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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 introduction 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 OBSERVATIONSOccasions 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, lets 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 points 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) isstandard time meridian. Each time meridian runs through the center of its time zone, which means that the zone extends for 730' on each side of the meridian. In each zone, the time is the same throughout the zone. There is a 1 hour difference in time between a particular zone and the adjacent zone. When determining time in different zones, it is helpful to remember this phrase: time is later as you move eastward. So, if it is 1200 in your zone, it is 1300 in the next zone to the east and 1100 in the next zone to the west |
 
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