating the same observation several times and taking the average of the readings. This reduces the errors due to graduation, centering and reading. Repeating theodolites are suitable for locations where the base is unstable or where space is limited.



  • Direction Theodolite: This type of theodolite measures angles by setting a zero direction and then measuring the angles from that direction. The readings are obtained from two opposite verniers and the difference between them is the angle. Direction theodolites are suitable for triangulation and traverse surveys.



  • Vernier Transit Theodolite: This type of theodolite has a telescope that can be transited or flipped over to allow back sighting and angle doubling. This eliminates the errors due to collimation and eccentricity. Vernier transit theodolites are suitable for construction and alignment surveys.



  • Digital Theodolite: This type of theodolite has an electronic display that shows the angles in digital form. It also has an optical encoder that converts the angular movement into electrical signals. Digital theodolites are more accurate, faster and easier to use than conventional ones.