What are Broad divisions of surveying or Different Types of Survey in Civil Engineering ? Discuss in brief.

 

 Classification of Surveys

 

Surveys can be classified into two classes depending upon whether the spherical shape of the earth is taken into account or not. In survey work requiring very high precision, as in triangulation for setting control stations, the curved shape of the earth’s surface is taken into account. It is then known as geodetic surveying. Survey conducted neglecting the curved shape of the earth’s surface is known as plane surveying.

 

Plane Survey

 

In plane surveying, the curved nature of the earth’s surface is neglected. A line on the earth’s surface is taken to be a straight line. When three points lying on the surface of the earth are joined, they form a plane triangle. Consequently, all plumb lines are assumed to be parallel. The methods of plane surveying are used when the extent of the survey is small and high precision is not required. This assumption is justified in the case of surveys of small areas. This is so because the difference between an arc of length 25 km and the corresponding chord is only 10mm. Further, the sum of the angles of a spherical triangle about 195 km2 in area exceeds 180° (spherical excess) by just i”. Surveys conducted for engineering projects fall under this category.

 

Geodetic Survey

 

Geodetic survey is survey done for works requiring high precision, as in triangulation surveys to establish control points and in surveys of large areas. The equipment used for geodetic surveying are of very high precision and the methods used also ensure a high degree of accuracy in the measurements. The curvature of the earth is accounted for in the measurements taken during such surveys. Surveys can be further classified into categories depending on the purpose, instruments, techniques used, etc.

Instrument-based Surveys

linrlier times. few surveying instruments Vt ere in use, and surxevs “ere classified based upon the instruments used. Chaiiis. compasses, plane tables, and-theodolites were the only instruments used. either alone or in connmction With one another. Many more instruments are available now.

 

1. Chain survey:

Chain survey is done using a chain or tape only. A chain or tape measures linear distances. The chain triangulation of a small area is done by leIdlltg the area to be surveyed into a number of triangles. Using the sides of the triangles. other details are worked out. Chains and tapes meant for survey work are available in lengths of 20 or 30 m. If the length of the line to be measured is more than that of the chain length, then intemediate points have to be set up by ranging. For locating details, perpendiculars to a chain line can be set by many methods.

 

2. Compass Survey:

A magnetic compass works on the principle that a freely suspended magnetic needle points in the magnetic north-south direction. This gives a reference direction, which remains parallel at all stations if the area is free from magnetic influences. A compass, together with a chain or tape, can be used to survey a given area by many methods such as triangulation or traversing.

 

3. Plane table survey:

Plane table survey essentially combines fieldwork and plotting work. A plane table is a drawing board with modifications for attaching it to a tripod. With an alidade providing the direction, and with a chain or tape, points can be plotted on a sheet. A plan can be prepared or, with stations already marked on the table, further details can be filled in.

4. Theodolite survey:

A theodolite measures horizontal and vertical angles. It has a telescope which provides the line of sight and graduated horizontal and vertical circles, both with Vernier’s. Vernier theodolites with a least count of 20” are commonly used for ordinary work. Precision theodolites are made of larger diameter circles. Optic and electronic theodolites have greater precision and are easy to operate. The theodolite is a very versatile instrument and is used for all types of survey work, from triangulation to traversing and filling in details.

 

5. Levelling:

A level has a telescope for providing the line of sight and a supporting system for ensuring that the line of sight remains horizontal in all directions. A graduated rod, known as the levelling staff, is used with a level to find the difference in elevation between points.

6. Tachometry:

A tachometer is very similar to a theodolite but has a stadia diaphragm having three cross hairs, The reading taken by a stadia rod or levelling staff against all the three cross hairs enables the horizontal and vertical distances to be calculated.

 

7. EDM Survey:

Electromagnetic distance measurement involves the generation, transmission, and reception from a reflector at a station of light, radio, or microwave signals. The phase difference between the transmitted and received signals enables the distance between the instrument station and the reflector station to be calculated and displayed or stored.

 

8. Total station survey:

A total station is a combination of an electronic theodolite and an EDM. Horizontal distances and horizontal and vertical angles are determined using a total station. Total stations are used in triangulation surveys and other forms of surveys needing a very high level of precision. A total station can display and store the values required as well as transfer the data to a computer for further processing.

 

9. Satellite-based instrument survey:

The instruments and methods of remote sensing can be used for surveys conducted for various purposes. Global positioning systems, on the other hand, use an array of satellites strategically placed around the globe. Handheld trans-receivers or total stations with GPS capability receive signals from a minimum of four satellites and can determine the position of the receiver very accurately.

Depending upon atmospheric conditions and the terrain, such instruments can be used for surveys.

 

Method-based Surveys

 

There are different methods of surveying depending upon the instruments available, the terrain, and the purpose of the survey. The following are common methods. ‘

 

1. Triangulation:

Triangulation is a basic method of surveying. A triangle is a stable figure, and measuring one of the sides, known as the base line, and the three angles establishes a triangle on the ground. The other sides can be calculated using several formula. lf the three sides are measured; the method is known as trilateration. The triangle can be extended by adding two more sides each time to form another triangle, thus increasing the area covered.

 

2. Traversing:

A traverse can be open or closed. The sides and their directions are measured with a compass and chain or a theodolite and tape. A traverse can enclose a large area and the details along the sides or within it can be measured from the sides. An open traverse is run along narrow stretches of land such as river banks, roads, of railway lines. A closed traverse is a closed polygon-shaped figure and is run to enclose a large tract of land. Levelling: Levelling is a method of surveying used for determining the elevations of points. A level is used {ET this type of survey. When, along with the elevations, the horizontal locations of points are also determined, 5 method is known as contouring.

4. Tachometry:

Tachometry is a method of surveying wherein horizontal distances and differences in elevation between survey stations are determined Without directly measuring the distances. Tachometry uses the principle of stadia Surveying. The stadia diaphragm has three cross hairs and the readings taken on a staff against all the three hairs enables the calculation of distances and elevations.

 

5. Trigonometrical levelling:

In trigonometrical levelling, distances and elevations are determined by measuring vertical angles to graduations on staves using the principles of trigonometry. Elevations in many cases cannot be determined by ordinary levelling because of the great differences in elevation. Trigonometriical levelling provides a solution in such cases.

 

Purpose-based Surveys

 

Based upon their purpose. surveys can be classified into the following types.

 

1. Reconnaissance:

Reconnaissance operations are conducted to get an idea about the terrain and any special or difficult features that may be encountered during a regular survey. Generally, no instruments are used. Distances may be roughly estimated by pacing. Sufficient notes and sketches are drawn for future use during such surveys.

 

2. Preliminary surveys:

 

A preliminary survey is more detailed in its scope. It locates all the prominent features of a terrain as well as any particular features to be represented on a map.

 

3. Engineering surveys:

Engineering surveys are very detailed surveys required to locate engineering projects such as roads, railways, factories, and dams. More precise instruments and methods are used for such surveys.

4. Geographical surveys:

Geographical surveys are conducted to collect data for the preparation of geographical maps. These maps may be prepared to serve different purposes such as specifying national boundaries, land use, contours, and resources.

5. Mine surveys:

Mine surveys include both surface and underground surveys. Special techniques are required to transfer surface data to underground points. The surface maps show the general layout of the mine. Subsequently, underground maps are prepared for the design of tunnels and shafts in the mine, underground plans. geological maps, etc.

6. Route surveys:

Route surveys are surveys conducted for locating road or railway networks. The most convenient alignment of roads is decided based on such Surveys. Other engineering aspects such as road profile, earth work in cutting and banking, and road curves and super elevation are taken care of subsequently.

7. Location surveys:

Location survey are conducted to locate points on the ground based on the plans prepared.

8. Geological surveys:

Geological surveys are of economic importance, as both surface and sub-surface survey are conducted to locate ores and mineral deposit. In addition, geological features of the terrain such as folds and faults are located.

9. Defence surveys:

Such surveys are conducted by the military establishment to locate strategic positions in enemy area. Aerial surveys are conducted for this purpose. Knowledge of the terrain features from ground surveys are also important to prepare strategic plans for defence and attack.

10. Archaeological surveys:

Archaeological surveys are conducted to locate relics of antiquity, civilizations etc.

 

Place based Surveys

Based upon the place of survey. the following types of surveys can be identified.

1. Land Survey:

Land surveys are done on land to prepare plans and maps of a given area. Such surveys are conducted for the purpose of partitioning land, determine their areas, locate boundaries of properties, etc. Topographical surveys are surveys conducted to prepare plans or maps indicating the location of important features such as buildings, rivers, and woods, including the elevation of points from some datum. Cadastral surveys or public land surveys are conducted for the purpose of locating land features such as agricultural fields, buildings, houses, and other property lines. Both urban and rural areas are surveyed extensively to obtain such data. City surveys are similar surveys conducted in cities for similar purposes but with great refinement, as the cost of the land is very excessive and the exact location and demarcation of features become important.

2. Hydrographic survey:

A hydrographic survey deals with water bodies such as lakes, rivers, streams, and coastal areas. The objective is to obtain data to design water navigation systems, determine shorelines, help in the design of structures built in water or along shore lines, and obtain data about the ground surface underneath water.

3. Aerial survey:

An aerial survey is done from aircraft, which take photographs of the surface of the earth in overlapping strips of land. Extensive areas can be covered by such surveys. This form of survey is also known as a photogrammetric survey. The method is very expensive. It is, however, recommended in large projects in difficult terrain where ground surveys may be difficult or impossible.

4. Underground survey:

Underground surveys are done in the case of mines and tunnels. These are done by transferring ground points to the underground level and conducting surveys of tunnels, caves, mines, etc.

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