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
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