Levelling

Surveying

Surveying or land surveying is the technique, profession, and science of determining the terrestrial or three-dimensional position of points and the distances and angles between them. A land surveying professional is called a land surveyor. This article is about measuring positions on Earth. 

Survey

A field of applied statistics, survey methodology studies the sampling of individual units from a population and the associated survey data collection techniques, such as questionnaire construction and methods for improving the number and accuracy of responses to surveys.

Types Of Survey

Geography

·         Surveying, the technique and science of measuring positions and distances on Earth

·         photogrammetry, a method of collecting information using aerial photography

·         Cadastral surveyor, used to document land ownership, by the production of documents, diagrams, plats, and maps

·         Dominion Land Survey, the method used to divide most of Western Canada into one-square-mile sections for agricultural and other purposes

·         Public Land Survey System, a method used in the United States to survey and identify land parcels

·         Survey township, a square unit of land, six miles (~9.7 km) on a side, done by the U.S. Public Land Survey System

Earth sciences

·         Geological survey, an investigation of the subsurface of the ground to create a geological map or model

·         Geophysical survey, the systematic collection of geophysical data for spatial studies

·         Soil survey, the mapping of the properties and varieties of soil in a given area

·         Hydrographic survey, the gathering of information about navigable waters for the purposes of safe navigation of vessels

·         Cave survey, the three-dimensional mapping of underground caverns

Construction and mining

·         Construction surveying, the locating of structures relative to a reference line, used in the construction of buildings, roads, mines, and tunnels

·         Deviation survey, used in the oil industry to measure a borehole's departure from the vertical

Other

·         Archaeological field survey, the collection of information by archaeologists prior to excavation

·         Site survey

·         Vessel safety survey

Astronomy

·         Astronomical survey, imaging or mapping regions of the sky

·         Durchmusterung, a German word for a systematic survey of objects or data, generally used in astronomy

·         Redshift survey, an astronomical survey of a section of the sky to calculate the distance of objects from Earth

Organizations

·         The Institut Géographique National, a French state establishment which produces and maintains geographical information for France and its territories

·         The Survey of India, is India's central agency in charge of mapping and surveying

·         The Zoological Survey of India studies the fauna of India

·         The British Geological Survey, a body which carries out geological surveys and monitors the UK landmass

·         The Ordnance Survey, national mapping agency for Great Britain

·         The British Antarctic Survey, conducts scientific research in and around Antarctica for the United Kingdom

·         The United States Geological Survey, is a government scientific research agency which studies the landscape of the United States

·         The U.S. National Geodetic Survey, performs geographic surveys as part of the U.S. Department of Commerce

·         The MySurvey US Survey Panel, performs consumer research, survey results are used by famous organization such as Ad Council and USATODAY.

Surveyor 

A surveyor (land surveyor) is a professional who determines positions on or near the Earth's surface.

Professions and their activities

·         Marine surveyor or "ship surveyor", someone who inspects and examines ships

·         Quantity surveyor, someone who controls costs on construction projects

·         Pollster, someone who conducts surveys or opinion polls

Types of surveying

There are many types of disciplines in surveying and a surveyor during their career may decide to specialise in a particular discipline or may gain experience in all disciplines.  The main disciplines of surveying are:

Land surveying

Land surveying involves measuring and determining property boundaries, which are used as the basis for all property transactions including buying, selling, mortgaging and leasing.  Due to the importance of having a secure and strong property market, in NSW a land surveyor needs to be registered to be able to carry out a land survey. 

Engineering surveying

Engineering surveyors are engaged in the construction industry and ensure construction works are built in the correct location and as per their design.  They are generally found on construction sites setting out various types of works such as buildings, roads, bridges, tunnels and various other forms of infrastructure. 

Mining surveying

Mining surveyors are involved in the development and construction of mining operations and can generally be found above and underground taking measurements to determine volumes and setting out new excavations and tunnelling.  Mining surveyors in NSW are registered due to many safety issues involved in mining including ensuring mining does not encroach upon hazardous areas and future subsidence.

Hydrographic surveying

 Hydrographic surveying involves locating and measuring points under the sea and on the shore.  There measurements are used to design infrastructure such as docks and jetties as well as ensuring ships have enough clearance from the sea bed to safely travel around the world.  Using sonar scanners they are able to provide a picture of the sea bed without needing to get their feet wet and enabling the discovery of ship wrecks and other objects lost at sea.

Geodetic surveying

Geodetic surveyors are involved with undertaking very precise measurements to determine the shape and size of the world and track the movement of continents.  Their measurements are used in the monitoring of sea level rise, earthquakes, and the tracking of satellites.  Geodetic surveyors are involved in the development of co-ordinate systems and datums which are used in the productions of maps and plans.

Plane surveying

Plane surveys are the most commonly practised method of surveying and treat the earth as a flat or "plane" surface. Good enough for small-scale surveys in limited areas, the curvature of the earth has no effect on the results, thus the computations and results can be referenced to a plane or flat surface.

Photogrammetry and remote sensing

Photogrammetry and remote sensing involves taking measurements of the world via photography or other wavelength bands such as infra-red or ultra-violet.  Measurements may be sources from aerial photography or satellite imagery.  Photogrammetry and remote sensing is used to map large areas and determine changes in the world over time.

Chain Survey

Chain survey/surveying is an very old method of Surveying. This article includes definition of chain survey along with all detailed information with necessary images about various aspects of chain surveying.  

Chain survey is the simplest method of surveying. In chain survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. Here only linear measurements are made i.e. no angular measurements are made.This is most suitable adapted to small plane areas with very few details. If carefully done, it gives quite accurate results.

The necessary requirements for field work are

·                     Chain

·                     Tape 

·                     Ranging Rod 

·                     Arrows 

·                     Cross staff

Suitability of Chain Survey

Chain survey is suitable in the following cases: 

1.                  Area to be surveyed is comparatively small

2.                  Ground is fairly level

3.                  Area is open and

4.                  Details to be filled up are simple and less.

Survey Station

Survey stations are of two kinds 

1.                  Main Stations

2.                  Subsidiary or tie

 

Main Stations

Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations re called the main survey line or the chain lines.

Subsidiary or the tie stations

Subsidiary or the tie stations are the point selected on the main survey lines, where it is necessary to locate the interior detail such as fences, hedges, building etc.

Tie or subsidiary lines

A tie line joints two fixed points on the main survey lines. It helps to checking the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features, such as paths, building etc.

Base Lines

It is main and longest line, which passes approximately through the center of the field. All the other measurements to show the details of the work are taken with respect of this line.

Check Line

A check line also termed as a proof line is a line joining the apex of a triangle to some fixed points on any two sides of a triangle. A check line is measured to check the accuracy of the framework. The length of a check line, as measured on the ground should agree with its length on the plan. 

Offsets

Offsets are the lateral measurements from the base line to fix the positions of the different objects of the work with respect to base line. These are generally set at right angle offsets. It can also be drawn with the help of a tape. There are two kinds of offsets: 

1.                  Perpendicular offsets

2.                  Oblique offsets.

The measurements are taken at right angle to the survey line called perpendicular or right angled offsets. For setting perpendicular offsets any one of the following methods are used: 

·                     Swinging

·                     Using cross staffs

·                     Using optical or prism square

Perpendicular Offset by Swinging:
Chain is stretched along the survey line. An assistant holds the end of tape on the object. Surveyor swings the tape on chain line and selects the point on chain where offset distance is the least (Fig. 12.13) and notes chain reading as well as offset reading in a field book on a neat sketch of the object.
Perpendicular Offsets Using Cross Staffs

three different types of cross staffs used for setting perpendicular offsets. All cross staffs are having two perpendicular lines of sights. The cross staffs are mounted on stand. First line of sight is set along the chain line and without disturbing setting right angle line of sight is checked to locate the object. With open cross staff  it is possible to set perpendicular only, while with french cross staff), even 45º angle can be set. Adjustable cross staff can be used to set any angle also, since there are graduations and upper drum can be rotated over lower drum.

  

FIELD BOOK

All observations and measurements taken during chain surveying are to be recorded in a standard field book. It is a oblong book of size 200 mm × 120 mm, which can be carried in the pocket. There are two forms of the book (i) single line and (ii) double line. The pages of a single book are having a red line along the length of the paper in the middle of the width. It indicates the chain line. All chain-ages are written across it. The space on either side of the line is used for sketching the object and for noting offset distances. In double line book there are two blue lines with a space of 15 to 20 mm is the middle of each book. The space between the two lines is utilised for noting the chain-ages. Figure 12.17 shows typical pages of a field books. 

Procedure in chain survey 

1.                  Reconnaissance: The preliminary inspection of the area to be surveyed is calledreconnaissance. The surveyor inspects the area to be surveyed, survey or prepares index sketch or key plan.

2.                  Marking Station: Surveyor fixes up the required no stations at places from where maximum possible stations are possible.

Some of the methods used for marking are:

o        Fixing ranging poles

o        Driving pegs

o        Marking a cross if ground is hard

o        Digging and fixing a stone.

3.                  Then he selects the way for passing the main line, which should be horizontal and clean as possible and should pass approximately through the center of work.

4.                  Then ranging roads are fixed on the stations.

5.                  After fixing the stations, chaining could be started.

6.                  Make ranging wherever necessary.

7.                  Measure the change and offset.

8.                  Enter in the field the book.

Instruments A plane table mounted on a tripod stand and a number of accessories are used during plane table survey. The accessories consist of alidade, spirit level, trough compass, plumbing fork, plumb bob, drawing sheet.

Plane Tables

The plane table consists of a drawing board with arrangement for fixing on a tripod stand. There are various types of plane tables, depending upon the arrangement of fixing the boards to the tripod, leveling of the table and rotating arrangement in a horizontal plane.  Alidade An alidade is a device in which the vertical plane of the line of sight is maintained parallel to a straight-edge ruler on which the sighting arrangement is kept. It is used to draw a line parallel to the line of sight and thus provides the direction of the object to be plotted. Depending on the type of sighting arrangement, alidades are classified as Plain Alidade, Telescopic Alidade and digital alidade. Plain Alidade It consists of a straight-edge ruler, made of a metal or wood, with one of the edges is bevelled and graduated known as fiducial edge. It consists of two vanes which are perpendicular to its ends, fitted with hinges at their bases, known as sight vanes. These are kept folded down on the ruler when not in use. One of the sight vanes is provided with a narrow slit having three holes. This is used as eye vane. The other, used as object vane, is open     and carries a hair or thin wire at its centre. Thus, the line passing through the slit of the eye vane joining the thin wire of the object vane and passing beyond is known as the line of sight of a plane alidade. A string is fitted at the top of the sight vanes and is used for inclined sight. In some alidade, a compass needle as well as a spirit level gets fitted in a box engraved at its base. However, the plain alidade is not very accurate. Telescopic Alidade It consists of a telescope as an arrangement for sighting (similar to that present in the upper part of a theodolite). The telescope is fitted with a stadia diaphragm and can be used as tacheometer also for computations of horizontal distance and vertical elevations. The line of sight of the telescope is aligned along the fiducial edge. In this instrument, the object is sighted through the telescope and the distance is scaled off in that direction along the fiducial edge. The telescopic alidade is designed for greater precision and longer range of sights. It can be used with advantages for contouring and plotting of details during topographic surveying. Plumbing Fork A plumbing fork is a U-shaped piece of metal or wooded frame. The end of one of its arm is pointed and the other arm is having an arrangement for hanging a plumb bob. The frame is constructed in such away that the tip of the pointed arm and the plumb line lie in the same vertical line. At the time of use, the pointed arm is placed on the table and the other arm, with a plumb bob attached, is kept below the table. Plumbing fork with a plumb bob is used in large scale surveying for Centring of plane table and for Transferring of ground point. Spirit Level It consists of flat based tube with a small bubble either circular or tubular in shape . It is used to check the level of plane table by placing it on the board in two positions at right angles to each other. When the bubble tube remains in the centre at any point on the the table is considered to be properly leveled. Trough Compass A trough compass consists of a long, narrow rectangular box, covered with glass. Inside the box, at its centre, there is a magnetic needle resting on the pivot. At the extremities of the trough compass, there are graduated scales with zero at the centre and marking up to 5° on either side of the zero line.  The trough compass is used for marking the magnetic north line on the drawing sheet of the plane table. In this case, the magnetic needle point to 0° - 0° of the graduated scale and a line drawn parallel to the edge of the trough compass is along the magnetic meridian. A trough compass is also used to orient the plane table with respect to the magnetic meridian. Drawing Arrangements A drawing paper of good quality and well-seasoned is to be used for plotting. It should be able to withstand the rubbing of the alidade. The effect of changes in humidity and temperature of the atmosphere should be minimum thus reduction in the expansion or contraction of the paper, and minimum alteration in the scale of the map and distortion in the plan. A sharp, hard (4H or 6H) pencil is to be used for drawing lines. Soft pencils are used for lines to be erased. A good quality rubber eraser is used for erasing the lines. Basic Principle of Plane Table Surveying Plane table surveying is based on the principle that lines drawn during plotting always lie parallel to the corresponding lines actually present on the ground. For example, let us consider four ground stations A, B, C and D which on joining provides a rectangle ABCD. This has been plotted on a sheet of paper at a scale by plane table surveying. Here, the sides AB, BC, CD and DA are plotted in such a way that they are parallel to the sides actually available on the ground. Setting of Instruments At each station, the plane table is required to get set up before carrying out any plotting work. It basically consists of the three operations: fixing, centring and leveling. Fixing of the Plane Table In this operation, first the top of the tripod stand is fixed in level by eye estimation at convenient height with its legs uniformly spread and shoes fixed firmly into the ground. The board is fixed to the tripod head by tightening the clamping screw. Centring of the Plane Table There are two types of operations involved in centring of plane table. the centring with reference to already plotted position of the station. In this case, the pointed end of the upper arm of a U-fork is held at the plotted position of the                        station and the table is shifted and adjusted in such a way that the tip of the plumb bob points exactly over the ground station. the centring with reference to ground location of the                        station. In this case, the tip of the plumb bob attached to a U-frame is placed exactly over the ground point which is required to be transferred. The point corresponding to the tip of the pointed arm of the U-frame provides the transferred position on the table. However, the accuracy with which centring is being carried out depends upon the scale of plotting. Leveling the Plane Table The top of the table is leveled by moving the legs of the tripod. The level of plane table is first judged by eye estimation. Further, it is checked by keeping spirit level at different positions on the table and if required, legs are further adjusted. Temporary adjustment of Plane Table It is necessary to                   check whether the accessories satisfies some basic conditions and if required, necessary adjustments are to be done before starting any plane table surveying work. The operations involved in this are known as temporary adjustment of plane table. The conditions needed to be tested and subsequent adjustments are as follows: 1. The surface of the board should be a perfectly plane. Test : It is tested by placing a straight edge on the top surface of the plane table in different directions. If there is no gap between the base of the straight edge and the surface of the                   plane table then the surface is perfectly plane. Otherwise, the surface is not perfectly plane. Adjustment : If the gaps are minute, those are removed by rubbing with sand paper and for more gaps, the table should be replaced. 2. The fiducial edge of the alidade should be straight. Test : It is tested by drawing a fine line on the paper along the fiducial                   edge of the alidade. Then, by reversing the alidade, end for end, and placing against the line drawn, a line is to be drawn again along the fiducial edge. If the two lines coincide, the edge is straight. Otherwise, the edge is not straight. Adjustment : The fiducial edge of the alidade is to be made straight by filing and then test is repeated till satisfactory outcome. 3. In fully opened condition, the sight vanes of the alidade should be perpendicular to its base. Test : Hang a plumb bob at a distance of about 5 to 10 m from the plane table. Bisect the string of the plumb bob through the alidade placed on properly leveled plane table. If the sighting slit, the object vane hair and the plumb bob string lie the same vertical line, the vanes of the alidade are perpendicular to the base of the alidade. Otherwise, it requires adjustment. Adjustment : is being carried out by inserting packing under the base of the sight vanes or by filing the base, as required. The test and adjustment get repeated till satisfactory outcome is achieved. 4. The telescopic                   alidade if used should be in perfect adjustment. The testing and adjustment of the telescopic alidade are to be carried as discussed in case of theodolite in Module 6. Orientation of PlaneTable The objective of this operation is to maintain the orientation of the table constant at all the stations in any particular plane table surveying i.e., the four edges of the plane table will always be in the same direction at all the stations. Thus, all lines plotted on the plane table sheet will maintain parallism to their corresponding lines on the ground. During orientation, the leveling of the plane table generally gets disturbed so it is usually carried out with leveling simultaneously iteratively. The orientation of plane table can be carried                   out by: Using a trough compass; backsighting; resection. Orientation by using Trough Compass In this method, the edge of the trough compass is placed along the magnetic meridian (drawn at the starting station) and the plane table is rotated till the needle points to zero-zero of the scale. Once it is achieved, the table is said to be oriented and thus clamped. This method of orientation is not very accurate and also may get affected by local attraction and is generally used for small-scale survey. Orientation by Backsighting In this method, the fiducial edge of alidade is laid along a ray drawn from previous station to the present station and the plane table is then rotated till the line of sight of alidade bisects exactly the ranging rod placed at previous station. The plane table is then clamped and said to be oriented. In this method, the level of the plane table has to be maintained identical in both the stations. Advantages of Plane Table Survey   It is suitable for location of details as well as contouring for large scale maps directly in the field. As surveying and plotting are done simultaneously in the field, chances of getting omission of any detail get less. The plotting details can immediately get compared with the actual objects present in the field. Thus errors as well as accuracy of the plot can be ascertained as the work progresses in the field. Contours and specific features can be represented and checked conveniently as the whole area is in view at the time of plotting. Only relevant details are located because the map is drawn as the survey progresses. Irrelevant details get omitted in the field itself. The plane table survey is generally more rapid and less costly than most other types of survey. As the instruments used are simple, not much skill for operation of instruments is required. This method of survey requires no field book. Disadvantages of Plane Table Survey The plane table survey is not possible in unfavorable climates such as rain, fog etc. This method of survey is not very accurate and thus unsuitable for large scale or precise work. As no field book is maintained, plotting at different scale require full exercise. The method requires large amount of time to be spent in the field. Quality of the final map depends largely on the drafting capability of the surveyor. This method is effective in relatively open country where stations can be sighted easily .

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