As
we know the aim of surveying is to make
plans and maps to show various objects on the ground at
their relative position to suitable scale. Various steps involved in making the
plans is explained in this Article. Contouring is the technique of showing the levels of ground in
a plan and Maping is the technique to prepare
the plan of the area surveyed both are will discuss in this article.
CONTOURS
A
contour line is a imaginary line which connects points of equal elevation. Such
lines are drawn on the plan
of an area after establishing reduced levels of several points in the area. The
contour lines in an area are drawn keeping
difference in elevation of between two consecutive lines constant.
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| Contour lines |
Characteristics of
Contours
The
contours have the following characteristics:
1.
Contour lines must close, not necessarily in the limits of the plan.
2. Widely spaced
contour indicates flat surface.
3.
Closely spaced contour indicates steep ground.
4.
Equally spaced contour indicates uniform slope.
5.
Irregular contours indicate uneven surface.
6.
Approximately concentric closed contours with decreasing values towards center indicate a pond.
7.
Approximately concentric closed contours with increasing values towards center indicate hills.
8. Contour lines with
U-shape with convexity towards lower ground indicate ridge
9.
Contour lines with V-shaped with convexity towards higher ground indicate
valley.
10.
Contour lines generally do not meet or intersect each other.
11. If contour lines
are meeting in some portion, it shows existence of a vertical cliff
12. If contour lines
cross each other, it shows existence of overhanging cliffs or a cave indicate a pond
Uses of Contour Maps
Contour
maps are extremely useful for various engineering works:
1. A
civil engineer studies the contours and finds out the nature of the ground to
identify.Suitable site for the project works
to be taken up.
2.
By drawing the section in the plan, it is possible to find out profile of the ground
along that line.
It helps in finding out depth of cutting and filling, if formation level of
road/railway is decided.
3. Intervisibility of any two points can be found by drawing profile of the ground along that line.
4. The routes of the railway, road,
canal or sewer lines can be decided so as to minimize and balance
earthworks.
5. Catchment area and hence quantity of
water flow at any point of nalla or river can be found.
This study is very important in locating bunds, dams and also to find out flood
levels.
6. From the contours,
it is possible to determine the capacity
of a reservoir.
METHODS OF CONTOURING
However,
broadly speaking there are two methods of surveying:
1.
Direct methods
2.
Indirect methods.
Direct Methods
It
consists in finding vertical and horizontal controls of the points which lie on
the selected contour line.For
vertical control levelling instrument is commonly used. A level is set on a
commanding position
in the area after taking fly levels from the nearby bench mark. The plane of
collimation/height of
instrument is found and the required staff reading for a contour line is
calculated. The instrument man
asks staff man to move up and down in the area till the required staff reading
is found. A surveyor establishes
the horizontal control of that point using his instruments. After that
instrument man directs the
staff man to another point where the same staff reading can be found. It is
followed by establishing horizontal
control. Thus several points are established on a contour line on one or two
contour lines and suitably
noted down. Plane table survey is ideally suited for this work. After required
points are established from
the instrument setting, the instrument is shifted to another point to cover
more area. The level and survey
instrument need not be shifted at the same time. It is better if both are
nearby so as to communicate easily.
For getting speed in levelling some times hand level and Abney levels are also
used. This method is
slow, tedious but accurate. It is suitable for small areas.
Indirect Methods
In
this method, levels are taken at some selected points and their levels are
reduced. Thus in this method horizontal
control is established first and then the levels of those points found. After
locating the points on
the plan, reduced levels are marked and contour lines are interpolated between
the selected points.For
selecting points anyone of the following methods may be used:
(a) Method of squares,
(b) Method of cross-section, or
(c) Radial line method.
Drawing Contours
After
locating contour points smooth contour lines are drawn connecting corresponding
points on a contour
line. French curves may be used for drawing smooth lines. A surveyor should not
lose the sight of
the characteristic feature on the ground. Every fifth contour line is made
thicker for easy readibility.On
every contour line its elevation is written. If the map size is large, it is
written at the ends also.
MAPPING
After
completing field work in chain survey and compass survey lot of office work is
involved to prepare
the plan of the area surveyed. In plane table survey office work is less. The
office work involved consist
of
1.
Applying necessary corrections to measurements
2.
Drawing index plan
3.
Selecting scale
4.
Selecting orientation
5.
Drawing network of survey lines
6.
Distributing closing error
7.
Filling in the details
8.
Colouring the map
9.
Drawing graphical scale
10.
Writing index.
Applying Necessary
Corrections to Measurements
Necessary
tape and chain corrections and corrections for local attraction in case of
compass survey,should
be applied to the survey lines measured.
Drawing Index Plan
On a
rough sheet index plan also known as key plan is drawn. This need not be to the
scale but distances and
directions of network of survey lines should be approximately to a scale. This
plan is necessary to identify
the shape of the area to be plotted.
Selecting Scale
Depending
upon the type of survey, scale should be selected. In general, scale selected
should be as large
as possible, if a range of scale is recommended. It depends upon the size of
the paper as well as largest
linear measurement in the field.
Selecting
Orientation
Looking
at index plan, orientation of map is to be decided so that the map is placed in
the middle of the drawing
sheet with its larger dimension approximately along the length of paper. North
direction is selected
and marked.
Drawing Network of
Survey Lines
Studying
index map and orientation of paper, first station point of survey is marked.
Starting from here one
by one survey line is drawn to the scale in its direction. After drawing all
survey lines, it is clearly seen
whether the selected scale and orientation appropriate. If necessary they may
be changed and network
of survey lines is redrawn.
Distributing Closing
Error
Sometimes
in closed traverse, the last point may not coincide with the plotted position
of first point.The
difference between the plotted position is known as closing error. Before
adjusting closing error it is
necessary that there are no plotting errors. If it is due to field work error
and the error is reasonably small
it can be adjusted in the office. If error is large, one has to go back to the
field and check doubtful measurements.
In the office closing error is adjusted distributing it suitably to all lines
graphically or by mathematical
calculation of corrected coordinates of station points. After adjusting closing
error network of
survey lines are drawn as per the convention.
Filling in the
Details
Surveyor
has to go through details of one by one survey lines. One by one point of
object noted in the field
is marked on the drawing sheet by converting the change and offsets to the
scale. Main scale and offset
scales will be quite useful for this work. After marking the salient points of
the objects like building,
boundary lines, roads, culvert ends, trees, electric poles etc. the respective
lines are joined to mark
the object. The field book will be useful in identifying the objects. If the
object is building, the measurements
may be only for salient points near the survey lines looking at overall
dimensions of the building
and scaling down, complete building may be shown in the plan. Thus attending to
the field observations
of each survey lines all details may be shown. Standard conventions should be
used in showing
the objects.
Colouring the Map
If
coloured maps are to be made, the recommended light washes of standard shades
as listed is IS 962-1989
(Chapter 7) may be applied.
Drawing Graphical
Scale
As
the drawing sheet may shrink and the measurement taken from shrunk sheet may
mislead the distances between
any two objects on the map, it is necessary to draw a graphical scale of 150 to
270 mm long just over
the space for indexing the drawing, which is right hand side lower corner of
the sheet.
Writing Index
Index
is the details giving the description of the area plotted, scale used, name of
leader of survey party and
the person drawing the plan/map. etc. It is normally written in the right hand
side lower corner of the
drawing sheet. North direction is shown neatly at the right hand side top
corner.
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