Surveying – Objectives, Principles & Classification

Surveying is defined as an art to determine the relative position of points on, above or beneath the surface of the earth with respect to each other by measurement of horizontal and vertical distances, angles and directions.

1. To determine the dimensions and contours of any part of the Earth’s surface to prepare a plan or map.
2. To establish boundaries of the land.
3. To measure areas and volumes.
4. To select a suitable site for an engineering project.

There are two main Principles of Surveying

The main idea of working from whole to part is to localize the errors and present their accumulation on the other hand, if we work from part to whole, the errors accumulate and expand to a greater magnitude and consequently the survey becomes uncontrollable at the end.

This process consists in measuring the distance in parts, as the length of the chain is smaller than the distance to be measured.

In this method, the length between two points A and B are measured by establishing intermediate points such as C, D and E… etc. as shown in the figure with respect to end points A and B. If error introduced in establishing any intermediate point will be carried in establishing the other points.

For example, if point D has been established out of line AB (D^’) and C, E, F… etc. have been established correctly. The actual distances DC and DE will be in error (D^’C and D^’E) but all other distances AC, EF, FG, etc. are correct. Therefore the error in this procedure is localized at point D and it is not magnified.

On the other hand, if we work from part to whole say if intermediate points are established with respect to AC as shown in the figure as C is already established out of line AB, there will be an error in the actual length AC, therefore the remaining points C^’, D^’, E’…etc. will also be established out of line AB with increasing magnitude of error and finally, the survey will become uncontrollable. Therefore working from part to whole is never recommended.

Two control points are selected in the area and the distance between them is measured accurately. The line is joining these two points is plotted to the scale on the drawing sheet, now the desired point can be plotted by marking two suitable measurements from the given control points as described below.

Let A and B be two control points whose distance is already plotted on the drawing sheet. The position of C can be plotted by any of the following methods.

1. By measuring distance BC and angle.
2. By dropping a perpendicular from C on the line AB and measuring either AD and CD or BD and CD.
3. By measuring the distance AC and BC

For a true representation of the field conditions so as to plot the plans and sections with the desired accuracy, sufficient data should be obtained from fieldwork. It consists of adjusting instruments and taking due care of these, making surveying measurements, and recording the measurements in the field notebook in a systematic manner.

Field notes are the written records of the field work made at the time of work. A surveyor should keep in mind not only the immediate use of the data but also those which may be expected to arise in future. Therefore the field notes must be complete and accurate and should be given to accuracy, legibility, integrity, arrangement and clarity.

Accuracy – All the measurements should be accurate, depending upon precision desired.

Legibility – the notes may be utilized by someone else who never even visited the site of the survey. Therefore, all the notes should be legible and contain a professional touch.

Integrity – the notes should be complete in all respects before leaving the site of the survey. Even a single omitted measurement may pose a serious problem while computing or plotting in the office.

Arrangement – It should be made clear as to how the work began and ended. The note forms should be appropriate to the particular survey and should be arranged in the sequence of the work done in the field.

Clarity – Sketches and tabulation of field data should be clear and readable, as the notes may be used by someone else in the future. Ambiguous notes lead to mistakes in drafting and computation.

Field notes are usually recorded on standard ruling sheets in a loose leaf or bound field books. The format of the standard ruled sheet depends upon the type of the instrument used for surveying. Some general suggestions for the field book while recording field notes are –

1. Use a notebook that may stand hard usage.
2. A hard lead pencil (3H) should be used to record field notes so that indentations will be formed on the paper and later if the notes erased due to any reason, the data can still be ascertained by examining the indentations.
3. Eraser should never be made in the field book. If a value is recorded incorrectly, it should be cut by a horizontal line and the correct value should be recorded above the cut value.
4. The notes should be read from left to right and from bottom to top as in the working drawings.
5. The left page of the field book is used for the recording data, while the right page is used for the sketches.
6. All the calculations and the reductions made in the field should be indicated on additional sheets and may be cross – referred as and when required.
7. On the top of the field notes, names of the survey party, instruments used, date, weather etc. should be mentioned. This is particularly useful when the field notes are presented as evidence in court.
8. At the end of the day work, the notes should be signed by note keeper.

a. Plane Survey – Survey in which the earth surface is regarded as plane and not curved as it really is, is known as plane surveying.

Assumptions

• A level line is considered a straight line and turns the plumb line at a point is parallel to the plumb line at any other point.
• The angle between two such lines that intersect is a plane angle and not a spherical angle.
• The meridian through any two points is parallel.

Plane survey is used only for a small portion of the Earth’s surface. It is used for projects on large scale such as factories, bridges, dams, locations and construction of canals, highways, railways etc. and also for establishing boundaries.

b. Geodetic Survey – Survey in which the shape (curvature) of the Earth’s surface is taken into account and a higher degree of precision is exercised in linear and angular measurements is known as Geodetic Survey

Assumptions

• A line connecting two points is regarded as an arc.
• The distance between two points is corrected for curvature.
• The angle between intersecting lines are spherical angles.

Geodetic survey is used for fixing widely spaced control points which may afterwards be used as necessary control points for fixing minor points for plane survey.

Chain survey is used when a plan is to be made for a very small open field. Where the field work consists of linear measurements only. All the measurements are done with chain and tape.

Limitations

• It is difficult to adopt chain survey, when there are obstacles to chain like trees and shrubs.
• It is difficult to carry chain survey in densely built-up areas.

Suitability

It is recommended for plans involving the development of buildings, roads, water supply and sewerage schemes etc.

When the linear measurements are done with chain and tape and the directions or angles are measured with the compass or transit respectively, the survey is called Traversing.

In traversing, the speed and accuracy of the field work are enhanced.

It is used for measuring boundaries of a field accurately by a framework of lines forming an open traverse.

It is used while carrying out survey work in a densely populated area with a framework of lines forming a closed traverse. A traverse survey is very useful for large projects such as reservoirs and dams.

This is a method of surveying in which both the horizontal and vertical distances are determined by observing a graduated staff with a transit equipped with a special telescope having stadia wires and anallactic lens.

It is very useful when the direct measurements of horizontal distances are inaccessible. It is usually recommended for making contour planes of building estates, reservoirs etc.

This is a method of surveying in which relative vertical heights of the points are determined by using a level and graduated staff. In planning a constructional project, it is essential to know the depth of excavation for foundations, trenches, fillings, etc. this can be achieved by collecting complete information regarding relative heights of the ground by levelling.

It is a graphical method of surveying in which field work and potting are done simultaneously. A clinometer is used in conjunction with plane table to plot the contours of the area. This method is very advantageous as there is no possibility of omitting any necessary measurement, the field being in view while plotting. The details like boundaries, shore line, etc. can be plotted exactly to their true shapes, being in views.

The only disadvantage of plane tabling is that it cannot be used in humid climate.

When the area to be surveyed is of considerable extent, triangulation is adopted. The entire area is divided into a network of triangles. Any one side of any triangle is measured preciously and axis line is called Base Line. All the angles in the network are measured with a transit. The lengths of the sides of all the triangles are then computed from the measured length of the base line and the observed angles corrected with the help of Sine Formula.

Survey which are done to provide sufficient data for the design of engineering projects such as highways, railways, water supply, sewage disposal, reservoirs, bridges, etc. It consists of topographic survey of the area, measurements of the Earth work, providing grade and making measurements of the completed work till date. These are also known as construction surveys.

Surveys that are carried out in the military to provide strategic information that can decide the course of war. Aerial and Topographic maps of the enemy area indicating important routes, airports, ordinance factories, missile sites, early warning and other types of radars, anti-aircraft positions and other topographic features can be prepared.

Aerial surveys can also provide vital information on locations, concentration and movement of troops and armaments. This information may be used for preparing tactical and strategic plans for defence and attack.

In this, both surface and sub-surface surveying are required to determine the location, extent and reserve of different minerals and rock types. Different types of geological structures like folds, faults, and unconformities may help to locate the possibility of the occurrence of economic minerals, oils etc.

Surveys conducted to provide sufficient data for the preparation of geographical maps are known as geographical surveys. The maps may be prepared to depict the land use efficiency, sources and intensity of irrigation, physiographic regions and waterfalls, surface drainage, slope height curve and slope profile and contours as well as the general geology of the area.

In this survey, both surface and underground surveys are required. It consists of a topographic survey of mine and making a surface map, making underground surveys to delineate fully the mine working and constructing the underground plans, fixing the positions and directions of tunnels, shafts, drifts, etc. and preparation of a geological map.

These are done to unearth the relics of antiquity, civilizations, kingdoms, towns, villages, forts, temples, etc. buried due to earthquakes, landslides or other calamities and are located, marked and identified. These provide vital links on understanding the evolution of the present civilization as well as human beings.

These surveys are carried out to locate and select the adopted line on the ground for a highway or railway and to obtain all the necessary data.

The sequence of operation in the route survey is as follows:-

1. Reconnaissance Survey – A visit is made to the site and all the relevant information is collected. It includes the collection of existing maps of the area, tracing the relevant map portion over a paper, incorporating details of the area, if missing, collected by conducting a rough survey.
2. Preliminary Survey – it is the topographical survey of the area in which the project is located. Sometimes an aerial survey is done if the area is extensive. It includes showing exact locations of all prominent features and fixing the position of the structure on the map.
3. Control Survey – It consists of planning a general control system for the preliminary survey which may be triangulation or traversing. For location survey, it consists of triangulation.
4. Location survey – it consists of planning a general establishing the points exactly on the ground for which the computations have been done in the control survey for location.

It consists of re-running old land lines to determine their lengths and directions, sub dividing the land into predetermined shapes and sizes and calculating their areas and setting monuments and locating their positions land survey includes.

1. Topographical Survey – This is a survey conducted to obtain data to make a map indicating inequalities of land surface by measuring elevations and to locate the natural and artificial features of the Earth e.g. rivers, woods, hills, etc.
2. Cadastial Survey – This is referred to extensive urban and rural surveys made to plot the details such as boundaries of fields, houses and property lines.
3. City Survey – An extensive survey of the area in and around a city for fixing reference monuments, locating and improving property lines, and determining the configuration and features of the land is known as city survey.

It deals with survey of water bodies like streams, lakes, coastal waters, and consists of acquiring data to chart the shore lines of water bodies. It also determines the shape of the area underlying the water surface to assess the factors affecting navigation, Water supply, sub-aqueous construction etc.

This is referred to as the proportion of underground plans, fixing the positions and directions of tunnel, shafts and drifts, etc. this consists of transferring bearings and coordinates from a surface baseline to an underground baseline. Ex – Mine survey.

When the survey is carried out by taking photographs with a camera fitted in an aeroplane or drone, it is called aerial or photogrammetric surveying. It is useful for making large scale maps of extensive constructional schemes with accuracy. Though expensive, this survey is recommended for the development of projects in places where ground survey will be slow and difficult because of a bury or complicated area.