Lesson Note On Methods of Grid Levelling
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.
To determine the positions of the contours
on a plan the leveling methods are used to find the relative positions of
points on the surface of the ground with reference to certain benchmarks at
regular intervals. This technique of making observations and plotting on a plan
is called as grid leveling.
on a plan the leveling methods are used to find the relative positions of
points on the surface of the ground with reference to certain benchmarks at
regular intervals. This technique of making observations and plotting on a plan
is called as grid leveling.
Contour Interval:
The vertical distance between any two
consecutive contours is known as a contour interval. For example, if the
various consecutive contours are 100m, 98m, 96 m etc., then the
contour interval is 2m. This interval depends upon,
consecutive contours is known as a contour interval. For example, if the
various consecutive contours are 100m, 98m, 96 m etc., then the
contour interval is 2m. This interval depends upon,
·
The
nature of the ground
The
nature of the ground
·
The
scale of the map and
The
scale of the map and
·
The
purpose of survey
The
purpose of survey
Contour intervals for flat country are
generally small, eg. 0.25m, 0.5m, 0.75 m etc. For a steep slope in hilly
area is greater, eg. 5m, 10m, 15 m etc.
generally small, eg. 0.25m, 0.5m, 0.75 m etc. For a steep slope in hilly
area is greater, eg. 5m, 10m, 15 m etc.
Again ,for a small-scale map, the interval
may be of 1m,2m,3m etc. and for large scale map,it may be of 0.25m,0.50m,0.75m
etc.
may be of 1m,2m,3m etc. and for large scale map,it may be of 0.25m,0.50m,0.75m
etc.
It should be remembered that the contour interval
for a particular map is constant.
for a particular map is constant.
Contouring:
The process of locating these contour
lines on the surface of the earth is known as contouring.
lines on the surface of the earth is known as contouring.
Methods of contouring:
The method of establishing / plotting
contours in a plan or map is known as contouring. It requires planimetric
position of the points and drawing of contours from elevations of the plotted
points. Contouring involves providing of vertical control for location of
points on the contours and horizontal control for planimetric plotting of
points. Thus, contouring depends upon the instruments used (to determine the
horizontal as well as vertical position of points). In general, the field
methods of contouring may be divided into two classes:
contours in a plan or map is known as contouring. It requires planimetric
position of the points and drawing of contours from elevations of the plotted
points. Contouring involves providing of vertical control for location of
points on the contours and horizontal control for planimetric plotting of
points. Thus, contouring depends upon the instruments used (to determine the
horizontal as well as vertical position of points). In general, the field
methods of contouring may be divided into two classes:
1.
Direct
methods
Direct
methods
2.
Indirect
methods
Indirect
methods
Direct
Method:
Method:
In the direct method, the contour to be
plotted is actually traced on the ground. Points which happen to fall on a
desired contour are only surveyed, plotted and finally joined to obtain the
particular contour. This method is slow and tedious and thus used for large
scale maps, small contour interval and at high degree of precision.
plotted is actually traced on the ground. Points which happen to fall on a
desired contour are only surveyed, plotted and finally joined to obtain the
particular contour. This method is slow and tedious and thus used for large
scale maps, small contour interval and at high degree of precision.
Vertical
control :
control :
In this method, a benchmark is required in
the project area. The level is set up on any commanding position and back sight
is taken on the bench mark. Let the back sight reading on the bench mark be
1.485 m. If the reduced level of the bench mark is 100 m, the height of
instrument would be 100 + 1.485 = 101.485 m.
the project area. The level is set up on any commanding position and back sight
is taken on the bench mark. Let the back sight reading on the bench mark be
1.485 m. If the reduced level of the bench mark is 100 m, the height of
instrument would be 100 + 1.485 = 101.485 m.
To locate the contour of 100.5 m value,
the staff man is directed to occupy the position on the ground where the staff
reading is 101.485 -100.500 = 0.985 m. Mark all such positions on the ground
where the staff reading would be 0.985 m by inserting pegs. Similarly locate the
points where the staff reading would be 101.485 -101 = 0.485 m for 101m
contour.
the staff man is directed to occupy the position on the ground where the staff
reading is 101.485 -100.500 = 0.985 m. Mark all such positions on the ground
where the staff reading would be 0.985 m by inserting pegs. Similarly locate the
points where the staff reading would be 101.485 -101 = 0.485 m for 101m
contour.
The contour of 101.5 m cannot be set from
this setting of the instrument because the height of instrument for this
setting of the instrument is only 101.485 m. Therefore, locating contours of
higher value, the instrument has to be shifted to some other suitable position.
this setting of the instrument because the height of instrument for this
setting of the instrument is only 101.485 m. Therefore, locating contours of
higher value, the instrument has to be shifted to some other suitable position.
Establish a forward station on a firm
ground and take fore sight on it. This point acts as a point of known
elevation, for shifting the position of the instrument to another position,
from where the work proceeds in the similar manner till the entire area is
contoured.
ground and take fore sight on it. This point acts as a point of known
elevation, for shifting the position of the instrument to another position,
from where the work proceeds in the similar manner till the entire area is
contoured.
Horizontal
control :
control :
The horizontal control is generally
provided by method of plane table surveying or locating the positions of
points.
provided by method of plane table surveying or locating the positions of
points.
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.
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