Introduction to Surveying
Surveying is the science and art of determining the relative positions of various points above, on or below the surface of the earth.
The relative positions in surveying are determined by measuring horizontal distances, vertical distances (elevations}, horizontal angles and vertical angles accurately by using various surveying instruments.
Important Objectives of Surveying :
- To determine areas, volumes and other related quantities.
- To layout or to mark the positions of the proposed structure on the ground.
- To take measurements and to determine the relative positions of the existing features on or near the ground.
History of Surveying :
- Surveying in one form or the other had been used for distinguishing one man’s land from that of another since time immemorial.
- Surveying techniques were also used for setting out and establishing points and lines on the ground for the construction of pyramids and huge buildings.
- Surveying in some form was used in India and Egypt to divide the land for taxation purposes even 1400 B.C.
- Surveying methods were used to control points in the Nile Valley Civilization.
- Romans developed improved methods of surveying during the Roman period.
- They used measuring wheels (odometers), for measuring distances, and groma, for laying off right angles.
- In Twentieth century, the surveying methods and instruments were furthered improved.
- Electronic Distance measuring (E.D.M) and Total Station instruments were introduced for very accurate measurements of distances and angles.
- Precise levelling instrument and modem optical theodolites were invented.
Primary Divisions of Surveying :
- The actual shape of the earth is an oblate spheroid. It is ellipsoid of revolution, flattened at the poles an bulging at the equator. It is taken as sphere approximately.
- The length of the polar axis is about 12713.168 km and that of the equatorial axis is about 12756.602 km, as computed by Clark in 1866. Thus the polar axis is shorter than the equatorial axis by about 43.434 km. Relative to the diameter of the earth, the difference in the lengths of the two axis is a very small quantity (about 0.34%).
- Average radius of earth is taken as 6370 km for all calculations.
Surveying Is Generally Divided Into 2 Types :
(a) Plane Surveying.
(b) Geodetic Surveying.
(a) Plane Surveying :
It is the type of surveying in which the curvature of the earth is neglected and it is assumed to be a fiat surface. Plane Surveying can safely be used when one is concerned with a small portions of the earth surface and the areas involved are less then 250 sq.km or so.
It is worth noting that the difference between an arc distance of 18.5 km on the surface of the earth and the corresponding chord distance is less than 10 mm.
In plane surveying, the angles of polygons and triangles are considered as plane angles. It is used for relatively small areas.
A curved line on the surface of the earth is considered as mathematically straight.
The directions of the plumb lines at various points are assumed to be parallel to one another.
The spherical angles are considered as plane angles and the standard of accuracy is lower than that in geodetic surveying.
(b) Geodetic Surveying :
It is the type of surveying in which the curvature of the earth is taken into consideration and a very high standard of accuracy is maintained. It is to determine the precise location of a system of widely spaced points on the surface of the earth. The point so located are used as control station of the primary surveys.
It is used for large areas and it is used for establishing precise points of reference or control points. The surface of the earth is considered as curved. The directions of plumb lines at various points are different. The earth’s mean sea level is perpendicular to the direction of gravity indicated by plumb bobs. The standard of accuracy is very high and very precise instruments are used.
Classification of Surveys :
Functional Classification of Survey
1. Control Survey :
It consists of establishing the horizontal and vertical positions of widely spaced control points using the principles of geodetic surveying.
2. Land Survey :
It is used to determine the boundaries and areas of tracts of land. Surveys are also used to provide data for making a plan of the area. A cadastral survey is done for marking legal boundaries of land.
3. City Survey :
These surveys are conducted within the limits of a city for urban planning. These are required for the purpose of layout of streets, building, sewers, pipes, etc.
4. Topographical Survey :
Topography is defined as the shape. These surveys are required to establish horizontal locations of the various points as well as their vertical locations. It is carried out to describe the existing natural features like rivers, mountains, forests and man made features (villages, building, roads & transmission lines) etc.
5. Route Survey :
These are special types of topographical surveys conducted along a proposed route for a highway, railway, sewer line, etc. This also includes the staking out and calculations of the earth work.
6. Mine Survey :
It will be carried out to determine the relative positions and elevations of mines, shafts, bore holes etc for underground works.
7. Hydrographic Survey :
It is carried out on or near the body of water, such as lakes, rivers, bays, harbours. Marine surveys are special types of hydrographic Surveys. These cover a broader area near sea for offshore structures, navigations, and tides etc.
8. Engineering Survey :
It is conducted to collect data for designing and planning of engineering works such as buildings, roads, bridges, dams, reservoirs, sewers and water supply line.
9. Astronomic Survey :
It is carried out for determination of latitudes, longitudes azimuths, local time etc for various places on the earth by observing heavenly bodies (the sun or stars).
10. Satellite Survey :
It is conducted to obtain intercontinental, inter-datum and inter-island geodetic ties all the world over by artificial earth satellites.
11. Geological Survey :
It is carried to obtain information about different strata of earth’s surface for geological studies.
12. Construction Survey :
After the plans have been prepared and the structures designed, the construction survey is conducted.
13. Miscellaneous Surveys :
- Archaeological Survey : These are done to unearth relics of antiquity.
- Military Survey : These are conducted to determine the routes and points of strategic importance.
- Gravity Survey : These are conducted to measure the intensity of the gravitational force at various points.
- General Survey : These are used to acquire and accumulate qualitative information and quantitative data for a specific purpose by observing, counting, classifying and recording according to the need. For example soil survey and traffic survey.
Classification Based on Instruments Used :
1. Chain Surveying :
This is the simple type of surveying in which only linear measurements are taken with a chain or tape. It is generally used when high accuracy it required.
2. Compass Surveying :
Horizontal angles are measured with the help of a magnetic compass. The compass survey is not very accurate.
3. Levelling :
It is used for determination of relative elevations (RL) of various points in the vertical plane.
Map is prepared the field while viewing the terrain after determining the directions of various line and taking the linear measurements with. a chain or a tape. The accuracy of the plane table surveying is low.
5. Theodolite Survey :
A theodolite is a very precise instrument for measuring horizontal and vertical angles. These can be broadly classified into Traverse and Triangulation.
6. Tacheometric Survey :
Horizontal angles, horizontal distances and elevations are measured with a tacheometer. These are not very accurate but these are extremely convenient for determining topographical details.
7. Photogrammetric Survey :
It is used for topographic mapping of vast areas. These are extremely useful for obtaining topographical details of areas which are difficult to reach.
8. EDM Survey :
Trilateration is a type of triangulation in which all the three sides of each triangle are measured accurately with EDM instruments. The angles are computed indirectly from the known sides of the triangles. Thus all the sides and angles are determined.
Basic Measurements in Surveying :
- Horizontal distances.
- Vertical distances.
- Horizontal angles.
- Vertical angles.
Principles of Surveying :
Location of a point by measurement from two points of reference :
The relative positions of the points to be surveyed should be located by measurement from at least two points of reference, the positions of which have already been fixed.
Working from whole to part :
It is very essential to establish first a system of control points and to fix them with higher precision. It can be used to prevent the accumulation of errors and to control and localize minor errors.