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Time and Date - 14071_354
Figure  15-14.Sun-observation  example  field  notes. - 14071_356

Engineering Aid 2 - Intermediate Structural engineering guide book
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Table 15-5.—GHA for the Sun and Polaris for O Hours Universal Time Now that you know how to compute for the GHA and eventually the LHA, the declination of the sun, and the latitude and longitude of your location, you are ready for the field procedure for determining the azimuth of a line. Field Procedure Horizontal angles from a line to the sun are obtained from direct and reverse pointings taken on the backssght mark  of  the  sun.  It  is  suggested  that  repeating  theodolites be used as directional instruments, with the sighting sequence being as follows: direct on mark direct on sun, reverse on sun, and reverse on mark. Times are recorded for each pointing on the sun. Since a large difference usually exists in vertical angles between the backsight mark and the sun, it is imperative that both direct and reverse pointings be taken to eliminate instrument errors. WARNING DIRECT   VIEWING   OF   THE   SUN WITHOUT   A   PROPER   FILTER   WILL CAUSE SERIOUS EYE DAMAGE. You must NOT observe the sun directly through the telescope without using an eyepiece or objective lens filter. If you do not have a filter, you can project the image of the sun and the cross hairs of the instrument onto a blank white surface held approximately 1 foot behind the eyepiece. The eyepiece and the telescope focus must be adjusted to obtain a sharp image. Usually only that position of the cross hair system situated within the  sun’s  image  is  clearly  visible.  Although  this  method of sun observation works, viewing the sun with the aid of a falter is more convenient and slightly improves pointing   accuracies.   (When   using   a   total-station instrument, you must use an objective lens filter to protect   the   electronic   distance   meter   (EDM) components.) Accurate pointings of the telescope cross hairs in the center of the sun is impractical. Rather than pointing to the center, you may take direct and reverse pointings on opposite edges (fig. 15-13). Pointings are made with the single portion of the vertical cross hair without regard to the location of the horizontal cross hair. You point the trailing edge of the sun by allowing it to move into the vertical cross hair. You point the leading edge by moving the vertical cross hair forward until the cross Figure  15-13.—Pointing  the  sun. 15-19







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