Techniques
The techniques used for surveying and land measurement are dependent
upon the type of survey being conducted. In this section there are two
choices:
Types of survey
Techniques
A simple classification is:
1. Land surveys,
which fix property lines, calculate land areas and assist with the
transfer of real property from one owner to another.
2. Engineering surveys,
which collect the data needed to plan and design engineering projects.
The information ensures the necessary position and dimension control
on the site so that the structure is built in the proper place and as
designed.
3. Informational surveys
obtain data concerning topography, drainage and man-made features of a
large area. This data is portrayed as maps and charts.
Another way to make a simple classification is:
1. Geodetic surveys
are precise and over large areas require the curvature of the earth to
be considered. Distances and angle measurements must be very, very
accurate. A wide variety of techniques are used including
triangulation, traversing, trilateration, levelling and astronomical
direction fixing.
2. Plane surveys,
which consider the surface of the earth to be a plane. Curvature is
ignored and calculations are performed using the formulas of plane
trigonometry and the properties of plane geometry. These may be
considered accurate for limited areas.
Sub-categories of the major classes provide more insight into the
various fields of surveying as follows:
· Property surveys
determine boundary lines, property corners, rights-of-way provide data
necessary for the preparation of land sub-divisions.
· Cadastral surveys
are executed by the Federal Government in connection with the disposal
of vast areas of land known as the public domain.
· Route surveys
are necessary for the design and construction of various engineering
projects such as roads, railways, pipelines, canals and powerlines.
· Industrial surveys,
or optical metrology,
are used in the aircraft and other industries where very accurate
dimensional layouts are required.
· Topographic surveys
are performed to gather data necessary to prepare topographic maps.
These are multicolour contour maps portraying the terrain; and rivers;
highways, railways, bridges and other man-made features.
· Hydrographic surveys
map the shorelines of bodies of water; chart the bottom of streams,
lakes, harbours and coastal waters; measure the flow of rivers; and
assess other factors affecting navigation and water resources. The
sounding of depths by radar is involved in this type of survey.
· Mine surveys
determine the position of underground works such as tunnels and
shafts, the position of surface structures and the surface boundaries.
· Aerial surveys
use photogrammetry to produce a mosaic of matched vertical
photographs, oblique views of landscape and topographic maps drawn
from the photographs.
· Construction surveys
fix elevations, horizontal positions and dimensions for construction
projects.
· Control surveys
provide basic horizontal and vertical position data. These are called
datum. For most surveying work the vertical position of points in
terms of height above a curved reference surface is mean sea level.
Techniques
1.
Triangulation
2.
Trilateration
3.
Traverse
4. Leveling
5.
Radiation
Triangulation
consists of a series of connected triangles which adjoin or overlap
each other, angles being measured from determined fixed stations.
Triangulation reduces the number of measures that need to be taped and
for this reason is often a preferred method of survey. A known
base-line measurement is required. Three examples of triangulation
systems are shown below.
A single chain of triangles is a rapid and economical system for
covering a narrow strip of land. A chain of quadrilaterals is more
accurate with checks being made by various combinations of angles and
sides as the survey proceeds. Larger areas use a central point
arrangement. A point to note is that all angles should be more than 20°.
Angles less than 20° are not considered valid for fixing position.
They introduce inaccuracies. This is much the same in navigation where
a fix by two bearings requires an angle of intersection of
approximately 90°, and for three bearings approximately 60°. Angles
less than 30° are not acceptable.
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Trilateration
uses electronic distance measuring equipment to directly measure the
lengths of the sides of triangles from which the angles can be
calculated. It is a very useful method for rough terrain where
positions can be accurately carried forward and is seen as an
alternative method to triangulation.
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A traverse
consists of a series of lines, whose lengths and directions are
measured, connecting points whose positions are to be determined. The
route of the traverse line can be adjusted for obstacles such as rough
or timbered terrain, swampy land, buildings and areas of heavy
traffic. A traverse may be either open or closed as shown below.
An open traverse
begins at a point of known position and ends at a station whose
relative position is unknown. This type of traverse is frequently used
for preliminary surveys for highways. A closed traverse begins and ends at the same point whose
position is known. An example of this type of traverse is a perimeter
survey of a tract of land.
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Leveling
is the operation of determining differences of elevation by measuring
vertical distances directly on a graduated rod with the use of a
leveling instrument such as a dumpy level, transit or Theodolites.
This method is called direct leveling or differential leveling.
Indirect leveling can be done using the principle that differences in
elevation are proportional to the differences in atmospheric pressure.
The difference in elevation between two points can also be determined
trigonometrically using vertical angles and horizontal or inclined
distances.
Differential leveling
Benchmarks
are very important in leveling. They are permanent objects of known
elevation located where there is least likelihood of disturbance. They
may be a metal or concrete post set close to the ground, a notch in a
tree root or a peg or spike in a tree.
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Radiation
is a surveying technique often used in conjunction with a plane table. From a
fixed position directly above the corresponding location on the ground
bearings are taken to various points at the boundary of the survey
area. The bearing lines are drawn on the paper on the table. Distances
to the points are measured and then converted to the required scale on
the survey sheet. Radiation surveying can be performed in a similar
way directly on the ground taking bearings and distances from a fixed
position and recording them for later work. Similarly plane table
surveying may be used with other techniques such as a traverse or
triangulation.
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