land surveying chapter 2 contouring

Chapter 4 : Contouring

4.1 Contour Characteristics

4.2 Method of Contouring

4.3 Choice of Vertical Interval

4.4 Interpolating the Contours

Contents

(1.) Contouring line A line drawn on a plan joining

all points of same height aboveor below some datum.

Contour lines are continuouslines and cannot meet or crossany other contour line, nor canany one line split or join anyother line, except in the caseof a cliff or overhang.


(1.) Contouring line Contour plan and section of an island.


(2.) Gradients The height between successive contours is called the

vertical interval or contour interval and is always constant over a map or plan.


Example:In Figure 4.1, determine the gradient of the ground between

(a) the point A and C, and; (b) the point D and E.


Solution:

(a) Gradient between the point A and C = 10/100 = 1/10 = 1 in

10

(b) Gradient between the point D and E = 10/30= 1/3

= 1 in 3


(3.) Reading Contours It should be clear from the above examples that the

gradient is steep where the contours are close together and conversely flat where the contours are far apart.

Regular gradient


(3.) Reading Contours



Steep Slope Moderate slope

Uniform slope




Saddle


(3.) Reading ContoursSaddles are a natural travel funnel between two high points of ground. A saddle can just be a dip in the elevation between two ridges or mountain peaks; they do not have to be a drastic change in elevation.

Mark the following topographic features on a contour map shown:

(a)Valley(b)Saddle points(c)Ridge line


Draw section A1A2, B1B2, and C1C2 from the contour map shown.


A1A2

C2

B2

B1

C1

There are 3 methods of determining contours.


(1.) Direct Method The point of required elevation are directly located on

the field with the help of levelling and positions of these points are then surveyed by either chain offset method or by the method traversing.

The contours of the required elevations are then directly obtained by joining those points.

Levelling.


(1.) Direct Method Direct method of locating contours.


(2.) Indirect Method The contouring spot levels of the ground are taken at

random and the positions of those points are determined by either chain offset method or be the method of traversing and plotted on the paper.

After the points are plotted on the paper with their elevations written against each point the contours of required elevations are then drawn by interpolation.


(2.) Indirect Method


(3.) Square Method It is an indirect method of contouring in which instead

of taking levels of points at random the whole area is divided into regular squares of convenient size depending upon the size of the plot to be contoured and levels taken at the junction of each square.


(3.) Square Method


Comparison of direct and indirect methods of contouring :


Direct Method Indirect Method

Most accurate but slow and tedious. Not very accurate but quicker and less tedious.

Expensive. Very cheap.

Used for small projects requiring higher accuracy, e.g. layout of building, factory, structural foundations, etc.

Used for large projects requiring lower accuracy, e.g. layout of highway, railway, canal, etc.

Unsuitable for hilly areas. Suitable for hilly terrain using tacheometry.

Calculation to be done in the field. Calculation in the field not required.

After contouring, calculation can not be checked.

Calculations can be checked as and when needed.

The vertical interval of the contour lines on any plan depend on several factors, namely:

• Purpose and Extent of Survey.• Scale of the Map or Plan.• Nature of Terrain.


Purpose and Extent of Survey If the survey is intended for detailed design work or for

accurate earthwork calculations, a small contour interval will be required.

The extent of survey in such cases will generally be small.

In the case of location surveys, for lines of communications, and for reservoir and drainage areas, where the extent of survey is large, a large contour interval should be used.


Scale of the map or plan On small-scale maps the vertical interval has to be

fairly large. If not, some essential details might be obscured by the

large number of contour lines produced by a small vertical interval.

The contour interval is normally inversely proportional to the scale of map.

If the scale is large, the contour interval should be small, and vice versa.


Nature of Terrain

A contour interval chosen for a flat ground will be

highly unsuitable for undulating ground.

For flat ground, a small contour interval is chosen

whereas for undulating and broken ground, larger

contour interval should be adopted.


Contour lines may be interpolated on the

grid either mathematically or graphically.


In the figure below, the levels of the grid are indicated.Draw a 7m contour line by interpolating mathematically.


Figure below represents a contouring survey using 10 mx10 m grid squares. Find the offset distances X, Y, and Z.


6.50 7.15 7.35

6.65 7.05

7.65 7.95

8.25

9.25

X

Y

Z

7.00 7.50

7.00

7.50

8.00

8.00

land surveying chapter 2 contouring

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