What is counter line?
. An imaginary line, on the ground, joining the points of equal elevation above the assumed datum is known as a contour line. and the process of plotting points of equal elevation or process of locating contours for different geometrical land is known as contouring in surveying.
Counter Line is a plan projection of the plane passing through the points of equal height on the surface of the earth.
To get full knowledge about “Contouring in surveying” please read carefully step by step.
The vertical distance between any two consecutive contours is known as the contour interval. It is kept constant for a contour map to depict the correct topography of the terrain.
The least horizontal distance between two consecutive contours, is called the horizontal equivalent. It is different for different contours and is dependent on the slope of the ground surface. Contours are comparatively less in hills than in plains.
Characteristics of Contours
- Two contours of different elevations don’t cross each other except in the case of an overhanging cliff.
- Contours of different elevations don’t unite to form one contour except in the case of a vertical cliff.
- Contours drawn closer indicate a steep slope and if drawn far apart represent a gentle slope.
- Contours equally spaced indicate a uniform slope. When contours are parallel, equidistant slope and straight, these represent an inclined plane surface.
- A contour at any point is perpendicular to the line of the steepest slope at that point.
- A contour line must close itself but not be necessary within the limit of the map itself.
- A set of ring contours with higher values inside represent a hill whereas a set of ring contours with lower values inside; represent a pond or a depression without an outlet.
- When contours cross a ridge or valley, they form the curve of V- shape across them. If the concavity of higher value contours lies towards the next lower value contours, they
- represent a ridgeline whereas if the concavity of lower value contours lies towards the
- higher-value contours, they represent a valley line.
- The same contour must appear on both sides of a ridge or a valley.
- Contours don’t have sharp turnings.
Method of contouring in Surveying / Method of locating contours:-
1. Direct method
2. Indirect method
1) Direct method:
In this method of contouring in surveying, the contour to be plotted is actually traced on the ground. Only those points are surveyed which happen to be plotted.
After surveyed those points, they are plotted and contours are drawn through them. This method is slow and is used for small areas and where great accuracy is required.
Steps of Direct method
- Vertical control:- the location of the points on the contour.
- Horizontal control:- plotting of the points by a plane table or compass or theodolite survey.
2) Indirect method
ln, this method of contouring in surveying, sufficient numbers of points are given spot levels. The location of such.points can be conveniently plotted on a plane table section as these generally form the corners of well-shaped geometrical figures i.e. squares, rectangles, triangles, etc. the contours in-between spot levels are interpolated and drawn.
This method is commonly used in small scale surveys of extensive areas.
This method is cheaper, quicker and less tedious as compassed with a direct method of contouring.
Methods of Direct Indirect Method
- By square method
- By cross-section method
- By tacheometric method
A) By Square method:-
- When the area to be surveyed is small and the ground is not very much undulating without any vegetative covers, this method is used for contouring.
- In this method, the entire area is divided into a number of squares, the side of which may vary from 5 m to 25 m depending upon the nature of the ground, contour interval and scale of the map. The corners of the squares are marked on the ground and spot levels of these points are given with a level. The squares are plotted on the desired scale of the map and R.L of the corners is entered. Then the contours of desired values are interpolated.
B) By cross-section method:
- This method is suitable for preparing a contour map of a road, railway or canal alignment.
- In this method, cross-sections perpendicular to the counter line of the area is set out.
- The spacing of the cross-section depends upon the contour interval, the scale of the map and the nature of the ground.
- Generally, the spacing of the cross-section at 20 min a hilly area and 100 in the flat area is adopted.
- Plot the center line and cross-section on the desired scale and enter their R.L. Then, the contours are interpolated w.r.t these R.L.
C) By Tachometric method :
- This method is suitable for contouring the area of long strips with mountainous or undulation where direct chaining is difficult.
- In this method, a number of radial lines at known angular intervals are drawn on the ground and positions of the points at equal distances are marked. Then the vertical angle and
- staff readings (stadia hair readings) are observed and RL. and their horizontal distances
- from the instrument,t positions calculated using tachometric formulae.
- The contours are drawn by interpolating the spot levels.
- The radial lines and the position of the points on each line are plotted on the desired scale. and their spot levels are entered.
Uses of counter maps
- Contour maps are used to study the general character of the tract of the contouring without visiting the ground.
- Counter maps are used to decide the most economical and suitable sites for engineering work such as canals, sewers, reservoirs, roads, railways e.t.c.
- Contours maps are used to determine the catchment area of the drainage basin and the capacity of the proposed reservoir.
- Contour maps are used to compute the earthwork for filling and cutting along the linear alignment of projects such as a canal, roads e.t.c.
- Counters’ maps are used to ascertain the indivisibility of the points.
- Counters map are used to trace a contour gradient for the road alignment.
- Contours maps are used to draw a longitudinal section and x- section to ascertain the nature of the ground.
- Contours maps are used to calculate the water capacities of the reservoir.
- Contours maps are used to decide the best position of the guns, the line of march and camping grounds by the army commanders during wars.
I hope you understood all about contouring in surveying.
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