Transcript techniques in basic surveying - GTU e

TECHNIQUES IN
BASIC SURVEYING
Patel Dixit K. (130400106054)
Patel Dhruv P.(130400106051)
Civil Engineering department
Sankalchand Patel college of Engineering, Visnagar
Basic ideas
• Surveying - the creation of a scale representation of
the ground surface - is a basic activity in many areas
of environmental management.
• A survey will be one of of two types:
– Primary survey - to establish the position of objects in three
dimensions when no previous information exists
– Secondary survey - to add extra information to existing
data or to measure changes over an interval of time
Basic ideas
• The task of three dimensional position fixing
is normally broken into two parts:
 Determining plan position
 Determining elevation
Basic ideas
• Each of these determinations may be either:
 absolute - made in terms of a fixed co-ordinate
system
 relative - made in terms of local co-ordinates
which may later be converted to absolute coordinates if required.
• The majority of surveys carried out for
environmental management are thus
secondary relative surveys
Plan Position Fixing
• The plan position of a station can be
established in a number of ways:
 By reference to the apparent positions
of astronomical objects when viewed
from that station
• This method gives the absolute location of the
station in terms of latitude and longitude,
which can be converted to local systems such
as the National Grid.
 By the measurement of the angles between lines
of sight to the unknown station from other known
positions
 By the intersection of lines of sight from the
unknown station to other objects whose
positions are already known
• These two methods both rely on the simple Euclidean
geometry of the plane. (Hence the term plane surveying).
The first procedure is termed triangulation and the
second resection.
The basic principle of triangulation
Measured angle
A
Measured angle
Baseline
B
The basic principle of resection
Known position
Known position
Measured
angle
Unknown position
Measured
angle
Measured
angle
Known position
 By measurement of distances between
the unknown station and other objects
of known positions
• This last method includes a number of
particular cases:
 measurements of offset distances from
a base line.
 trilateration - the distance equivalent of
triangulation.
 tacheometry - an optical method of
distance measurement along a known
bearing
The basic principle of trilateration
Measured side
A
Measured side
Baseline
B
Plane Surveying: Theory
• Plane surveying relies on the basic
concepts of Euclidean geometry, and in
particular the properties of triangles.
• The most important (for our purposes)
of these are:
Plane Surveying: Theory
 The internal angles of a triangle sum to 180
 The sides of an equilateral triangle are equal
and the internal angles are all 60°
 The base angles and opposing sides of an
isosceles triangle are equal
60º
The equilateral triangle
All sides equal in length
All angles equal (= 60º)
60º
60º
The isosceles triangle
Two sides equal in length
Two angles equal
a
a
Plane Surveying: Theory
 If the respective angles in two triangles are
equal then the triangles are similar and their
sides are all in the same proportion
 If two triangles have two angles and one side
equal (or vice versa) then they are congruent
and all their other respective angles and
sides are equal.
 Two triangles are also congruent if all their
sides are equal.
Similar triangles have corresponding angles equal
but are of differing size
Conguent triangles are identical
• two angles and one side equal
• two sides and one angle equal
• all three sides equal
Plane Surveying: Theory
• Congruent triangles are unique - you cannot
draw two different triangles from the same set
of measurements
• This means that a complete set of survey
data must define the positions of objects
uniquely.
Plane Surveying: Theory
 Any closed polygon can be subdivided into a
series of contiguous triangles
• These properties are repeatedly used in the
procedure of triangulation in which stations
are surveyed in a pattern of contiguous
triangles.
Any closed polygon can be subdivided into contiguous triangles
These should be chosen to make as many of the triangles as
close to equilateral as possible
Plane surveying: practical aspects
• In practice, most plane surveys are carried out
in a straightforward way following an
established sequence:
1. A reconnaisance survey will establish the
dimensions of the area, relative levels,
significant features, accessibility, obstacles etc
Plane surveying: practical aspects
2. Establish an accurate baseline by measurement
from existing survey points, natural features,
buildings etc. If none are available then the baseline
must be fixed by absolute methods.
3. Establish as required any further control points by
triangulation or trilateration from the base-line.
Plane surveying: practical aspects
4. Incorporate detail by tacheometry, traversing, tape &
offset or whatever other method is appropriate.
5. The intermediate stations should where appropriate
be cross-checked with the control points by resection
and all traverses should be closed at a control point.
6. Inaccessible detail should be incorporated by
triangulation or plane tabling from the ends of the
baseline.
ILLUSTRATION OF THE USE OF OFFSETS
Baseline
Plane surveying: practical aspects
7. If a topographic survey is being undertaken, levelling
traverses should be carried out around the survey
stations and the baseline tied to the local benchmark
by a closed traverse.
8. The use of a theodolite or total station will enable
both the position and the elevation of stations to be
found simultaneously by combined tacheometry and
triangulation or by trilateration
THE “CHAIN” SURVEY
How to establish relative plan positions
Chain survey
• Simplest of all survey techniques
• Relies on linear measurements; slopes
>3o require some adjustment to technique
• Usually requires a clear line of sight
• The triangles used should be equilateral
or approximately so
Terminology
• Trilateration is the measurement of
sides of a triangle
• whereas triangulation refers to the
measurement of the angles of the
triangle
Basic equipment
• Ranging poles
• Survey pegs and ‘arrows’
• Chain & tape measure or other distance
measuring instrument
• Plumb line
• Compass
Chain survey components
•
•
•
•
Base line: the longest line
Chain /survey lines
Survey stations
Offset lines
Order of events
• “Range out” survey stations with ranging rods
• Establish base line and measure accurately
• Measure remaining distances between other
survey stations
• Measure offset lines whilst measuring
between survey stations
Sloping ground
• If the ground slopes by more than about 3°,
this must be allowed for in the survey.
• The measured distances are thus slant
distances and must be corrected to true
horizontal distances.
• This requires that the vertical angle between
the stations is known
Ground distance determined
X
h
a
Sloping ground
• For an approximate survey, it may be
sufficient to step up or downhill using a
series of horizontal and vertical lines
• If the drop is measured at the same
time, some estimate of the slope profile
can be obtained
Chain surveying (“stepping”)
a
w
b
x
c
y
z
Sloping ground
• If stepping is not appropriate, more
sophisticated methods must be used to
measure the slant distance and the
vertical angle simultaneously
• Requires optical sighting equipment:
usually either a clinometer, Abney level
or theodolite
Basic levelling in chain surveys
h
a
h
Correcting for horizontal distance:
the “hypotenusal allowance”
correction factor = xy - yz
x
= xy(1 - cosa)
h
a
z
y