To plot the contours of an area by method of squares (or gridding).

Aim:

To plot the contours of an area by method of squares (or gridding).

Equipment and Accessories:

Automatic Level, tripod, Levelling staff, chain, tape, cross staff, arrows, ranging rods, pegs, hammer, Levelling book, pencil and eraser.

Principle:

The method of squares is also called coordinated method or gridding of locating contours. The entire area is divided into squares or rectangles forming a grid. The reduced levels of the corners are then determined by differential Levelling. The reduced levels are then plotted on the plan showing the same grid drawn to a suitable scale. After choosing suitable elevation values for contours to be drawn, the required contours are plotted by means of interpolation.

Procedure:

  1. On the parcel of the ground for which contour map has to be made establish a square grid of points at a suitable interval ( anywhere between 3m to 20m) by means of direct ranging and fix arrows as shown in figure given below.
  2. Carry out differential Levelling to find the R.L.’s of every arrow point and enter the readings in table.
  3. Draw the same grid that is established on the ground on the drawing sheet to a suitable scale.
  4. Indicate on the drawn grid the reduced levels of each grid point.
  5. Choose suitable values for elevations of contour lines.
  6. Draw contour lines on the grid by interpolation.

Fig.27 Contouring

Observations & Calculations:

TABLE :

S. No. Grid point B.S.

(m)

I.S.

(m)

F.S.

(m)

H.I.

(m)

R.L.

(m)

Remarks

Result:

Contours of the given area are plotted.

 

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