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Chapter: Civil Surveying : Theodolite Surveying

Measurement of Horizontal and Vertical Angles in Theodolite

The objective lens has to be focused whenever an object is sighted, as this depends upon the distance between the instrument and the object.

Measurement of Horizontal and Vertical Angles in Theodolite



The objective lens has to be focused whenever an object is sighted, as this depends upon the distance between the instrument and the object. A focusing screw on the side of the telescope is operated to focus the objective. This operation brings the image of the object in the plane of the cross hairs. This helps to exactly bisect the object, be it a ranging rod or an arrow. To focus the objective, swing the instrument to bring the object into view by looking over the telescope. Rotate the focusing knob till the object is in sharp view along with the cross hairs.


Using the Theodolite


The theodolite is mainly used to measure horizontal and vertical angles, even though many other operations can be done with the instrument. It is a delicate and sensi-tive instrument and needs to be handled carefully. The following points should be noted while using the instrument.


1.  The theodolite should be set up and levelled at every station. This is a funda-mental, necessary operation and should be carried out carefully.

2.  In measuring horizontal angles, the inclination of the telescope is not significant The line of sight is arranged to bisect the object clearly.


3.  The graduated circle plate gives the outer axis and the vernier plate provides the inner axis. Both the axes coincide if the instrument is properly adjusted and form the vertical axis.


4.  There are three clamp screws each with its own tangent screw. The lower clamp screw releases the lower plate, the upper clamp screw releases the upper ver-nier plate, and the third vertical circular clamp releases the vertical circle. One should be familiar with the location of the clamp screws and the corresponding tangent screws.


5.  Each clamp screw releases one plate. The lower plate is released by the lower clamp screw. When this plate is released, swinging the instrument or rotating it in a horizontal plane causes no change in the reading of the circle, as both the plates move together. This is used when an object has to be sighted with the zero setting of the circle or with any other reading without changing the reading.


6.  Both the clamp screws should not be released together. When the lower clamp is tight and the upper clamp screw is released, the upper plate moves relative to the lower plate and the reading changes. This is done when one has to measure an angle.

The clamp screws should be tightened very near to their final postion so that only a a very small movement has to be effected by the tangent screw. For each clamp  screw, the corresponding tangent screw should be for final adjustment.

8. To set the instrument to zero at the plate circle, release the upper clamp and rotate the instrument about the vertical axis. On the vernier A, make the zero of the circle coincide with the zero of the vernier. Tighten the upper clamp and using the upper tangent screw, make the zeros exactly coincide. This can be verified by look in g th rough the magnifying gl assessand seeing that the gradua tions on either side are symmetrical. Verify the condition on vernier B as well, where the 180 o graduation should coincide with the zero of the vernier.


9.  While bisecting the signals or setting the zero reading, keep the line of sight in such a position that the tangent screw moves the sight in the same direction as the movement of the instrument. If the movement is clockwise, then the

tangent screw is adjusted to move the cross hairs from left to right.


10.            Operate a tangent screw only after clamping the corresponding clamp screw.

11.            The magnifying glasses are so fixed that they can be moved along the circle.

Read the circle by bringing the glass over the reading and looking directly over the reading to avoid any parallax error.


12.            While bisecting stations with the theodolite, the station mark should be very clear and must be a point. Bisect either the cross marks on pegs at their inter-section or the ranging rod and arrow at their lowest pointed end.


13.            Clamp screws and tangent screws need careful handling. Do not apply great force on these screws and handle them delicately during survey work.


Measuring Horizontal Angles


To measure the horizontal angle between two lines, the following procedure is adopted.


1.     Referring to Fig. 6.5, the angle POQ is to be measured. Set up the theodolite at O.


2. Set the instrument to read 0 o 00 00?. This is not strictly required, as the angle can be determined as the difreadings. However, it is convenient to make the initial reading zero. For this, release the upper clamp and      O rotate the instrument to make the Q reading approximately zero. Clamp     the upper plate and using the upper          Fig. 6.5   Measuring a horizontal angle    tangent screw, make the readingcompassexactly zero. Vernier A reads zero and vernier B reads 180 o 00 00?.


3.  Release the lower plate and rotate the instrument to bisect the station P. After approximately bisecting it, clamp the lower plate and using the lower tangent screw, bisect the signal exactly. The readings on the plates do not change as both the plates move together in this operation. Check that the readings on vernier A and B are zero and 180 o , respectively.


4.  Release the upper plate by loosening the upper clamp. Rotate the instrument to screw, exactly bisect the signal at Q. Read both the verniers A and B. The reading at A will give the angle directly. The reading at B will be 180 o + -POQ.


5.  If there is any difference, take the average of the two values as the correct angle.


Horizontal angles are measured this way for ordinary work. The accuracy can be improved by reading the angles with face-left and face-right observations and taking the average of the two. For more precise work, the angles are repeatedly measured with both the faces and the average taken. This method is known as the repetition method and is described below.

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