Surveying (Part-3) (General Engineering) (SSC JE)

Direct method of controlling is the most accurate method.

Direct method–In direct method, the contour to be plotted is actually traced on the ground. Only those points are surveyed which happen to be plotted and contour are drawn through them. The method is slow and tedious and is used for small areas and where great accuracy is required.

The method of orienting a plane table with two inaccessible points is know as the two-point problem. In this method, the surveyor orients the plane table by aligning it with two points that are not directly visible from the survey location.

The surveyor establishes a line of sight to each of these two point and then rotates the plane table until the lines of sight align with the corresponding points on the map.

• The diagonal scales give us three successive dimensions that is a unit, a sub unit and a sub-division of a sub unit.

• We have seen that the plain scales give only two dimensions such as a unit and its sub unit or its fraction.

A line ranger is not a tool used for measuring perpendicular offsets. It is a type of surveying instrument used to fixed straight line between two points.

Invar tape– Invar tape are made of alloy of nickel 36% and steel 64% having very low co-efficient of thermal expansion.

• It is 6 mm wide and generally available in length of 30 m, 50 m and 100 m.
• It is not affected by change of temperature therefore, it is used when high degree of precision is required.

Area shows on map = 256 cm²
Actual area = 64 km²
∵ 256 cm² shows = 64 km²

In an internal focusing telescope eyepiece and objective lens are fixed and focusing is achieved by movement of double concave lens mounted in a short tube.

Invar tape is used for a high degree of precision measurement. Generally, invar tape is used for base line measurement.