| Azimuth Traverse |
In this method, at each traverse station the back azimuth of the preceding line and the azimuth of the forward line are measured using a transit. The reference meridian may be either true or assumed. In this method, successive stations are occupied, beginning with the line of known or assumed azimuth. At each station the transit is "oriented" by setting the A vernier or horizontal circle index to read the back azimuth (forward azimuth ± 180°) of the preceding line and then back sighting to the preceding traverse station. The instrument is then turned on the upper motion, and a foresight on the following traverse station is secured. The reading indicated by the horizontal circle index on the clockwise circle is the azimuth of the forward line. Any angular error of closure of a traverse becomes evident by the difference between initial and final observations taken along the first line. The method is used extensively for topographic and other surveys where a large number of details are located by angular and linear measurements from the traverse stations.
Figure 28.8 shows a closed-loop traverse run by the azimuth method. The traverse is begun at station A with a backsight along line AB for which the azimuth is known to be 62°00'00 " by astronomical observation or using GNSS receiver. With the instrument set over station A, (the A vernier is set to 62° 00' 00" clockwise) and a backsight is made on station B using the lower motion. The instrument is then turned on the upper motion and a foresight is taken on station E. Since zero is oriented toward true north (Figure 28.8), the clockwise circle read at the A vernier (143° 30' 20") is the azimuth of line AE. When the instrument is moved to point B, the back azimuth of line AB is computed by adding 180° to the forward azimuth (62° 00' 00" + 180° = 242° 00' 00"), and this value is set on the clockwise circle before a backsight, using the lower motion, is taken on station A. The circle is oriented with zero toward true north and when the instrument is turned on the upper motion for a foresight on station C, the angle observed on the circle at the A vernier (135° 48' 40") is the azimuth of line BC from true north. This operation is repeated for each successive traverse station. When the final setup is made over station A, the azimuth observed on the foresight along line AB should equal the azimuth of line AE plus180°. Any deviation between this last observation and the fixed direction of the closing line is the angular closure. The error of closure in azimuth may be adjusted as discussed in interior angle method of traversing.
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