Surveying is the art of determining the relative positions of points on, above or beneath the surface of the earth by means of direct or indirect measurements of distance, direction and elevation. Levelling is a branch of surveying the object of which is,
1. to find the elevations of points with respect to a given or assumed datum, and
2. to establish points at a given elevation or at different elevations with respect to a given or assumed datum.

In general, surveying deals with measurements in an horizontal plane where as levelling deals with measurements in a vertical plane.

Primary Divisions of Survey

The two primary divisions of survey are,

1. Plane surveying
2. Geodetic surveying

Plane Surveying

It is that type of surveying in which the mean surface of the earth is considered as a plane and the spheroidal shape is neglected. All triangles formed by survey lines are considered as plane triangles. The level line is considered as straight and plumb lines are considered parallel. In other words, when small areas are to be surveyed, the curvature of the earth’s surface is ignored and such a surveying is called plane surveying.

Geodetic Surveying

It is that type of surveying in which the curved shape of the earth is taken in to account. The object of geodetic survey is to determine the precise position on the surface of the earth, of a system of widely distant points which form control stations in which surveys of less precision may be referred.

Classification

Surveys may be classified based on the nature of the field of survey, object of survey and instruments used. Some of the important surveys are,

1. Topographical surveys

They are carried out determine the position of natural features of a region such as rivers, streams, hills etc. and artificial features such as roads and canals. The purpose of such surveys is to prepare maps and such maps of are called topo-sheets.

1. Hydrographic Survey

Hydro-graphic survey is carried out to determine M.S.L. (Mean Sea Level), water spread area, depth of water bodies, velocity of flow in streams, cross-section area of flow etc.

1. Astronomical Survey

The Astronomical Survey is carried out to determine the absolute location of any point on the surface of earth. The survey consists of making observations to heavenly bodies such as stars.

1. Engineering Survey

This type of survey is undertaken whenever sufficient data is to be collected for the purpose of planning and designing engineering works such as roads, bridges and reservoirs.

1. Archeological Survey

This type of survey is carried out to gather information about sites that are important from archeological considerations and for unearthing relics of antiquity.

1. Photographic survey

In this type of survey, information is collected by taking photographs from selected points using a camera.

1. Aerial Survey

In this type of survey data about large tracks of land is collected by taking photographs from an aero-plane.

1. Reconnaissance Survey

In this type of survey, data is collected by marking physical observation and some measurements using simple survey instruments.

Principles of Surveying

The fundamental principle upon which the various methods of plane surveying are based can be stated under the following two aspects.

1. Location of a point by measurement from two points of reference.

According to this principle, the relative position of a point to be surveyed should be located by measurement from at least two points of reference, the positions of which have already been fixed.

If P and Q are the two reference points on the ground, any other point, such as R, can be located by any of the direct methods shown in the above figures. But, although a single method is sufficient to locate the relative position of ‘R’ with respect to reference points P and Q, it is necessary to adopt at least any two methods to fix the position of point ‘R’. While the measurements made in the either of the first method or second method will be helpful in locating the point ‘R’, the measurements made in the other method will act as a check.

1. Working from whole to part

According to this principle, it is always desirable to carryout survey work from whole to part. This means, when an area is to be surveyed, first a system of control points is to be established covering the whole area with very high precision. Then minor details are located by less precise methods.

The idea of working this way is to prevent the accumulation of errors and to control and localize minor errors which, otherwise, would expand to greater magnitudes if the reverse process is followed, thus making the work uncontrolled at the end.