Surveying is the process of determining the relative positions of different objects on the surface of the earth by measuring horizontal distances between them and preparing a map to any suitable scale.
Measurements are taken in the horizontal plane only. 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 to find the elevations of points with respect to a given or assumed datum and to establish points at a given elevation or at different elevations with respect to a given or assumed datum.
The survey is performed to prepare a map of relative positions on the surface of the earth. It shows the natural feature of a country such as towns, villages, roads, railways, etc. They may also include details of different engineering works such as bridge, canal, dam etc.
Before commencement of any major engineering project, the survey is carried out to determine the location of the site and its surrounding. To understand the topography of the site and natural resource availability. A survey is carried out to prepare the cadastral map showing property boundaries.
Principles of Surveying
The fundamental principle upon which the various methods of plane surveying are based can be stated under the following two aspects.
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 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.
Working from whole to part
Method of Surveying in Civil Engineering
Primary types of Surveying are:
- Plane surveying
- Geodetic surveying
1. Plane surveying
Plane surveying is conducted by state agencies as well as private agencies. As we know earth is spherical in shape but its diameter is big enough to consider a plane in small dimensions. 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 plane triangles. The level line is considered straight and the plumb lines are considered parallel. Plane surveying is done of the area of survey is less than 250 km2.
2. Geodetic surveying
The geodetic survey is conducted by the survey department of the country. It is that type of surveying in which the curved shape of the earth is taken into 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. . Line joining two points is considered as curved line and angles are assumed as spherical angles. It is carried out if the area exceeds over 250 km2.
Secondary classification of Surveying
Surveys may be classified based on the nature of the field of survey, the object of survey and instruments used.
1) Surveying based on Nature of Survey
a) Topographical Surveys
They are carried out to 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 are called topo-sheets.
b) Hydrographic Survey
The hydrographic survey is carried out to determine M.S.L. (Mean Sea Level), water spread area, depth of water bodies, the velocity of flow in streams, the cross-section area of flow etc.
c) Astronomical Survey
The Astronomical Survey is carried out to determine the absolute location of any point on the surface of the earth. The survey consists of making observations of heavenly bodies such as stars.
d) 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.
e) Archeological Survey
This type of survey is carried out to gather information about sites that are important from archaeological considerations and for unearthing relics of antiquity.
f) Photographic Survey
In this type of survey, information is collected by taking photographs from selected points using a camera.
g) Aerial Survey
In this type of survey data about large tracks of land is collected by taking photographs from an aero-plane.
h) Reconnaissance Survey
In this type of survey, data is collected by marking physical observation and some measurements using simple survey instruments.
2) Surveying based on Type of Instruments
a) Chain Surveying
Chain surveying is the basic and oldest type of surveying. The principle involved in chain survey is triangulation. The area to be surveyed is divided into a number of small triangles. Angles of triangles must not be less than 30 degrees and greater than 120 degrees. Equilateral triangles are considered to be ideal triangles. No angular measurements are taken, tie line and check lines control accuracy of the work.
This method is suitable on level ground with little undulations and the area to be survey is small.
b) Compass Surveying
Compass survey uses the principle of traversing. This method does not require the need to create triangles. It uses a prismatic compass for measuring the magnetic bearing of the line and the distance is measured by the chain. A series of connecting lines is prepared using a compass and measuring distances using a chain. Interior details are located using offset from main survey lines.
They suitable for large area surveying crowded with many details. It can be used to survey a river course.
c) Plane Table Surveying
The principle of the plane table survey is parallelism. They are plotted directly on paper with their relative position. The rays are drawn from the station to the object on the ground. The table is placed at each of the successive station parallels to the position of the last station.
They are basically suitable for filling interior detailing and is recommended when great accuracy is not required.
d) Theodolite Surveying
The theodolite is an instrument used mainly for accurate measurement of the horizontal and vertical angles. They are accurate to measure up to 10″ or 20″ angles.
Theodolite can be used to measure:
- Horizontal angles
- Vertical angles
- Deflection angle
- Magnetic bearing
- The horizontal distance between two points
- Vertical height between two points
- Difference in elevation
Nowadays theodolite is shadowed and replaced by the use of Total Station which can perform the same task with greater ease and accurate results
e) Tacheometric Surveying
Tachometer is a branch of surveying in which horizontal and vertical distances are determined by taking angular observations with an instrument known as a tacheometer. Tacheometer is nothing but a transit theodolite fitted with a stadia diaphragm and an anallatic lens. There is no need for chaining in such a survey. The principle of the Tacheometer is based on the property of the isosceles triangle, where the ratio of the distance of the base from the apex and the length of the base is always constant.
A different form of stadia diaphragm commonly used:
f) Photographic Surveying
The photographic survey is based on the technique of taking photographs from a different angle to prepare topographic details with relatively high speed.
There are two types of photographic surveying
i). Terrestrial or ground photogrammetry
In terrestrial photogrammetry, maps are prepared from ground photographs from different points on the earth surface for measurement purpose.
ii). Aerial photogrammetry
In aerial photogrammetry, maps are produced from the air from an aeroplane or helicopter.
Photogrammetry encompasses two major areas of specialization.
- Metrical photogrammetry
- Interpretive photogrammetry
Metrical photogrammetry is of principal interest to surveyors since it is applied to determine distances, elevations, areas, volume, etc. to compile topographic maps made from measurements on photographs.
Intuitive photogrammetry involves objects from their photographic image and their significance. Critical factors considered in identifying an object of shape, sizes, patterns, shadow.
