Method of reiteration for measurement of horizontal angle is usually adopted in case several angles of well distributed points/ objects are to be measured from the same instrument station with high precision. In this method, angles are measured successively starting from a point termed as initial station (Figure 22.2). The angle between the terminating station and the initial station is the last observation during a set of measurement of horizontal angle by method of reiteration. This process of measuring the angles at an instrument station round the point is to obtain a check on their sum being equal to 360° and is called closing the horizon. When the horizon is closed, the final reading of the vernier should be the same as its initial reading if there is no discrepancy. Figure 22.2 shows a instrument station O where the angles POQ, QOR and ROS have to be measured by method of reiteration. The steps involved in the measurement of the horizontal angles by method of reiteration are as follows:
Steps 1 to 7 are same as given in measurement of horizontal angle and record readings in Table 22.3.
8. Loosen the upper plate clamp screw and turn the telescope clockwise until the station R is sighted. Tighten the upper clamp screw. Use the upper tangent screw for placing the object R on the vertical cross hair. Read both the verniers, and record readings in the Table 22.3. Compute the angle QOR. And note down in the table.
9. Likewise, determine the angle ROS.
10. Finally, close the horizon by sighting the reference object P again. Note down the readings. The vernier A should now read zero (or 360°).
11. Now change the face left of the instrument to the face right by transiting (plunging) the telescope and swinging it through 180°. Repeat steps 3 to 10 in the anti-clockwise direction.
12. The average value of each angle obtained with the face left and the face right provides the observed values of the angles.
