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Field Procedure - 14071_355
Field Procedure - CONTINUED - 14071_357

Engineering Aid 2 - Intermediate Structural engineering guide book
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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







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