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Surveying is the science and
art of measuring distances and angles on or near the surface of the
earth. It is an orderly process of acquiring data relating to the
physical characteristics of the earth and in particular the relative
position of points and the magnitude of areas. Evidence of surveying
and recorded information exists from as long ago as five thousand
years in places such as China, India, Babylon and Egypt.
Ancient Egyptian surveyors were
called harpedonapata (rope-stretcher). They used ropes and knots, tied
at pre-determined intervals, to measure distances. The 3-4-5 triangle
(later formalized by Pythagoras) was discovered to give a right angle
easily by using a rope knotted at distances of 3,4 and 5 units (as
below) and shaped (stretched) to form a triangle with a knot at each
corner (vertex).
An early instrument for
leveling is shown below. It was made from three pieces of wood in the
form of an isosceles triangle. A plumb bob suspended from the apex of
the frame aligned itself with a notch at the midpoint of the base only
when the base was level.
Early Egyptian level
The great Pyramids were
presumably laid out using knotted ropes, simple levels and water
trough levels for the foundations. The Egyptians used these techniques
in the field whilst the Greeks (Pythagoras, Archimedes, Eratosthenes)
proved the geometric reasoning behind the principles and demonstrated
a clear relationship between mathematics and surveying.
Archimedes (by 250 B.C.)
recorded that the circumference of the earth is 30 myriads of stadia
(300 000 stadia). Stadia is an ancient Greek measure of a distance of
202 yards, or approximately 185 metres. Eratosthenes supported this
notion and by some complicated reasoning and calculations using the
summer solstice, the sun, angles of shadows and the known position of
two towns Syene and Alexandria (Syene was known to be 5 000 stadia due
south of Alexandria), found the circumference of the earth to be 25
000 miles. It is in fact 24 881 miles - not bad!
For leveling the Greeks
used a chorobate. Wooden poles were often used for linear
measurements.
Roman level (Chorobate)
Navigation skills were
needed for exploration. Lodestone (a naturally magnetized rock -
magnetite) was first used to locate magnetic north and in time the
magnetic compass developed for navigation on land and water. An
Englishman, Thomas Digges, used the word theodolite to describe an
instrument, graduated in 360 degrees, used to measure angles in the
mid-1500's. By 1590 the plane table, credited as the invention of Jean
Praetorius, was in use. It remained in similar form until the early
1900's. A telescope attached to a quadrant for measuring angles
permitted the development of the surveying procedures of triangulation
in the 1660's. Sextants, which are precision instruments made from
brass or aluminum, became very useful for ocean navigation where
celestial observations were taken to plot a ship's position. Sextants
accurately measure angles between celestial objects such as the stars,
moon and sun, and the horizon from which calculations of position can
be made. They are a sophisticated refinement of earlier instruments
(cross-staff, quadrant, octant to name a few). The sextant remains
today as a valuable tool for ocean navigation. The first dumpy levels
appeared in the first half of the 1700's combining a telescope with a
bubble level. In 1831, in Philadelphia, W. J. Young invented the
transit which exhibited marvelous improvements in accuracy of
surveying methods. It allowed the telescope to revolve (transit) on
its axis. This meant that both forward and backward sightings could be
accurately taken and by repeating the process errors were minimized.
The military requirements of
two world wars provided the motivation for vast improvements in the
design of surveying equipment and execution of surveying operations.
In the 1950's Electronic Distance Measurement (EDM) largely replaced
triangulation methods for distance measurements. Electronic data
collection brought many changes to surveying procedures. The transfer
of data collected electronically to computer plotting and drawing
systems has reduced time and relieved tedious manual drawing work. The
scope of surveying has extended beyond land measurement to include
environmental concern for such things as water resources, energy
requirements, marine exploration, demarcation of boundaries and
protection of the environment.
The use of GPS (Global
Positioning System) in surveying procedures is the most recent and
revolutionary change to impact land measurement. GPS is very accurate,
quick and reliable. However, in conversation with practicing
surveyors, I discovered that there is trend to keep older mechanical
dumpy levels. This is because they can always be relied on, and
trusted, when electronic equipment is faulty or fails, to complete the
job in hand. |
