Figure 15-14.—Sun-observation example field notes.
hair becomes tangent to the sun’s image. Averaging the
of the line (ZL) computed for each. A general equation
direct and reverse angles results in an angle to the center
of the sun.
Since the sun travels on a curved path, averaging the
angles introduces a systematic error-the magnitude
being a function of time between pointings. This error
can be eliminated by computing an azimuth for each
pointing and averaging the azimuths.
An alternate procedure is to take both direct and
reverse pointings on the same edge (usually the trailing
edge). A correction, dH, is calculated from the sun’s
semidiameter and is applied to the average horizontal
angle. The semidiameter of the sun is tabulated in the
ephemeris. The correction dH (a function of the sun’s
altitude), should be computed by using the following
formula (both observations, direct and reverse, should
be made within 4 minutes):
When you are pointing the left edge (left when
facing the sun), add dH to an angle right. When pointing
the right edge, subtract the left edge, which is always the
trailing edge at latitudes greater than 23.5°N; the left
edge is always the leading edge at latitudes greater than
23.5°S. The number of sets of data varies, depending on
accuracy requirements. For most applications, a
minimum of three sets should be taken and an azimuth
for ZL is
ZL can be normalized to between 0° and 360° by
adding or subtracting 360°.
After azimuths of the line have been computed, they
are compared and, if found to be within an acceptable
limit, averaged.
In working through the following example, refer to
the field notes shown in figure 15-14. These notes
illustrate the standard procedure of incrementing
horizontal circles and micrometer settings for a
directional theodolite.
EXAMPLE: Determine the true azimuth of a line
(line AB) on the ground from a celestial observation.
1. Set the transit at station A and train it on B.
2. Adjust the horizontal circle at zero and lock the
lower motion.
3. Train the telescope on the sun and record the
time at the instant the vertical cross hair is aligned on
the edge of the sun.
4. Read and record the horizontal angle.
5. Invert the telescope and take another reading.
6. Repeat Steps 3 and 4 until all the necessary sun
shots are completed.
7. Proceed with the computation
15-20
