3.1 PRINCIPLE OF CHAIN
SURVEYING
SURVEYING
The principle of chain
surveying is triangulation. This means that the area to be surveyed is divided
into a number of small triangles which should be well conditioned. In chain
surveying the sides of the triangles which should be well conditioned. In chain
surveying the sides of the triangles are measured directly on the field by
chain or tape, and no angular measurements are taken. Here, the tie lines and
check lines control the accuracy of work.
surveying is triangulation. This means that the area to be surveyed is divided
into a number of small triangles which should be well conditioned. In chain
surveying the sides of the triangles which should be well conditioned. In chain
surveying the sides of the triangles are measured directly on the field by
chain or tape, and no angular measurements are taken. Here, the tie lines and
check lines control the accuracy of work.
It should be noted that
plotting triangles requires no angular measurements to be made, if the three
sides are known.
plotting triangles requires no angular measurements to be made, if the three
sides are known.
Chain surveying is
recommended when:
recommended when:
1.
The ground surface is
more or less level
The ground surface is
more or less level
2.
A small area is to be
surveyed
A small area is to be
surveyed
3.
A small-scale map is to
be prepared and
A small-scale map is to
be prepared and
4.
The formation of
well-conditioned triangles is easy
The formation of
well-conditioned triangles is easy
Chain surveying is
unsuitable when:
unsuitable when:
1.
The area is crowded with
many details
The area is crowded with
many details
2.
The area consists of too
many undulations
The area consists of too
many undulations
3.
The area is very large
and
The area is very large
and
4.
The formation of
well-conditioned triangles becomes difficult due to obstacles
The formation of
well-conditioned triangles becomes difficult due to obstacles
A. Large-Scale and
Small-Scale Maps
Small-Scale Maps
When 1 cm of a map
represents a small distance, it is said to be a large-scale map.
represents a small distance, it is said to be a large-scale map.
For example,
When 1 cm of the map
represents a large distance, it is called a small-scale map.
represents a large distance, it is called a small-scale map.
For example,
A map having an RF of
less than 1/500 is considered to be large-scale. A map of RF more than 1/500 is
said to be small-scale.
less than 1/500 is considered to be large-scale. A map of RF more than 1/500 is
said to be small-scale.
3.2 WELL-CONDITIONED AND
ILL-CONDITIONED TRIANGLES
ILL-CONDITIONED TRIANGLES
A triangle is said to be
well-conditioned when no angle in it is less than 300 or
greater than 1200 . An equilateral triangle is considered to be
the best-condition or ideal triangle
well-conditioned when no angle in it is less than 300 or
greater than 1200 . An equilateral triangle is considered to be
the best-condition or ideal triangle
Well-conditioned
triangles are preferred because their apex points are very sharp and can be
located by a single ‘dot’. In such a case, there is no possibility of relative
displacement of the plotted point.
triangles are preferred because their apex points are very sharp and can be
located by a single ‘dot’. In such a case, there is no possibility of relative
displacement of the plotted point.
A triangle in which an
angle is less than 300 or more than 1200 is
said to be ill-conditioned
angle is less than 300 or more than 1200 is
said to be ill-conditioned
Well – conditioned
triangles are not used in chain surveying. This is because their apex points
are not sharp and well defined, which is why a slight displacement of these
points may cause considerable error in plotting.
triangles are not used in chain surveying. This is because their apex points
are not sharp and well defined, which is why a slight displacement of these
points may cause considerable error in plotting.
3.3 RECONNAISSANCE
SURVEY AND INDEX SKETCH
SURVEY AND INDEX SKETCH
Before the commencement
of any survey work, the area to be surveyed is thoroughly examined by the
surveyor, who then thinks about the possible arrangement of the framework of
survey. This primary investigations of the area is termed as reconnaissance
survey or reconnoitre.
of any survey work, the area to be surveyed is thoroughly examined by the
surveyor, who then thinks about the possible arrangement of the framework of
survey. This primary investigations of the area is termed as reconnaissance
survey or reconnoitre.
During reconnaissance
survey, the surveyor should walk over the area and note the various obstacles
and whether or not the selected stations are intervisible. The main stations
should be so selected that they enclose the whole area. The surveyor should
also take care that
survey, the surveyor should walk over the area and note the various obstacles
and whether or not the selected stations are intervisible. The main stations
should be so selected that they enclose the whole area. The surveyor should
also take care that
The neat hand sketch of
the area which is prepared during reconnaissance survey is known as the ‘index
sketch’ or ‘key plan’. The index sketch shows the skeleton of the survey work.
It indicates the main survey stations, sub-stations, tie stations, base line,
arrangement for framework of triangles and the approximate positions of
different objects. This sketch is an important document for the surveyor
and for the person who will plot the map. It should be attached to the starting
page of the field book
the area which is prepared during reconnaissance survey is known as the ‘index
sketch’ or ‘key plan’. The index sketch shows the skeleton of the survey work.
It indicates the main survey stations, sub-stations, tie stations, base line,
arrangement for framework of triangles and the approximate positions of
different objects. This sketch is an important document for the surveyor
and for the person who will plot the map. It should be attached to the starting
page of the field book
3.4 DEFINITIONS AND
ILLUSTRATIONS
ILLUSTRATIONS
A. Survey Stations
Survey stations are the
points at the beginning and the end of a chain line. They may also occur at any
convenient points on the chain line. Such stations may be:
points at the beginning and the end of a chain line. They may also occur at any
convenient points on the chain line. Such stations may be:
1.
Main stations
Main stations
2.
Subsidiary stations and
Subsidiary stations and
3.
Tie stations
Tie stations
2.
Main
stations Stations taken
along the boundary of an area as controlling points are known as ‘main
survey lines’. The main survey lines should cover the whole area to be
surveyed. The main stations are denoted by ‘ ’ with letters A, B, C, D, etc.
The chain lines are denoted by “__ … __ … __…__…__…__”.
Main
stations Stations taken
along the boundary of an area as controlling points are known as ‘main
survey lines’. The main survey lines should cover the whole area to be
surveyed. The main stations are denoted by ‘ ’ with letters A, B, C, D, etc.
The chain lines are denoted by “__ … __ … __…__…__…__”.
3.
Subsidiary
stations Stations which are
on the main survey lines or any other survey lines are known as “Subsidiary
stations”. These stations are taken to run subsidiary lines for dividing the
area into triangles, for checking the accuracy of triangles and for locating
interior details. These stations are denoted by ‘’ with letters S1,S2,S3,
etc.
Subsidiary
stations Stations which are
on the main survey lines or any other survey lines are known as “Subsidiary
stations”. These stations are taken to run subsidiary lines for dividing the
area into triangles, for checking the accuracy of triangles and for locating
interior details. These stations are denoted by ‘’ with letters S1,S2,S3,
etc.
4.
Tie
stations These are also
subsidiary stations taken on the main survey lines. Lines joining the tie
stations are known as tie lines. Tie lines are mainly taken to fix the
directions of adjacent sides of the chain survey map. These are also taken to
form ‘chain angles’ in chain traversing, when triangulation is not possible.
Sometimes tie lines are taken to locate interior details. Tie stations are
denoted by ‘’ with letters T1, T2, T3. Etc.
Tie
stations These are also
subsidiary stations taken on the main survey lines. Lines joining the tie
stations are known as tie lines. Tie lines are mainly taken to fix the
directions of adjacent sides of the chain survey map. These are also taken to
form ‘chain angles’ in chain traversing, when triangulation is not possible.
Sometimes tie lines are taken to locate interior details. Tie stations are
denoted by ‘’ with letters T1, T2, T3. Etc.
B. Base
Line
Line
The line on which the
framework of the survey is built is known as the ‘base line’. It is the most
important line of the survey. Generally, the longest of the main survey lines
is considered the base line. This line should be taken through fairly level
ground, and should be measured very carefully and accurately. The magnetic
bearings of the base line are taken to fix the north line of the map.
framework of the survey is built is known as the ‘base line’. It is the most
important line of the survey. Generally, the longest of the main survey lines
is considered the base line. This line should be taken through fairly level
ground, and should be measured very carefully and accurately. The magnetic
bearings of the base line are taken to fix the north line of the map.
C. Check
Line
Line
The line joining the
apex point of a triangle to some fixed point on its base is known as the ‘check
line’. It is taken to check the accuracy of the triangle. Sometimes this line
helps to locate interior details.
apex point of a triangle to some fixed point on its base is known as the ‘check
line’. It is taken to check the accuracy of the triangle. Sometimes this line
helps to locate interior details.
D. Offset
The lateral measurement
taken from an object to the chain line is known as ‘offset’. Offsets are taken
to locate objects with reference to the chain line. They may be of two
kinds – perpendicular and oblique.
taken from an object to the chain line is known as ‘offset’. Offsets are taken
to locate objects with reference to the chain line. They may be of two
kinds – perpendicular and oblique.
1. Perpendicular
offsets When the lateral
measurements are taken perpendicular to the chain line, they are known as
perpendicular offsets
offsets When the lateral
measurements are taken perpendicular to the chain line, they are known as
perpendicular offsets
Perpendicular offsets
may be taken in the following ways:
may be taken in the following ways:
(a) By setting a
perpendicular by swinging a tape from the object to the chain line. The point
of minimum reading on the tape will be the base of the perpendicular
perpendicular by swinging a tape from the object to the chain line. The point
of minimum reading on the tape will be the base of the perpendicular
(b) By setting a right
angle in the ratio 3 : 4 : 5
angle in the ratio 3 : 4 : 5
(c) By setting a right
angle with the help of builder’s square or tri-square
angle with the help of builder’s square or tri-square
(d) By setting a right
angle by cross-staff or optical square.
angle by cross-staff or optical square.
2. Oblique offsets Any offset not perpendicular to the chain
line is said to be oblique. Oblique offsets are taken when the objects are at a
long distance from the chain line or when it is not possible to set up a right
angle due to some difficulties. Such offsets are taken in the following manner.
line is said to be oblique. Oblique offsets are taken when the objects are at a
long distance from the chain line or when it is not possible to set up a right
angle due to some difficulties. Such offsets are taken in the following manner.
Suppose AB is a chain
line and p is the corner of a building. Two points ‘a’ and ‘b’ are taken on the
chain line. The chainages of ‘a’ and ‘b’ are noted. The distances ‘ap’ and ‘bp’
are measured and noted in the field book. Then ‘ap’ and ‘bp’ are the oblique
offsets. When the triangle abp is plotted, the apex point p will represent the
position of the corner of the the building.
line and p is the corner of a building. Two points ‘a’ and ‘b’ are taken on the
chain line. The chainages of ‘a’ and ‘b’ are noted. The distances ‘ap’ and ‘bp’
are measured and noted in the field book. Then ‘ap’ and ‘bp’ are the oblique
offsets. When the triangle abp is plotted, the apex point p will represent the
position of the corner of the the building.
Perpendicular offsets
are preferred for the following reasons:
are preferred for the following reasons:
(a) They can be taken
very quickly
very quickly
(b) The progress of
survey is not hampered
survey is not hampered
(c) The entry in the
field book becomes easy
field book becomes easy
(d) The plotting of the
offsets also becomes easy
offsets also becomes easy
3. Number of
offsets The offsets should be
taken according to the nature of the object. So, there is no hard and fast rule
regarding the number of offsets. It should be remembered that the objects
are to be correctly represented and hence the number of offsets should be
decided on the field. Some guidelines are given below:
offsets The offsets should be
taken according to the nature of the object. So, there is no hard and fast rule
regarding the number of offsets. It should be remembered that the objects
are to be correctly represented and hence the number of offsets should be
decided on the field. Some guidelines are given below:
(a) When the boundary of
the object is approximately parallel to the chain line, perpendicular offsets
are taken at regular intervals
the object is approximately parallel to the chain line, perpendicular offsets
are taken at regular intervals
(b) When the boundary is
straight, perpendicular offsets are taken at both ends of it
straight, perpendicular offsets are taken at both ends of it
(c) When the boundary
line is zigzag, perpendicular offsets are taken at every point of bend to
represent the shape of the boundary accurately. In such a case, the interval of
the offsets may be irregular
line is zigzag, perpendicular offsets are taken at every point of bend to
represent the shape of the boundary accurately. In such a case, the interval of
the offsets may be irregular
(d) When a road crosses
the chain line perpendicularly, the chainage of the intersection point is
to be noted
the chain line perpendicularly, the chainage of the intersection point is
to be noted
(e) When a road crosses
a chain line obliquely, the chainages of intersection points ‘a’ and ‘b’ are
noted. Then at least one offset is taken on both sides of the inter-section
points. More offsets may be taken depending on the nature of the road. Here,
perpendicular offsets are taken at ‘c’ and ‘d’
a chain line obliquely, the chainages of intersection points ‘a’ and ‘b’ are
noted. Then at least one offset is taken on both sides of the inter-section
points. More offsets may be taken depending on the nature of the road. Here,
perpendicular offsets are taken at ‘c’ and ‘d’
(f) When the building is
small, its corners are fixed by perpendicular or oblique offsets and the other
dimensions are taken directly on the field and noted in the field book.
small, its corners are fixed by perpendicular or oblique offsets and the other
dimensions are taken directly on the field and noted in the field book.
(g) When the building is
large, zigzag in shape and oblique to the chain line, then the corners are
fixed by perpendicular or oblique offsets. Then the full plan of the building
is drawn on a separate page along with all the dimensions. This page should be
attached with the field book at the proper place.
large, zigzag in shape and oblique to the chain line, then the corners are
fixed by perpendicular or oblique offsets. Then the full plan of the building
is drawn on a separate page along with all the dimensions. This page should be
attached with the field book at the proper place.
(h) When the object is
circular, perpendicular offsets are taken at short and regular intervals
circular, perpendicular offsets are taken at short and regular intervals
4. Limiting length
of offset The maximum length of
the offset should not be more than the length of the tape used in the survey.
Generally, the maximum length of offset is limited to 15m. However, this length
also depends upon the following factors:
of offset The maximum length of
the offset should not be more than the length of the tape used in the survey.
Generally, the maximum length of offset is limited to 15m. However, this length
also depends upon the following factors:
(a) The desired accuracy
of the map
of the map
(b) The scale of the map
(c) The maximum
allowable deflection of the offset from its true direction and
allowable deflection of the offset from its true direction and
(d) The nature of the
ground
ground
Problems on limiting
length of offset
length of offset
Problem 1 An offset was laid out 50 from
its true direction and the scale of the map was 20 m to 1 cm. Find the maximum
length of offset for the displacement of a point on the paper not to exceed
0.03 cm.
its true direction and the scale of the map was 20 m to 1 cm. Find the maximum
length of offset for the displacement of a point on the paper not to exceed
0.03 cm.
Solution Let AB be the actual length of offset
which was laid out 50 from its true direction. So, BC is
the displacement of the point.
which was laid out 50 from its true direction. So, BC is
the displacement of the point.
Let the maximum length of offset, AB = L m
or
BC = AB sin 50 = L sin 50 m (displacement
of the ground)
BC = AB sin 50 = L sin 50 m (displacement
of the ground)
Since the scale is 1 cm
to 20 m, 20 m on the ground represents 1 cm on the paper.
=
6.884 m
6.884 m
Therefore, the maximum length of offset should be 6.884 m.
Problem 2 The length of the offset is 15 m and the
scale of the plan 10 m to 1 cm. If the offset is laid out 30 from
its true direction, find the displacement of the plotted point on the paper
scale of the plan 10 m to 1 cm. If the offset is laid out 30 from
its true direction, find the displacement of the plotted point on the paper
(i) perpendicular to the
chain line, and
chain line, and
(ii) parallel to the
chain line.
chain line.
Solution Let AB be the actual length of offset,
which is 15 m long and deflected by 30 from its true direction.
which is 15 m long and deflected by 30 from its true direction.
Here,
BC = Displacement parallel to chain line
CD = displacement perpendicular to chain line
(i) CD = AD – AC =
AB – AC
AB – AC
= 15 – 15 cos 30
= 15 (1 – cos 30) m (displacement on the ground)
Since the scale is 1 cm to 10 m,
10 m on the ground = 1 cm on the map
= 0.002 cm on
the map
Required displacement perpendicular to chain line
= 0.002 cm (on paper)
(ii) BC = AB sin 30 =
15 sin 30 = 0.7850 m (displacement on ground)
15 sin 30 = 0.7850 m (displacement on ground)
E. Degree of Accuracy
Degree of accuracy is
determined before the starting of any survey work. It is worked out according
the following factors:
determined before the starting of any survey work. It is worked out according
the following factors:
(a) Scale of plotting
(b) Permissible error in
plotting
plotting
During reconnaissance survey,
the length of the main survey lines are approximately determined by the pacing
method. One pace or walking step of a man is considered to equal 80 cm. When
the length of the survey lines or the extent of area to be surveyed is
approximately known, the scale of the map may be assumed. Again, the
permissible error in plotting may be obtained from the concerned department.
Then the degree of accuracy in measurement is ascertained.
the length of the main survey lines are approximately determined by the pacing
method. One pace or walking step of a man is considered to equal 80 cm. When
the length of the survey lines or the extent of area to be surveyed is
approximately known, the scale of the map may be assumed. Again, the
permissible error in plotting may be obtained from the concerned department.
Then the degree of accuracy in measurement is ascertained.
Let us now consider an
example.
example.
Suppose the scale of
plotting is 5 m to 1 cm and the allowable error is 0.02 cm.
plotting is 5 m to 1 cm and the allowable error is 0.02 cm.
Then, 1 cm on the
map = 500 cm on the ground
map = 500 cm on the ground
0.02 cm on the map = 500 x 0.02 = 10 cm on the ground
So, the measurement
should be taken nearest to 10 cm.
should be taken nearest to 10 cm.
3.5 SELECTION OF SURVEY
STATIONS
STATIONS
The following points should
be remembered during the selection of survey stations:
be remembered during the selection of survey stations:
- The
stations should be so selected that the general principle of surveying may
be strictly followed. - The
stations should be intervisible. - The
stations should be selected in such a way that well-conditioned triangles
may be formed. - The
base line should be the longest of the main survey lines. - The
survey lines should be taken through fairly level ground, as far as
practicable. - The
main survey lines should pass close to the boundary line of the area to be
surveyed. - The
survey lines should be taken close to the objects so that they can be
located by short offsets. - The
tie stations should be suitably selected to fix the directions of adjacent
sides. - The
subsidiary stations should be suitably selected for taking check lines. - Stations
should be so selected that obstacles to chaining are avoided as far as
possible. - The survey lines should not be
very close to main roads, as survey work may then be interrupted by
traffic.
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