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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° 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° 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° 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°, 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°
+ –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.

Tags : Civil Surveying - Theodolite Surveying

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