A2.2NP1
Environmental Practical 1
TOPIC 1
TECHNIQUES IN
BASIC SURVEYING
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 co

ordinates 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
.
Baseline
A
B
The basic principle of triangulation
Measured angle
Measured angle
Unknown position
The basic principle of resection
Measured
angle
Measured
angle
Known position
Known position
Known position
Measured
angle
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
Baseline
A
B
The basic principle of trilateration
Measured side
Measured side
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
60º
60º
The equilateral triangle
All sides equal in length
All angles equal (= 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.
Baseline
ILLUSTRATION OF THE USE OF OFFSETS
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
>3
o
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
a
h
X
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”)
w
x
y
z
c
b
a
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
a
h
h
Correcting for horizontal distance:
the “hypotenusal allowance”
a
h
z
correction factor = xy

yz
= xy(1

cos
a
)
y
x
LEVELLING
How to destermine relative
elevations
Levelling:
accounting for slopes
Unlike chain surveys, levelling surveys
account directly for slope and
incorporate this data into the whole
measurement exercise
AIMS:
•
to determine height differences between
two points
•
to determine elevations for sections
•
The elevation of a station can be
established by:
inclined line of sight from chain survey stations
levelling from another point of known height
by inclined tacheometry
•
Levelling is the more accurate method but is
also the slower. Modern instruments are
capable of cm accuracy under normal
conditions over distances of 100’s metres.
•
The keys to successful levelling lie in the
setting up of the instrument, in the closure of
the traverses and in the careful recording
(booking) of the results.
•
Inclined tacheometry relies on the combined
measurement, by theodolite, of the slant
distance to the new station and the angle
relative to the horizontal.
•
The elevation change and horizontal distance
can then be found by simple trigonometry.
•
The accuracy of the method, using normal
instruments, is around 10’s cms in 100’s
metres.
Direct levelling
•
Most typical form used
Relies upon:
•
a horizontal line of sight, also termed
“the line of collimation”
•
a fixed datum level
Measurements to be taken
•
Backsight
•
Foresight
•
Intermediate sights
Booking your results
The “rise and fall” method
•
This method records the relative change
in level between successive stations
•
The changes are converted to the
reduced level of each station
•
The reduced level is relative to the local
datum
Booking the results
•
The method relies on recording your
results in a survey book in a standard
format
•
This allows you to check your work and
to identify any errors systematically
Reduced levels
The change of level is 2.312m

2.533m =

0.221m
2.533m
Datum line: 100.522m
(from OS Benchmark)
2.312 m
The
reduced
level of point B is
100.301m
B
A
The absolute (datum) level of point A is
100.522m
IP 1
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks
2.312
100.522
0.221
100.301
1.2
Rise and fall booking
Point A
Point B

2.533
Transfer of level
The new change of level is 1.674m

1.631m = + 0.043m
1.631m
1.674 m
The absolute level of point C is
100.344m
C
B
At the next stage, B becomes the backsight and C is the new foresight
IP 2
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks
2.312
100.522

0.221
100.301
1.2
Rise and fall booking (cont)
Point A
Point B

2.533
1.674
1.631
+ 0.043
100.344
Point C
•
Continuing this process, suppose we
end up with a set of results as follows:
•
This will enable us to check our working
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks
2.312
100.522

0.221
100.301
Rise and fall booking (cont)
Point A
Point B

2.533
1.674
1.631
+ 0.043
100.344
Point C
2.504
3.010
2.413
0.956
2.016
2.718




+ 1.548
+ 0.994

0.305
101.892
102.886




102.581
11.913
9.854
9.854
2.585

0.526

0.526
102.581

100.522
2.059
2.059
2.059
CHECKS
OK
Using an intermediate sight
•
Sometimes we wish to include a specific
feature but it is not convenient to set up
a new instrument position for this
•
The solution is to take a sighting onto
the staff when it is placed on this feature

this is called an
intermediate
sight
Intermediate sight
The new change of level is 1.674m

2.988m =

1.314m
2.988m
The absolute level of the intermediate point C is
98.987m
C
B
The intermediate sight is taken at the base of the channel between B and C
IP 2
Intermediate sight
1.674m
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks
2.312
100.522

0.221
100.301
1.2
Rise and fall booking
(intermediate sight)
Point A
Point B

2.533
1.674
1.631
+ 0.043
100.344
Point C
2.988
channel

1.314
98.987
Next FS
Optical distance measurement
•
It is often convenient to use the levelling
instrument itself to calculate the distance
between the instrument and staff positions
•
This is done using the
stadia lines
that are
visible in the viewfinder
•
These are arranged such that the distance to
the staff is
100x
the
stadia interval
that is
read on the staff between the two lines
•
This procedure is known as
tacheometry
Tacheometry
The viewfinder:
Stadia
lines
Multiply vertical
distance by 100
to obtain
horizontal distance
Inclined tacheometry
•
If the ‘level’ can be swung in a vertical
arc, the distance up an inclined sight
line can be obtained.
•
If the vertical angle is also measured,
the slant distance can be converted to
give both the change in height and the
true horizontal distance.
Inclined tacheometry
a
Change of height
Tacheometric distance
Measured angle
True horizontal distance
The theodolite
•
If such an instrument can also be swung in a
horizontal arc, and the angle of rotation can
be measured, we are able to determine the
angles of the sight lines between stations.
•
This allows both trilateration and triangulation
with the same instrument.
•
Such a versatile instrument exist and is called
a
theodolite
.
Summary
•
Chain surveys are suited to planimetric
surveys on low slopes. They rely upon
trilateration.
•
Levelling is used where terrain is more
uneven. Levelling surveys often use
tacheometry to fix station positions.
•
A theodolite survey permits levelling,
tacheometry or triangulation as required.
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