Surveying is the practice of measuring angles and distances on the ground so that they can be accurately plotted on a map.
Principles of Surveying
The fundamental principles upon which the surveying is being carried out are:
- Working from whole to part.
- After deciding the position of any point, its reference must be kept from at least two permanent objects or stations whose position have already been well defined.
The purpose of working from whole to part is:
- to localize the errors and
- to control the accumulation of errors
Classifications of Surveying
Based on the purpose (for which surveying is being conducted), Surveying has been classified into:
Control surveying: To establish horizontal and vertical positions of control points.
Land surveying: To determine the boundaries and areas of parcels of land, also known as property survey, boundary survey or cadastral survey.
Topographic survey: To prepare a plan/ map of a region which includes natural as well as and man-made features including elevation.
Engineering survey: To collect requisite data for planning, design and execution of engineering projects.
Three broad steps are
1) Reconnaissance survey : To explore site conditions and availability of infrastructures.
2) Preliminary survey : To collect adequate data to prepare plan / map of area to be used for planning and design.
3) Location survey : To set out work on the ground for actual construction / execution of the project.
Route survey: To plan, design, and laying out of route such as highways, railways, canals, pipelines, and other linear projects.
Construction surveys: Surveys which are required for establishment of points, lines, grades, and for staking out engineering works (after the plans have been prepared and the structural design has been done).
Astronomic surveys: To determine the latitude, longitude (of the observation station) and azimuth (of a line through observation station) from astronomical observation.
Mine surveys: To carry out surveying specific for opencast and underground mining purposes
Chain Survey: Chain survey is the simplest method of surveying. In this survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office.
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 and some time cross staff.
Types of Survey Stations
Survey stations are of two kinds i.e. main stations and subsidiary or tie stations.
Main Stations: Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations are 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.
Important Terms Related to Surveying
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 centre 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: These 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:
Perpendicular offsets, and oblique offsets.
The measurements are taken at right angle to the survey line called perpendicular or right angled offsets.
The measurements which are not made at right angles to the survey line are called oblique offsets or tie line offsets.
Procedure in chain survey
- Reconnaissance: The preliminary inspection of the area to be surveyed is called reconnaissance. The surveyor
inspects the area to be surveyed, survey or prepares index sketch or key plan. - Marking Station: Surveyor fixes up the required number of stations at places from where maximum possible stations are possible.
- Then he selects the way for passing the main line, which should be horizontal and clean as possible and should pass approximately through the centre of work.
- Then ranging roads are fixed on the stations.
- After fixing the stations, chaining could be started.
- Make ranging wherever necessary.
- Measure the change and offset.
- Enter in the field book.
Classifications of Surveying
Generally, surveying is divided into two major categories: plane and geodetic surveying. But there are other categories as well as per below explanation.
Plane surveying is a process of surveying in which the portion of the earth being surveyed is considered a plane.
The term is used to designate survey work in which the distances or areas involved are small enough that the curvature of the earth can be disregarded without significant error.
In general, the term of limited extent. For small areas, precise results may be obtained with plane surveying methods, but the accuracy and precision of such results will decrease as the area surveyed increases in size.
To make computations in plane surveying, you will use formulas of plane trigonometry, algebra, and analytical geometry.
A great number of surveys are of the plane surveying type. Surveys for the location and construction of highways and roads, canals, landing fields, and railroads are classified under plane surveying.
When it is realized that an arc of 10 mi is only 0.04 greater that its subtended chord; that a plane surface tangent to the spherical arc has departed only about 8 in. at 1 mi from the point of tangency; and that the sum of the angles of a spherical triangle is only 1 sec greater than the sum of the angles of a plane triangle for a triangle having an area of approximately 75 sq mi on the earth’s surface, it is just reasonable that the errors caused by the earth’s curvature be considered only in precise surveys of large areas.
Geodetic Surveying is a process of surveying in which the shape and size of the earth are considered.
This type of survey is suited for large areas and long lines and is used to find the precise location of basic points needed for establishing control for other surveys.
In geodetic surveys, the stations are normally long distances apart, and more precise instruments and surveying methods are required for this type of surveying than for plane surveying.
The shape of the earth is thought of as a spheroid, although in a technical sense, it is not really a spheroid.
In 1924, the convention of the International Geodetic and Geophysical Union adopted 41,852,960 ft as the diameter of the earth at the equator and 41,711,940 ft as the diameter at its polar axis.
The equatorial diameter was computed on the assumption that the flattening of the earth caused by gravitational attraction is exactly 1/297. Therefore, distances measured on or near the surface of the earth are not along straight lines or planes, but on a curved surface.
Hence, in the computation of distances in geodetic surveys, allowances are made for the earth’s minor and major diameters from which a spheroid of reference is developed.
The position of each geodetic station is related to this spheroid. The positions are expressed as latitudes (angles north or south of the Equator) and longitudes (angles east or west of a prime meridian) or as northings and castings on a rectangular grid.
Topographic Surveys are useful to gather survey data about the natural and man-made features of the land, as well as its elevations. From this information a three dimensional map may be prepared.
You may prepare the topographic map in the office after collecting the field data or prepare it right away in the field by plane table. The surveying method statement work usually consists of the following:
- Establishing horizontal and vertical control that will serve as the framework of the survey
- Determining enough horizontal location and elevation (usually called side shots) of ground points to provide enough data for plotting when the map is prepared
- Locating natural and man-made features that may be required by the purpose of the survey
- Computing distances, angles, and elevations
- Drawing the topographic map
Topographic surveys are commonly identified with horizontal and/or vertical control of third and lower-order accuracies.
Route Surveys
The term route survey refers to surveys necessary for the location and construction of lines of transportation or communication that continue across country for some distance, such as highways, railroads, open-conduit systems, pipelines, and power lines. Generally, the preliminary survey for this work takes the form of a topographic survey.
In the final stage, the work may consist of the following:
- Locating the center line, usually marked by stakes at 100-ft intervals called stations
- Determining elevations along and across the center line for plotting profile and cross sections
- Plotting the profile and cross sections and fixing the grades
- Computing the volumes of earthwork and preparing a mass diagram
- Staking out the extremities for cuts and fills
- Determining drainage areas to be used in the design of ditches and culverts
- Laying out structures, such as bridges and culverts
- Locating right-of-way boundaries, as well as staking out fence lines, if necessary
Special Surveys
As mentioned earlier special surveys are conducted for a specific purpose and with a special type of surveying equipment and methods. A brief discussion of some of the special surveys familiar to you follows.
Land Surveys sometimes called cadastral or property surveys are conducted to establish the exact location, boundaries, or subdivision of a tract of land in any specified area.
This type of survey requires professional registration in all states. Presently, land surveys generally consist of the following chores:
- Establishing markers or monuments to define and thereby preserve the boundaries of land belonging to a private concern, a corporation, or the government.
- Relocating markers or monuments legally established by original surveys. This requires examining previous survey records and retracing what was done. When some markers or monuments are missing, they are reestablished following recognized procedures, using whatever information is available.
- Rerunning old land survey lines to determine their lengths and directions. As a result of the high cost of land, old lines are remeasured to get more precise measurements.
- Subdividing landed estates into parcels of predetermined sizes and shapes.
- Calculating areas, distances, and directions and preparing the land map to portray the survey data so that it can be used as a permanent record.
- Writing a technical description for deeds.
Control Surveys provide “basic control” or horizontal and vertical positions of points to which supplementary surveys are adjusted. These types of surveys (sometimes termed and traverse stations and the elevations of bench marks.
These control points are further used as references for hydrographic surveys of the coastal waters; for topographic control; and for the control of many state, city, and private surveys.
Horizontal and vertical controls generated by land (geodetic) surveys provide coordinated position data for all surveyors. It is therefore necessary that these types of surveys use first order and second-order accuracies.
Hydrographic Surveys are made to acquire data required to chart and/or map shorelines and bottom depths of streams, rivers, lakes, reservoirs, and other larger bodies of water.
This type of survey is also of general importance to navigation and to development of water resources for flood control, irrigation, electrical power, and water supply.
As in other special surveys, several different types of electronic and radio-acoustical instruments are used in hydrographic surveys.
These special devices are commonly used in determining water depths and location of objects on the bottom by a method called taking soundings.
Soundings are taken by measuring the time required for sound to travel downward and be reflected back to a receiver aboard a vessel.
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