Fundamentals of Surveying by engineering docs

Defination of Surveying

• Surveying is the art and science of determining the relative position of various points or stations on, above on beneath the earth's surface by measuring the horizontal and vertical distances, angles and taking of the detail of these points and preparing a map or plan to any suitable scale

Levelling

• Levelling is a branch of Surveying that deals with the measurement of relative heights of different points on or below the surface of the earth.

• Thus, in leveling, the measurements (elevations) are taken in the vertical plane.

Primary Divisions of Surveying

• The approximate shape of the earth can best be defined as an oblateoblate (spheroid) : An oblate (spheroid) is a surface of revolution obtained by rotating an ellipse about its minor axis i.e., having equatorial radius greater than the polar radius. tri-axial ovaloid. Ovaloid : A pear shaped figure having dimension of one hemisphere larger than the other for earth, southern hemisphere is larger than the northern.

• But, most of the civil engineering works, concern only with a small portion of the earth which seems to be a plane surface.

• Thus, based upon the consideration of the shape of the earth, surveying is broadly divided into two types.

1) Plane Surveying

• Plane surveying is that type of Surveying in which the earth's surface is considered as a plane & the curvature of the earth is ignored.

• Line joining any two stations is considered to be straight.

• The triangle formed by any three points is considered a plane triangle and the angles of the triangles are considered plane angles.

• Results from Plane surveying are less accurate than geodetic.

• It is carried out for a small area. Area less than 250 $\bf km^2$

$\bf Area \le 250 km^2$

• Thus for most of the Civil Engineering projects, methods of plane surveying are valid.

• It is carried out by local or state agencies like the Road & Building, Irrigation, Railway department etc.

2) Geodetic Surveying

• In this method of Surveying curvature of the earth is taken into account.

• It generally extends over large areas. The line joining any two stations is considered a curved line.

• The triangle formed by any three points is considered spherical and the angles of the triangles are considered to be spherical angles.

• Geodetic Surveying is conducted by the survey of India (SOI) department and is carried out for a large area exceeding 250 km²

$\bf Area \ge 250 km^2$

Geodetic Surveying - Engineering Docs