Measuring Horizontal Angles
To measure a horizontal angle, loosen both the upper and lower clamps and set the 0of the vernier close to 0on the circle. Then tighten the upper L clamp by means of the upper slow-motion screws.
Set the index of the vernier exactly opposite 0on the circle, and direct the telescope at one of the objects to be sighted. When the object is in the field of view and near the vertical hair, tighten the lower clamp. Set the vertical hair exactly on the object by using the
lower slow-motion screw. The telescope should be focused carefully.
Now, loosen the upper clamp and sight the telescope on the second object. Tighten the upper clamp and use the slow-motion screw to bring the object on the vertical crosshair. You can now read the angle by adding to the circle, reading the minutes read on the vernier. To eliminate instrument errors, measure the angle again with the telescope inverted, and take the mean of the measured angles. Another procedure is to measure the angle by repetition, once with the telescope direct and once reversed, with the mean of the two measurements taken as the measurement of the angle. The vernier is read in the direction the angle is turned.
Measuring Vertical Angles
To measure a vertical angle, carefully level the instrument and direct the telescope at the object. When the object is observed in the telescope, clamp the vertical motion, and, using the vertical motion tangent screw, set the middle horizontal crosshair exactly on the point. The reading on the vertical arc is the vertical angle When the point is above the
horizontal plane, the angle IS a positive angle or angle of elevation; when the point is below the horizontal plane, the angle is a negative angle or angle of depression. In the survey notes, angles are designated by a t sign or a -- sign. Tie in important points or stations by establishing offset ties. No fewer than three horizontal measurements
to the nearest 0.01m (0.10ft) should be taken to the station from readily identifiable permanent points. Carefully record the measurements by means of a sketch in the notes as in figures 1-30and 1-3qa).In an area survey, the transit station may have many points to which measurements are taken to obtain adequate information for construction of a
map. Sometimes details are collected as the work of laying out the transit line proceeds. For other surveys the details are obtained after the transit line has been established and checked, especially if the survey covers a large territory. If the instrument line of sight and the axis of the telescope bubble are not in adjustment, it is impossible to obtain a correct vertical angle with the vertical arc only. If the instrument has a full vertical circle, the error can be eliminated by reading the vertical angle first with the telescope direct and then with it reversed, and then taking the average of the two readings. In figure 1-28, a 1 minute vernier is shown with its 0 opposite the 0 (360) of the circle, ready to measure an angle. The vernier lines on both sides of 0fail to match the lines on the circle by 1 minute;
the next lines of the vernier fail to match the lines of the circle by 2 minutes, and so on. To set exactly at 0, note that the first vernier lines on either side of 0fail to match the circle divisions by the same amount. Figure 1-29 shows a 1 minute vernier reading 17O25' from left to right and 342'35' right to left. 
Lesson 25