Chapter 4 : Contouring
Contents 4.1 Contour Characteristics 4.2 Method of Contouring 4.3 Choice of Vertical Interval 4.4 Interpolating the Contours
4.1 Contour Characteristics (1.) Contouring line
A line drawn on a plan joining all points of same height above or below some datum. Contour lines are continuous lines and cannot meet or cross any other contour line, nor can any one line split or join any other line, except in the case of a cliff or overhang.
4.1 Contour Characteristics
4.1 Contour Characteristics (1.) Contouring line
Contour plan and section of an island.
4.1 Contour Characteristics (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:
4.1 Contour Characteristics
In Figure 4.1, determine the gradient of the ground between (a) the point A and C, and; (b) the point D and E.
4.1 Contour Characteristics 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
=
4.1 Contour Characteristics (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
4.1 Contour Characteristics (3.) Reading Contours
4.1 Contour Characteristics (3.) Reading Contours
Steep Slope
Moderate slope
Uniform slope
4.1 Contour Characteristics (3.) Reading Contours
4.1 Contour Characteristics (3.) Reading Contours
4.1 Contour Characteristics (3.) Reading Contours
Saddle
4.1 Contour Characteristics (3.) Reading Contours Saddles 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.
4.1 Contour Characteristics (3.) Reading Contours
4.1 Contour Characteristics Mark the following topographic features on a contour map shown: (a)Valley (b)Saddle points (c)Ridge line
4.1 Contour Characteristics B 2
C2
A2
A1
C1 B1
Draw section A1A2, B1B2, and C1C2 from the contour map shown.
4.2 Method of Contouring
There are 3 methods of determining contours.
4.2 Method of Contouring (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.
4.2 Method of Contouring (1.) Direct Method
Direct method of locating contours.
4.2 Method of Contouring (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.
4.1 Contour Characteristics (2.) Indirect Method
4.2 Method of Contouring (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.
4.2 Method of Contouring (3.) Square Method
4.2 Method of Contouring 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.
4.3 Choice of Vertical Interval 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.
4.3 Choice of Vertical Interval 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.
4.3 Choice of Vertical Interval 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.
4.3 Choice of Vertical Interval 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.
4.4 Interpolating the Contours
Contour lines may be interpolated on the grid either mathematically or graphically.
4.4 Interpolating the Contours In the figure below, the levels of the grid are indicated. Draw a 7m contour line by interpolating mathematically.
4.4 Interpolating the Contours Figure below represents a contouring survey using 10 mx10 m grid squares. Find the offset distances X, Y, and Z.
6.50
7.15
X
7.00
6.65
7.05
7.00
7.50 8.00
8.25
Y 8.00
7.50 7.65
7.35
7.95
Z 9.25
