GATE Civil Engineering (CE) 2027 Test: Traverse Surveying & Contouring

In the loose needle method, the direction of the magnetic meridian is established at each traverse station and the direction of the line is determined with reference to the magnetic meridian. In other words, the magnetic bearing of each line is determined directly. A theodolite fitted with a compass is used for determining the magnetic bearing of the traverse line. The loose needle method is also known as free needle method.

The principles of traversing say that, a traverse should consist of a series of straight lines connected together to form an open or a closed polygon. The points, defining the ends of the traverse lines are called traverse stations or traverse points.

Correction of departure of a line according to Bowditch Rule = 
⇒ = l₁ traverse side length 
∑l = total traverse side length 
∑l = l₁ + l₂ + l₃.... 
given ∑l = L 
∑D = total departure = -[ L₁/L x ΔD] 

Equally spaced contour lines are formed by uniform slope.
Contours are always perpendicular to the steepest slope.
A series of closed contours on map indicates a depression, if the higher values are outside.
Contours never run into one another except in the case of a vertical cliff. In the vertical cliff case, several contours coincide at a point and the horizontal equivalent becomes zero.