CONTOURING - Civil Engineering psc short notes

KERALA PSC NOTES BASED ON CONTOURING

CONTOUR / CONTOUR LINE 
• Imaginary line passing through points of equal elevation 
• Formed by the intersection of level surface with the ground 
• Maps showing contour lines are called as contour maps 
• Contour lines underwater called as : fathom , bathymetric curves, submarine contour 

CONTOUR INTERVAL 
• The vertical distance between any two consecutive contours is called the contour interval 
• For important work CI will be shorter 
• Constant contour interval is needed throughout the map 
• Contour interval ∝ slope ∝ 1/scale 

HORIZONTAL EQUIVALENT 
• The horizontal distance between any two consecutive contours 

CONTOUR GRADIENT / GRADE CONTOUR 
• A line lying on the ground surface and having a constant inclination to horizontal 
• Infinite number of contour gradient can be set out through a point 

COMMONLY USED CONTOUR INTERVALS 
  • for building sites : 0.2 - 0.5
  • For town planning scheme & reservoirs : 0.5 - 2
  • For location survey : 2 - 3

CHARACTERISTICS OF CONTOUR LINE 
 Contour lines cannot cross one another on the map, except in the case of an overhanging cliff. 
 A uniform slope is indicated when contour lines are uniformly paced 
 Contour lines, cross ridge lines or valley lines at right angles 
 A ridge line is shown when the higher values are inside the loop in the contour 
 A valley is shown when the lower values are inside the loop 
 A vertical cliff is indicated when several contours coincide 
 Contour lines cannot end anywhere, but close on themselves either within or over the limits of the map 
 series of closed contours with lower values are inside them indicates a depression 
 A series of closed contours higher values are inside them indicates a hill 
 Depression between summits are called saddle 
 Line passing through the saddles and summits gives the divide line or watershed line. 

METHODS OF CONTOURING 

1. DIRECT METHOD 
• Only the points lying in the contour are surveyed 
• More accurate than indirect method but consumes more time 
• Direct method involves 

• Vertical control : Finding the points lying on various contours 

• Horizontal control : surveying these established points and plotting them on map 

2. INDIRECT METHOD 
• Used in case of undulating ground 
• Elevation of random points are found out and contours are prepared by interpolation 

i. Method of squares 
• Suitable for small open areas 
• Area is divided into squares and grid points are marked on the ground, elevation of these grid points are determined and contours are drawn using interpolation 

ii. Method of cross-section 
• Suitable for roads, railways, canals 
• Elevation of points on cross-section along a particular direction is found and contours are plotted 

iii. Tachometric method /radial line method 
• Suitable for hilly areas 
• Elevation of points on radial line are determined using a tachometer 

INTERPOLATION OF CONTOURS 

1. BY ESTIMATION : rough method 

2. BY COMPUTATION \ ARITHMETIC CALCULATION 
• Most accurate method 
• Most time consuming 

3. GRAPHICAL METHOD 
• Interpolation is done using a tracing paper or cloth of graphical construction


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