Aquaculture Engineering (2+1)

Methods of locating contours - Direct method

Unit 1- Surveying

Contour surveying

Methods of locating contours

Direct method
The direct method is divided into two forms.

• Vertical control : Location points
• Horizontal control: Survey of those points.

Vertical control
The points on the contours are traced with the help of a level and staff. The level is set at a point to command the area as much as possible and is levelled. The staff is kept on the B.M and the height of the instrument is determined. If the B.M is not nearby fly levelling may be performed to establish a temporary bench mark (T.B.M.) in that area. Having known the H.I the staff reading is calculated so that the bottom of the staff is at an elevation equal to the valve of the contour.

E.g.: H.I =101.80 meter, the reading to get a point on the contour of 100.00 m elevation will be 1.80 m. Taking one contour at a time, the staff man is directed to keep the staff on the points on contour so that reading of 1.80 m is obtained every time.

Horizontal control
After having located the points on various contours they are to be surveyed with a suitable control system. The system to be adopted depends mainly on the type and extent of areas. For small areas chain surveying may be used and the points may be located by off sets from the survey lines. For larger work the theodolite compass or plane table may be used.