Aquaculture Engineering

Practical - 8
Experiment No 1 - Drawing of contour lines by direct method

Aim
To draw the contour lines for given elevations (Direct method)

Apparatus
Dumpy level, Levelling staff, Tape, Plane table with accessories, Arrows etc.

Procedure

• Set up the level at a convenient place, so that all the points and bench mark are visible properly.
• Do the “temporary adjustments” properly and make the line of sight exactly horizontal.
• Take the back sight by focusing the telescope towards the staff held on the bench mark.
• Calculate the elevation of line of sight as height of instrument.
• The staff reading is calculated so that the bottom of the staff is at an elevation equal to the value of contour. Eg: when the height of instrument is 101.80mt, the staff reading to get a point on the contour of 100.00 m will be 1.8 m.
• Taking one contour at a time, the staff man is directed to keep the staff on the ground so that reading of 1.8 m is obtained every time (say A, B, C, D etc.)
• Set the plane table on the ground near to the levelling instrument at instrument station say 'X’. Level it. Transfer the ground point on to the sheet by means of plumbing fork, thus getting ‘x’ representing ‘X’.
• With the help of trough compass mark the north direction on the sheet Clamp the table.
• Pivoting the alidade about ‘x’, sight it to the first point say ‘A’ on the ground. Draw a ray along the fiducial edge of the alidade.
• Measure the distance between ground station ‘X’ and the point A. Plot this distance to suitable scale along the ray marked.
• Similarly sight the other points on the ground. Measure their distance and plot it on the sheet for same scale (say B, C, D etc.)
• Join these points to get the contour line for an elevation of 100 m.
• By using same procedure draw the other contour lines.