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QUESTIONS - CONTINUED - 14071_336
ELEMENTS OF FIELD ASTRONOMY - 14071_338

Engineering Aid 2 - Intermediate Structural engineering guide book
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CHAPTER 15 FIELD ASTRONOMY AND TRIANGULATION This chapter provides information that will aid you in carrying out your duties involving field astronomy and in establishing horizontal control using triangula- tion  methods. In  regards  to  field  astronomy,  we  will  explain  the basic  elements  of  field  astronomy  and  the  use  of different kinds of time-such as solar time, zone time, and Greenwich mean time-in determining direction from  celestial  observations.  You  will  also  learn  how  to determine latitude using a transit and how to determine the true azimuth of a line on the ground from celestial observation. In  the  discussion  of  triangulation,  we  will  explain the purpose and kinds of triangulation networks, the steps  involved  in  a  triangulation  survey,  and  the computations   involved   in   establishing   horizontal control  points  using  triangulation. Also included in this chapter is a very brief intro- duction to satellite surveying systems. That discussion includes types of satellite surveying systems and the basic  principles  involved  in  locating  point  positions  on the surface of the earth from observations taken on satellites. DIRECTION  FROM  CELESTIAL OBSERVATIONS Occasions may occur when you must determine the direction of the true meridian (astronomic north) in an area where no usefully located station monuments exist. In  a  case  like  this,  you  have  to  rely  on  astronomic observations  taken  on  one  of  the  celestial  bodies,  such as the sun or a star. To do this, you must understand the astronomical  and  trigonometric  principles  of  field astronomy. To begin, let’s first discuss time as it applies to  field  astronomy. TIME Before you can understand the procedure involved in determining direction from celestial observations, you   must   have   some   knowledge   of   different designations of time. Solar  Time The sun is the most commonly used reference point for reckoning time, and time reckoned by the sun is solar time. Time reckoned according to the position of the actual physical sun is solar  apparent time. When the sun is directly over a meridian, it is noontime, local apparent  time, along that meridian. At the same instant it is midnight, local apparent time, on the meridian 180° away from that meridian, on the opposite side of the earth. The time required for a complete revolution of the earth on its axis is a constant 24 hours with regard to a particular point on the earth; however, this time varies slightly with regard to the point’s position with relation to the actual sun. Therefore, days reckoned by apparent time (that is, the position of the actual sun) vary slightly in  length.  This  difficulty  can  be  avoided  by  reckoning time according to a mean position of the sun, and this is called mean time. By mean time the interval from noon  to  noon  along  any  meridian  is  always  the same-24   hours. We know that the earth, not the sun, actually moves, but for the purposes of this explanation, we will assume that the earth is motionless, with the heavenly bodies moving westward around it. As the sun moves along its course, it takes noontime with it, so to speak. In other words, when the mean sun is on a particular meridian, it is noontime along that meridian, not yet noon at any point west of that meridian, and already past noon at any point east of that meridian. This means that, by local mean time, the time is different at any two points lying in different longitudes. To avoid the obvious disadvantages of a system in which the time is different at the opposite ends of a short street running  east-west,  the  nations  of  the  earth  have generally established  zone or standard time. Zone Time Under the zone time system, the earth has been divided along meridians into 24 time zones. The starting point is the Greenwich meridian, lying at 0° longitude. Every  meridian  east  or  west  of  Greenwich  that  is numbered 15° or a multiple of 15° (such as 30° east or west, 45° east or west, 60° east or west, and so on) is 15-1







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