Contouring in surveying

Contouring 

Content

• Contour or contour line
• Contouring 
• Contour map 
• Contour interval 
• Horizontal equivalent 
• Contour gradient 
• Use of contour map 
• Characteristics of contour map 
• Contour interval depends upon following factors
• Scale of the map
• Nature of the ground 
• The purpose of the map 
• Time & expense of the field work
• Instruments used for measurements of area any plan
• Polar planimeter 
• Digital planimeter 

Contour or contour line 

An imaginary line which joints the all points of equal elevation on the surface of earth is termed as Contour or contour line. 

Contouring 

The process of drawing the contour line is termed as contouring. 

Contour map

The map showing the all contour lines are termed as contour line. 

Contour interval

The constant perpendicular distance between two successive  contours is termed  as contour interval. 

Horizontal equivalent 

The shortest horizontal distance between two consecutive contour is termed as horizontal equivalent. It is variable and depends upon the slope of the ground. 

Contour gradient 

The constant slope between two consecutive contour with a horizontal is termed as contour gradient. It is denoted by TanΦ and represented in the ratio of vertical distance to the horizontal distance.

i.e.

tanΦ = Contour interval / Horizontal equivalent 

Uses of contour map

• The study of general character of the region of the country without visiting the ground. 
• To decide the most economical section and sides for engineering work such as bridges, canals, reservoirs, etc. 
• To compute the sufficient cutting or filling of soil for the projects like canals, roads, etc.
• To trace  (draw) a contour gradient for roads, and railway alignment. 
• To calculate the water capacity of the reservoirs approximately. 

Characteristics of contour map 

The following characteristics may be used while reading the contour plan:-

• All points in a contour line has a same elevation or height.                                      
• When the contour lines are uniformly spaced it indicates a standardized slope.                                                    When the control lines are straight, and equal spaced then it indicates an inclined plan surface. 
• When control lines are widely, it indicates a flat surface and when  they run close together it indicates steep slope.                                                               
• A series of closed control lines on a map indicates a hill if the higher values are inside.
• If the higher values of contour lines are outside then a series of closed contour lines on a map shows a valley.
• Contour lines across the ridge line and valley lines at right angle.
• If the higher values are inside the loop or bent in the contour, it indicates a ridge line and convexity of contour lines are downwards.
• If the higher values are outside the loop in the contour line it indicates a valley line and convexity of the contour lines are upwards.

The contour always become closed either  within or outside the map.

A contour of a different elevation cannot be cross each other either except in the case of overhanging cliff.

Contour never unites (collapse) to each other except in the case of vertical cliff.

 

1. What is surveying and its classification
2. Principle of surveying and types of scale
3. Accessories of linear measurement

For more information go through above link 👆 

Contour interval depends upon following factors

1. Scale of the map
2. Nature of the ground 
3. The purpose of the map 
4. Time & expense of the field work 

1. Scale of the map 

Contour interval is inversely proportional to the scale of the map that is a large the scale, smaller the contour interval and small the scale, larger the contour interval.

2. Nature of the ground  

For flat surface contour interval should be small, and for hilly surface it should be large.

3. The purpose of the map 

Contour interval largely depends upon purpose of the map. For small  areas and designing work, contour interval is small but for large areas such as roadway, railway, Canals, etc is taken large.

4. Time & expense of the field work 

If  more time is available contour interval is taken  small but if less time is available contour interval is taken large.

Instruments used for measurements of area of any plan 

1. Polar planimeter

It is an instrument which is used in surveying to compute the area of any given plan. Planimeter only needs plan drawn on the sheet to calculate the area.

Generally it's very difficult to see the realm of irregular plot. So by using planimeter we will easily calculate the given area of any shape. 

There are following parts of planimeter 

1. Tracing arm
2. Tracing point 
3. Anchor arm
4. Anchor point
5. Measuring wheel
6. Vernier scale
7. Counting disc
8. Index mark
9. Weight & needle point
10. Adjustable support 

2. Digital planimeter 

The digital planimeter is a computerized instrument with polar arm and roller. This instrument is used for measure the area of map of any scale. 

The digital planimeter is widely used nowadays, it helps in providing the foremost accurate results. Digital planimeter can measure any area with  100 times larger than ordinary planimeter. 

This instrument (Contouring) is generally utilized by surveyor's, foresters, geologist, engineers, and architectures.