Surveying in Civil Engineering

Surveying is one of the most important aspect of civil engineering. It involves the assessment of site and recording details about land. Measurement of angles, vertical and horizontal distance between points all these data help us to plan construction projects.

The main principle of surveying is working from whole to part i.e., It is very essential to establish first a system of control points and to fix them with higher precision. Minor control points can then be established by less precise methods and the details can then be located using these minor control points by running minor traverses etc. It is for the prevention the accumulation of errors and to control and localize minor errors.

Surveying is a method to determine relative position between two points on, above or beneath the earth surface and it also includes the art of establishing by predetermined angular and linear measurements.

Levelling is a branch of surveying in which elevations of points are found with respect to a given or assumed datum. Unlike levelling, surveying is a broader topic which finds the elevation between any two points (not necessary that one point should be datum).

Classes of Surveying

Primarily, surveying can be divided into two classes:

1. Plane Surveying
2. Geodetic Surveying

Plane Surveying

That type of surveying in which the surface of earth is considered as a plane for all calculations and spheroidal shape is neglected. Suppose, a triangle on earth's surface should be considered as a plane triangle. Level lines are considered straight and plumb lines are parallel. It is less precise as compared to Geodetic Surveying.

Geodetic Surveying

In this type of surveying, the actual shape of earth is taken into consideration for all calculations. Lines are treated as curved lines and triangles as spherical triangles. This surveying is generally done for larger magnitude of work and high degree precision.

There are also different classification of surveying in civil engineering based upon the nature of the field (like marine survey, topographical survey, cadastral survey, etc.), object of survey (engineering survey, military survey, mine survey and geological survey) and the instruments used (chain survey, compass survey, theodolite survey, etc.).

Now, solving any questions in a GATE exam or any college exams requires a good understanding of Bearings, Meridians and Local attraction among stations.

Bearings and Angles

In surveying, the direction of a line can be established (a) with relation to each other, or (b) with relation to any meridian. The first one (a) will give angle between two lines and second (b) will give the bearing of the line.

Bearing of a line is its direction relative to a given meridian and a meridian can be in any direction but of 3 types: Magnetic meridian, True meridian and Arbitrary meridian.

Meridian	Bearing types	About Meridians
True meridian	True bearing	Passes through true north and south poles
Magnetic meridian	Magnetic bearing	Any convenient direction towards a permanent mark or signal
Arbitrary meridian	Arbitrary bearing	Any convenient direction towards a permanent mark or signal

Fore Bearing and Back Bearing

Consider a line AB, if the bearing of line is measured from A towards B, then it is Forward or Fore Bearing. And, if the bearing of line AB is measured from B towards A then, it is Backward or Back Bearing.

FB of AB = Θ, BB of AB = Φ
BB = FB + 180, if FB < 180 degrees
BB = FB - 180, if FB > 180 degrees

In surveying, these two points are known as stations. Stations which are free from Local attraction have exact 180 degrees difference between FB and BB of line.