To layoff a line in the field it is usually accomplished through laying off a horizontal angle between the reference and considered line. The following procedure may be used to layoff an angle say POQ (Figure 22.1). Let us consider that the value of the angle is 31° 23' 20".
- Set up the instrument at O and carry out temporary adjustment of the instrument.
- Set the vernier A to 0° (or 360°) using the upper plate clamp and tangent screws.
- With upper plate clamp screw fixed and lower plate clamp screw open, turn the instrument and sight station P. Tighten the lower plate clamp screw and bisect P accurately using the lower tangent screw. The vernier readings should be as it was in step2.
- Loosen the upper clamp and turn the upper plate until the vernier A reads about 31° 23' 20". Clamp the upper plate, and use the upper tangent screw to set it exactly at 31° 23' 20". The line of sight is now in the required direction OQ.
- Depress the telescope and establish point Q on the ground along the line of sight. The angle POQ laid on the ground is equal to the required angle.
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