3. Mark the next point with a marker, and follow the procedures for establishing a point or station.

Similarly, you may use the procedures above to measure a horizontal angle by sighting on two existing points and reading their interior angle. In addition, the following hints may help you when you are taking horizontal measurements:

1. Make the centering of the line of sight as close as possible by hand so that you will not turn the tangent screw more than one or two turns. Make the last turn of the tangent screw clockwise to compress the opposing springs.

Figure 13-12.-Sample field notes from a deflection angle transit-tape survey.

2. Read the vernier with the eye directly over the top of the coinciding graduations to eliminate the effects of parallax.

3. Take the reading of the other vernier as a check. The readings should be 180 apart.

4. Check the plate bubbles before measuring an angle to see if they are centered, but do not disturb the leveling screws between the initial and final settings of the line of sight. If an angle is measured again, the plate may be releveled after each reading before sighting again on the starting point.

5. Make sure that the rodman is holding the range pole truly vertical when you sight at it. When the bottom of the range pole is not visible, let the rodman use a plumb bob.

6. Avoid accidental movement of the horizontal circle; for instance, moving the wrong clamp or tangent screw. If a number of angles will be observed from one setup without moving the horizontal circle, you should sight at some clearly defined distant object that will serve as a reference mark and take note of the angle. Occasionally, you should recheck the reading to this point during measurement to see if there is any accidental movement.

An example of a horizontal deflection angle measurement is shown in figure 13-12. The field notes contain data taken from a loop traverse shown in the sketch. The transit was first set up at station A, and the magnetic bearing of AB was read on the compass. Then the deflection angle between the extension of EA and AB was turned in the following manner:

1. The instrumentman released both clamps, matched the vernier to zero by hand, tightened the upper motion clamp, and set the zero exactly with the upper tangent screw.

2. With the telescope plunged (inverted position), the instrumentman sighted the range pole held on station E. Then he tightened the lower motion clamp and manipulated the lower motion tangent screw to bring the vertical cross hair to exact alignment with the range pole.

3. The instrumentman replunged the telescope and trained on the extension of EA. (Notice that the telescope is in its normal position now.) He then released the upper motion and rotated the telescope to the right until the vertical cross hair came into line with the range pole held on station B. He further set the upper motion clamp screw and brought the vertical cross hair into exact alignment with the range pole by manipulating the upper motion tangent screw.

4. The instrumentman then read the size of the deflection angle on the A vernier (8901). Since the angle was turned to the right, he recorded 8901R in the column headed "Defl. Angle." Likewise, he recorded the chained distance between stations A and B and the magnetic bearing of traverse line AB under their appropriate headings.

Figure 13-13.-Sample field notes for closing the horizon.

The instrumentman used the same method at each traverse station, working clockwise around the traverse to station E. Note that the algebraic sum of the measured deflection angle (angles to the right considered as plus, to the left as minus) is 35059. For a closed traverse, the algebraic sum of the deflection angles from the standpoint of pure geometry is 36000. Therefore, there is an ANGULAR ERROR OF CLOSURE here of 001. This small error would probably be considered a normal error. A large variance would indicate a larger mistake made in the measurements. In the example just presented, the general accuracy of all the angular measurements was checked by comparing the sum of the deflection angles with the theoretical sum. The accuracy of single angular measurement can be checked by the, procedure CLOSING THE HORIZON. The method is based on the fact that the theoretical sum of all the angles around a point is 36000. The field notes in figure 13-13 show the procedure for closing the horizon. The transit was set up at station A, and angle BAC was turned, measuring 5115. Then the angle from AC clockwise around to AB was turned, measuring 30845. The sum of the two angles is 36000. The angular error of closure is therefore 000, meaning that perfect closure is obtained.