Lesson Note On Contours- Map
preparation
The art of determining relative altitudes
of points on the surface of the earth of beneath the surface of earth is called
LEVELLING.
of points on the surface of the earth of beneath the surface of earth is called
LEVELLING.
A contour is defined as an imaginary line
of constant elevation on the ground surface. It can also be defined as the line
of intersection of a level surface with the ground surface. For example, the
line of intersection of the water surface of a still lake or pond with the
surrounding ground represents a contour line.
of constant elevation on the ground surface. It can also be defined as the line
of intersection of a level surface with the ground surface. For example, the
line of intersection of the water surface of a still lake or pond with the
surrounding ground represents a contour line.
Indirect method of contouring:
In this method, the spot levels of
selected guide points are taken with a level and their levels are computed. The
horizontal positions of these points are measured or computed and the points
are plotted on the plan. The contours are then drawn by a process called
interpolation of contours from the levels of the guide points. The following
are the indirect methods are commonly used for locating contours.
selected guide points are taken with a level and their levels are computed. The
horizontal positions of these points are measured or computed and the points
are plotted on the plan. The contours are then drawn by a process called
interpolation of contours from the levels of the guide points. The following
are the indirect methods are commonly used for locating contours.
1.
Squares
or Grid method
Squares
or Grid method
2.
Cross
section method
Cross
section method
Square or
grid method:
grid method:
In this method, the area to be surveyed is
divided into a grid or series of squares. The grid size may vary from 5 m x 5 m
to 25 m x 25 m depending upon the nature of the terrain, the contour interval
required and the scale of the map desired. Also, the grids may not be of the
same size throughout but may vary depending upon the requirement and field
conditions. The grid corners are marked on the ground and spot levels of these
comers are determined by leveling. The grid is plotted to the scale of the map
and the spot levels of the grid corners are entered. The contours of desired
values are then located by interpolation. Special care should be taken to give
the spot levels to the salient features of the ground such as hilltops, deepest
points of the depressions, and their measurements from respective corners of
the grids, for correct depiction of the features. The method is used for large
scale mapping and at average precision.
divided into a grid or series of squares. The grid size may vary from 5 m x 5 m
to 25 m x 25 m depending upon the nature of the terrain, the contour interval
required and the scale of the map desired. Also, the grids may not be of the
same size throughout but may vary depending upon the requirement and field
conditions. The grid corners are marked on the ground and spot levels of these
comers are determined by leveling. The grid is plotted to the scale of the map
and the spot levels of the grid corners are entered. The contours of desired
values are then located by interpolation. Special care should be taken to give
the spot levels to the salient features of the ground such as hilltops, deepest
points of the depressions, and their measurements from respective corners of
the grids, for correct depiction of the features. The method is used for large
scale mapping and at average precision.
Cross
section method:
section method:
In these sections, a base line, centre
line or profile line is considered. Cross sections are taken
perpendicular to this line at regular intervals. After this points are
marked along the cross sections at regular intervals. A temporary bench
mark is set up near the site. Staff readings are taken along the base
line and the cross sections. The readings are entered in the level book the
base line and the cross sections should also be mentioned. The RL of each
of the points calculated. Then the base line and cross sections are
plotted to a suitable scale. Subsequently the RLs of the respective
points are noted on the map, after which the required contour line is drawn by
interpolation
line or profile line is considered. Cross sections are taken
perpendicular to this line at regular intervals. After this points are
marked along the cross sections at regular intervals. A temporary bench
mark is set up near the site. Staff readings are taken along the base
line and the cross sections. The readings are entered in the level book the
base line and the cross sections should also be mentioned. The RL of each
of the points calculated. Then the base line and cross sections are
plotted to a suitable scale. Subsequently the RLs of the respective
points are noted on the map, after which the required contour line is drawn by
interpolation
This method is suitable for route survey,
when cross sections are taken transverse to the longitudinal section.
when cross sections are taken transverse to the longitudinal section.
Method of interpolation of contours:
The process of locating the contours
proportionately between the plotted points is termed interpolation.
Interpolation may be done by:
proportionately between the plotted points is termed interpolation.
Interpolation may be done by:
1.
Arithmetical
calculation
Arithmetical
calculation
2.
The
graphical method
The
graphical method
By
arithmetical calculation
arithmetical calculation
Let A and B be two corners of the
squares. The RL of A is 98.75 m, and that of B 100.75 m. the
horizontal distance between A and B is 10m.
squares. The RL of A is 98.75 m, and that of B 100.75 m. the
horizontal distance between A and B is 10m.
Horizontal distance between A and B = 10m
Vertical difference A and B =
100.75-98.75=2m
100.75-98.75=2m
Let a contour of 99.00 m be required.
Then,
Then,
Difference of level between A and 99.00m
contour = 99.00-98.75=0.25m
contour = 99.00-98.75=0.25m
Therefore, distance of 99.00 m contour
line form A= 10/2 *0.25=1.25m
line form A= 10/2 *0.25=1.25m
This calculated distance is plotted to the
same scale in which the skeleton was plotted to obtain a point of RL of 99.00
m.
same scale in which the skeleton was plotted to obtain a point of RL of 99.00
m.
Similarly, the other points can be
located.
located.
By
graphical method
graphical method
On a sheet of tracing paper, a line AB is
drawn and divided into equal parts. AB is bisected at C and a
perpendicular is drawn at this point. A point O is selected on this
perpendicular line and then radial lines are drawn from O to the divisions on
AB. After this lines 1-1, 2-2, 3-3….are drawn parallel to AB. These
lines serve as guide lines. Boundary line and every fifth the line is
marked with a thick or red line.
drawn and divided into equal parts. AB is bisected at C and a
perpendicular is drawn at this point. A point O is selected on this
perpendicular line and then radial lines are drawn from O to the divisions on
AB. After this lines 1-1, 2-2, 3-3….are drawn parallel to AB. These
lines serve as guide lines. Boundary line and every fifth the line is
marked with a thick or red line.
Suppose we have to interpolate a 2m
contour between two points a and b of RLs 92.5 and 100.75m.
contour between two points a and b of RLs 92.5 and 100.75m.
Let us consider the lowest radial line OB
to represent an RL of 90.00. So, every fifth line will represent 95,100,105,
etc. The tracing paper is moved over the plan until ‘a’ lies at 92.5 and
‘b’ at 100.25. Line ‘ab’ should be parallel to AB. Now the points 94, 96,
98,100 are picked through to obtain the positions of the required contours.
to represent an RL of 90.00. So, every fifth line will represent 95,100,105,
etc. The tracing paper is moved over the plan until ‘a’ lies at 92.5 and
‘b’ at 100.25. Line ‘ab’ should be parallel to AB. Now the points 94, 96,
98,100 are picked through to obtain the positions of the required contours.
Method of interpolation of contours:
The process of locating the contours
proportionately between the plotted points is termed interpolation.
Interpolation may be done by:
proportionately between the plotted points is termed interpolation.
Interpolation may be done by:
1.
Arithmetical
calculation
Arithmetical
calculation
2.
The
graphical method
The
graphical method
By arithmetical calculation
Let A and B be two corners of the
squares. The RL of A is 98.75 m, and that of B 100.75 m. the
horizontal distance between A and B is 10m.
squares. The RL of A is 98.75 m, and that of B 100.75 m. the
horizontal distance between A and B is 10m.
Horizontal distance between A and B = 10m
Vertical difference A and B =
100.75-98.75=2m
100.75-98.75=2m
Let a contour of 99.00 m be
required. Then,
required. Then,
Difference of level between A and 99.00m
contour = 99.00-98.75=0.25m
contour = 99.00-98.75=0.25m
Therefore, distance of 99.00 m contour
line form A= 10/2 *0.25=1.25m
line form A= 10/2 *0.25=1.25m
This calculated distance is plotted to the
same scale in which the skeleton was plotted to obtain a point of RL of 99.00
m.
same scale in which the skeleton was plotted to obtain a point of RL of 99.00
m.
Similarly, the other points can be
located.
located.
By graphical method
On a sheet of tracing paper, a line AB is
drawn and divided into equal parts. AB is bisected at C and a
perpendicular is drawn at this point. A point O is selected on this
perpendicular line and then radial lines are drawn from O to the divisions on
AB. After this lines 1-1, 2-2, 3-3….are drawn parallel to AB. These
lines serve as guide lines. Boundary line and every fifth the line is
marked with a thick or red line.
drawn and divided into equal parts. AB is bisected at C and a
perpendicular is drawn at this point. A point O is selected on this
perpendicular line and then radial lines are drawn from O to the divisions on
AB. After this lines 1-1, 2-2, 3-3….are drawn parallel to AB. These
lines serve as guide lines. Boundary line and every fifth the line is
marked with a thick or red line.
Suppose we have to interpolate a 2m
contour between two points a and b of RLs 92.5 and 100.75m.
contour between two points a and b of RLs 92.5 and 100.75m.
Let us consider the lowest radial line OB to represent an RL of 90.00.
So, every fifth line will represent 95,100,105, etc. The tracing paper is
moved over the plan until ‘a’ lies at 92.5 and ‘b’ at 100.25. Line ‘ab’ should
be parallel to AB. Now the points 94, 96, 98,100 are picked through to
obtain the positions of the required contours.
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