CIVIL ENGINEERING BLOG: Surveying

Surveying

INTRODUCTION

The art of determining relative position of objects on the surface of the earth by taking measurments in the horizontal and vertical planes, is called surveying. The surveying may primarily be divided into following two classes :

Plane Surveying :- The surveys in which the curvature of earth is not taken into consideration are known as plane surveys. These surveys extend over small area. The degree of accuracy obtained in this type of surveying is comparatively low. Generally areas less than 250 sqkm are treated as plane. When the surveys extend over limited areas, the difference in length between the arc and substended chord for any two points on the surface of the earth is only about 0.1 m for a distance of 18.2 km, 0.3 m for 54.3 km and 0.5 m for 91 km. Similarly, the difference between the sum of the angles of a spherical triangle on the earth's surface and that of the angles of the corresponding plane triangle is only one second for every 195.5 sqkm of area.
Geodetic Surveying :- The surveys in which the curvature of earth is taken into considration are known as geodetic surveys (also called trigonometrical surveying). These surveys extend over large areas (more than 250 sqkm) and where the degree of accuracy required is great.

GENERAL CLASSIFICATION OF SURVEYS

The surveys may be classified in different ways as follows :-

According to the instrument used :- The surveys, according to the instrument used are chain survey, compass survey, plane table survey, theodolite survey etc.
According to the purpose of survey :- The surveys, according to the purpose are mine survey, geological survey, archaeological survey and military survey.
According to the method employed :- The surveys, according to the method employed are triangulation survey and traverse survey.
According to the place of work :- The survey according to the place of work are land survey , hydrographical or marine survey.

The land survey may be further sub divided into following classes :-

Topographical Survey :- The survey is carried out for determining the natural features of the country such as hills, valleys, rivers, lakes, woods and nallahs etc. It is also used for artificial objects such as canals, railways, roads, towns and villages etc.
Cadastral survey :- This survey is performed to determine the additional details such as boundaries of feilds, houses and other property.
City Survey :- This survey is performed in connection with town planning schemes such as drainage , water supply etc. and for laying out plots, roads, streets etc.
Engineering Survey :- This survey is performed for determining and for collecting data for the design of engineering works such as roads, railways, reservoirs or works in connection with water supply, sewerage etc. The engineering survey may be further sub-divided into the following classes :-

Reconnaissance surveys for determining the feasibility the rough cost of scheme
Preliminary surveys for collecting more precise data to choose the best location for the work.
Location surveys for setting out the work on the ground.

PRINCIPLES OF SURVEYING

The following are the two fundamental principles of surveying which should be kept in view while determining the relative position of points on the surface of earth.

To work from whole to part :- The working from whole to the part is done in surveying in order to ensure that errors and mistakes of one portion do not effect the remaining portion.
To fix the position of new stations by at least two independent processes. The new stations are fixed from points already fixed by linear measurements, angular measurements or by both linear and angular measurements.

SCALES

The drawings of objects are not usually made full sized. It is generally necessary to draw them to a convenient scale. The ratio of the distance on the map or drawing to the corresponding distance on the ground is called representative fraction (R.F). For Example, when 1 cm on a map represents 10 m on the ground, then the representative fraction of the scale is 1/1000.

The scales may be plain scale, diagonal scale, comparative scale, shrunk scale and vernier scale.

A plain scale is used to read only two dimensions such as meters and decimeters.
A diagonal scale is used to read three dimensions such as meters, decimeters and centimeters.
A comparative scale has a common representative fraction but read in different measures.
The vernier scale is used for measuring fractional parts of smallest division of the main scale. It consists essentially of a small scale called the vernier scale which moves with its graduated edge along the graduated edge of a long fixed scale called the main or primary scale. The readings with a simple vernier can be taken in one direction only. There are two types of vernier i.e. direct vernier and retrograde vernier.

In a direct vernier, the smallest division of a vernier is shorter than the smallest division of its primary scale. In a retrograde vernier, the smallest division of a vernier is longer than the smallest division of its primary scale.

The ratio of the smallest division on the main scale to the number of divisions on the vernier is called least count of the vernier. For example, if x is the smallest division on the main scale and n are the numbers of division on the vernier, then the least count of the vernier is x/n.

NOTE :- If by chance measurements are taken with a wrong scale, then the true measurements may be found by using the following relations :