Among all the celestial bodies, the sun is the most prominent body that can be observed easily and accurately. Thus, the sun observation provides surveyors a convenient method for determination of astronomic azimuth. In the hour-angle method, a horizontal angle from a line to the sun is measured. Knowing accurate time of the observation and geographic position of the observation station, the sun's azimuth is computed using the relations of astronomical triangle. This azimuth and horizontal angle are combined to yield the line's azimuth as explained in Lesson 26.
The sun is observed through the telescope fitted with either an eyepiece sun glass or an objective lens filter. For total stations, an objective lens filter is mandatory (to protect EDMI components). It is to be noted that the sun's image is large in diameter-approximately 32 min of arc-making accurate pointing on the center impractical. Thus, in lieu of pointing the centre, observation to the sun are usually taken with the edges tangential to both the horizontal and vertical cross hairs. It is usually achieved by allowing sun's trailing edge to move onto the vertical cross hair or the leading edge is pointed by moving the vertical cross hair forward, until it becomes tangent to the sun's image.
The field work involved in the determination of azimuth of a line from sun observation consists of following steps:
1. Carry out temporary adjustment of a theodolite at the observation station with face left condition.
2. Open the lower plate main screw and swing the telescope to bisect the reference object. Fix the lower plate main screw and bisect accurately using the lower plate tangent screw. Note down the horizontal circle reading.
3. Swing the telescope by opening the upper plate main screw. Bring the image of the sun into the upper left quadrant of the diaphragm (Figure 27.1a). Close the upper plate main screw and then bring the vertical hair tangent to right limb of the sun using the upper plate tangent screw. As soon as the lower limb of the sun makes contact with the horizontal hair, the chronometer time is recorded. Note down the horizontal (as well as vertical, if altitude of the sun is required to be observed) circle readings.
4. Next, bring the image of the sun into lower right quadrant using the upper and vertical plate tangent screws. The vertical hair is kept in tangent to left limb of the sun using by the upper plate tangent screw. As soon as the upper limb of the sun makes contact with the horizontal hair, the chronometer time is recorded (Figure 27.1b). Note down the horizontal (as well as vertical, if altitude of the sun is required to be observed) circle readings.
5. Open the upper plate main screw and swing back the telescope to the reference object. Close the upper plate main screw and bisect the reference object. Note down the horizontal circle reading.
6. Change the face left of the instrument into the face right condition and repeat step 2 with the instrument in face right condition.
7. Swing the telescope by opening the upper plate main screw. Bring the image of the sun into the upper right quadrant of the diaphragm (Figure 27.1c). Close the upper plate main screw and then bring the vertical hair tangent to left limb of the sun using the upper plate tangent screw. As soon as the lower limb of the sun makes contact with the horizontal hair, the chronometer time is recorded. Note down the horizontal (as well as vertical, if altitude of the sun is required to be observed) circle readings.
8. Next, bring the image of the sun into lower left quadrant using the upper and vertical plate tangent screws. The vertical hair is kept in tangent to right limb of the sun using the upper plate tangent screw. As soon as the upper limb of the sun makes contact with the horizontal hair, the chronometer time is recorded (Figure 27.1d). Note down the horizontal (as well as vertical, if altitude of the sun is required to be observed) circle readings.
9. Repeat step 5.
Thus, a set of observation is to be taken. It consists of four instants of time and four horizontal circle readings (and four vertical circle readings). An azimuth of the line AZL should be computed for each pointing on the sun.
For any field observation, it may consist of one or more sets, but a minimum of three sets is recommended.