Introduction to surveying
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Introduction:
Surveying is defined as taking a general view of, by observation and measurement determining the boundaries, size, position, quantity, condition, value etc. of land, estates, building, farms mines etc. and finally presenting the survey data in a suitable form”. This covers the work of the valuation surveyor, the quantity surveyor, the building surveyor, the mining surveyor and so forth, as well as the land surveyor.
Surveying can also be defined as the act of making measurement of the relative position of natural and manmade features on earth’s surface and the presentation of this information either graphically or numerically.
The process of surveying is in three stages namely:
(i) Taking a general view
This part of the definition is important as it indicates the need to obtain an overall picture of what is required before any type of survey work is undertaken. In land surveying, this is achieved during the reconnaissance study.
(ii) Observation and Measurement
This part of the definition denotes the next stage of any survey, which in land surveying constitutes the measurement to determine the relative position and sizes of natural and artificial features on the land.
(iii) Presentation of Data:
The data collected in any survey must be presented in a form which allows the information to be clearly interpreted and understood by others. This presentation may take the form of written report, bills of quantities, datasheets, drawings and in land surveying maps and plan showing the features on the land.
Types of Surveying
On the basis of whether the curvature of the earth is taken into account or not, surveying can be divided into two main categories:
Plane surveying: is the type of surveying where the mean surface of the earth is considered as a plane. All angles are considered to be plane angles. For small areas less than 250 km2 plane surveying can safely be used. For most engineering projects such as canal, railway, highway, building, pipeline, etc constructions, this type of surveying is used. It is worth noting that the difference between an arc distance of 18.5 km and the subtended chord lying in the earth’s surface is 7mm. Also the sum of the angles of a plane triangle and the sum of the angles in a spherical triangle differ by 1 second for a triangle on the earth’s surface having an area of 196 km2.
Geodetic surveying: is that branch of surveying, which takes into account the true shape of the earth (spheroid).
Classification of surveying
Classification on the Basis of Instruments Used.
i) Chain tape surveys: This is the simple method of taking the linear measurement using a chain or tape with no angular measurements made.
ii) Compass surveys: Here horizontal angular measurements are made using magnetic compass with the linear measurements made using the chain or tape.
iii) Plane table surveys: This is a quick survey carried out in the field with the measurements and drawings made at the same time using a plane table.
iv) Theodelite surveys: Theodolite survey takes vertical and horizontal angles in order to establish controls.
v) Leveling : This is the measurement and mapping of the relative heights of points on the earth’s surface showing them in maps, plane and charts as vertical sections or with conventional
symbols.
Classification based on the surface and the area surveyed
(1) Land survey
Land surveys are done for objects on the surface of the earth. It can be subdivided into:
(a) Topographic survey: This is for depicting the (hills, valleys, mountains, rivers, etc) and manmade features (roads, houses, settlements…) on the surface of the earth.
(b) Cadastral survey is used to determining property boundaries including those of fields, houses, plots of land, etc.
(c) Engineering survey is used to acquire the required data for the planning, design and Execution of engineering projects like roads, bridges, canals, dams, railways, buildings, etc.
(d) City surveys: The surveys involving the construction and development of towns including roads, drainage, water supply, sewage street network, etc, are generally referred to as city survey.
(2) Marine or Hydrographic Survey: Those are surveys of large water bodies for navigation, tidal monitoring, the construction of harbours etc.
(3) Astronomical Survey: Astronomical survey uses the observations of the heavenly bodies (sun, moon, stars etc) to fix the absolute locations of places on the surface of the earth.
CLASSIFICATION BASED ON THE METHOD USED
1. Triangulation Survey
In order to make the survey, manageable, the area to be surveyed is first covered with series of triangles. Lines are first run round the perimeter of the plot, then the details fixed in relation to the established lines. This process is called triangulation.
ii. Traverse survey:
If the bearing and distance of a place of a known point is known: it is possible to establish the position of that point on the ground. From this point, the bearing and distances of other surrounding points may be established. In the process, positions of points linked with lines linking them emerge. The traversing is the process of establishing these lines, is called traversing, while the connecting lines joining two points on the ground. Joining two while bearing and distance is known as traverse. A traverse station is each of the points of the traverse, while the traverse leg is the straight line between consecutive stations. Traverses may either be open or closed.
1. Closed Traverse :
When a series of connected lines forms a closed circuit, i.e. when the finishing point coincides with the starting point of a survey, it is called as a ‘closed traverse’. Closed traverse is suitable for the survey of boundaries of ponds, forests etc. Here ABCDEA represents a closed traverse.
Figure: closed traverse
2. Open Traverse :
When a sequence of connected lines extends along a general direction and does not return to the starting point, it is known as ‘open traverse’ or (unclosed traverse). Open traverse is suitable for the survey of roads, rivers etc. Here ABCDE represents an open traverse.
Figure: Open Traverse
BASIC PRINCIPLES IN SURVEYING
PRINCIPLE OF WORKING FROM WHOLE TO PART
• It is a fundamental rule to always work from the whole to the part. This implies a precise control surveying as the first consideration followed by subsidiary detail surveying.
• This surveying principle involves laying down an overall system of stations whose positions are fixed to a fairly high degree of accuracy as control, and then the survey of details between the control points may be added on the frame by less elaborate methods.
• Once the overall size has been determined, the smaller areas can be surveyed in the knowledge that they must (and will if care is taken) put into the confines of the main overall frame.
• Errors which may inevitably arise are then contained within the framework of the control points and can be adjusted to it.
Surveying is based on simple fundamental principles which should be taken into consideration to enable one get good results.
(a) Working from the whole to the part is achieved by covering the area to be surveyed with a number of spaced out control point called primary control points called primary control points whose pointing have been determined with a high level of precision using sophisticated equipments. Based on these points as theoretic, a number of large triangles are drawn. Secondary control points are then established to fill the gaps with lesser precision than the primary control points. At a more detailed and less precise level, tertiary control points at closer intervals are finally established to fill in the smaller gaps. The main purpose of surveying from the whole to the part is to localize the errors as working the other way round would magnify the errors and introduce distortions in the survey.
(b) Using measurements from two control parts to fix other points. Given two points whose length and bearings have been accurately determined, a line can be drawn to join them hence surveying has control reference points.
i) Using points A and B as the centers, ascribe arcs and fix (where they intersect).
ii) Draw a perpendicular from D along AB to a point C.
iii) To locate C, measure distance AB and use your protractor to equally measure angle ABC.
iv) To locate C the interior angles of triangle ABC can be measured. The lengths of the sides AC and BC can be calculated by solving the triangle.
Linear measurements
A linear measurement assigns a numerical value for the length of an object or between objects. Units of linear measure include inch, foot, meter, kilometer and mile. Linear measurements have one dimension, whereas square measurements have two dimensions and cubic measurements have three.
In plane surveying, linear measurement is done in measuring of all the distances. The commonly used instruments for linear measurements are chains, tapes.
Supporting instruments in linear measurements
Ranging Rods: The ranging rods are used to mark points which must be seen from a distance and also for ranging the survey lines. They are 2 to 3 meters long with a steel point at the bottom and painted alternately black and white or red and white. They are made up of wood, steel and bamboo. Colored cloth flags may be fastened to the top of the rod to facilitate its visibility when ranging over long distances.
Chaining/Arrows/Pins: Arrows are used to mark the end of the tape or chain in measuring distance. They are made up of stout wire, pointed at one end for sticking into the ground and with a loop at other end. They are usually 30 to 40 cm long and sometimes they are made long to be visible in grass or stubble.
Plumb Bob: The plumb bob is used for locating point’s directly below or other points. It is made up of a pointed metal weight suspended by a string. It is made of brass with a replaceable tip of wear resistant alloy steel.
Ranging Survey Lines
Direct ranging
Indirect ranging
a) To range a line across a valley
b) To range over a hill
c) To range on undulating steep slope
Process of Chaining
These two topics are discussed in class
Chain Surveying
This is the simplest and oldest form of land surveying of an area using linear measurements only. It can be defined as the process of taking direct measurement, although not necessarily with a chain.
The basic instruments used in chain surveying are as follows:
Chains
Chains are the commonly used instruments for linear measurements. The different types of chains are as follows:
Suveyor’s chain: This chain is most popular instrument for linear measurement. Chains are made up of galvanized iron wire and consist of 100 or 150 links. The end of each links are bend into rings and connected with the ring of the next piece by two or three oval rings which afford flexibility to the chain.
Metric chain: Metric chains are usually made up in length of 20 and 30 meters. Each link in metric chain is 20 cm long. Tags are fixed at every five meter length and small brass rings are provided at every meter length, except where tags are attached.
Engineer’s chain: This chain is 100 feets long and is divided into 100 links each links of 1 foot long.
Gunter’s chain: Gunters chain was introduced by Edung Gunter of England in 1620 AD. It is 66 feet long and divided into 100 links, each links of 66 feet or 7.92 inches.
Tapes
Tapes are used in linear measurements because of their high accuracy compared to chains. Tapes are available in different sizes such as 5, 10, 20, 50 and 100 meters. The common types of tapes used are as follows:
Metallic cloth Tape: it consists of a varnished strip of woven linen about 12 mm wide with a number of strands of copper wire woven into it to minimize stretching or twisting. They are available in length of 10, 20 and 30 meters.
Steel Tapes: Steel tapes are made up of steel and used for accurate measurements. They are marked in various ways such as etched, stamped on clamps, soldered sleeves or stamped on bosses. They are available in length up to 100 meters or even longer.
GENERAL PROCEDURE IN MAKING A CHAIN SURVEY
1. Reconnaissance: Walk over the area to be surveyed and note the general layout, the position of features and the shape of the area.
2. Choice of Stations: Decide upon the framework to be used and drive in the station pegs to mark the stations selected.
3. Station Marking: Station marks, where possible should be tied - in to a permanent objects so that they may be easily replaced if moved or easily found during the survey. In soft ground wooden pegs may be used while rails may be used on roads or hard surfaces.
4. Witnessing: This consists of making a sketch of the immediate area around the station showing existing permanent features, the position of the stations and its description and designation. Measurements are then made from at least three surrounding features to the station point and recorded on the sketch.
The aim of witnessing is to re-locate a station again at much later date even by others after a long interval.
5. Offsetting:- Offsets are usually taken perpendicular to chain lines in order to dodge obstacles on the chain line.
6. Sketching the layout on the last page of the chain book, together with the date and the name of the surveyor, the longest line of the survey is usually taken as the base line and is measured first.
Optical Square:
This instrument is used for setting out lines at right angle to main chain line. It is used where greater accuracy is required. There are two types of optical square, one using two mirrors and the other a prism.
ERRORS IN SURVEYING
Surveying is a process that involves observations and measurements with a wide range of electronic, optical and mechanical equipment some of which are very sophisticated.
Despite the best equipments and methods used, it is still impossible to take observations that are completely free of small variations caused by errors which must be guided against or their effects corrected.
TYPES OF ERRORS
1. Gross Errors
These are referred to mistakes or blunders by either the surveyor or his assistants due to carelessness or incompetence.
On construction sites, mistakes are frequently made by in – experienced Engineers or surveyors who are unfamiliar with the equipment and method they are using.
These types of errors include miscounting the number of tapes length, wrong booking, sighting wrong target, measuring anticlockwise reading, turning instruments incorrectly, displacement of arrows or station marks etc.
Gross errors can occur at any stage of survey when observing, booking, computing or plotting and they would have a damaging effect on the results if left uncorrected.
Gross errors can be eliminated only by careful methods of observing booking and constantly checking both operations.
2. Systematic or Cumulative Errors
These errors are cumulative in effect and are caused by badly adjusted instrument and the physical condition at the time of measurement must be considered in this respect. Expansion of steel, frequently changes in electromagnetic distance (EDM) measuring instrument, etc are just some of these errors.
Systematic errors have the same magnitude and sign in a series of measurements that are repeated under the same condition, thus contributing negatively or positively to the reading hence, makes the readings shorter or longer.
This type of error can be eliminated from a measurement using corrections (e.g. effect of tension and temperature on steel tape).
Another method of removing systematic errors is to calibrate the observing equipment and quantify the error allowing corrections to be made to further observations.
Observational procedures by re-measuring the quantity with an entirely different method using different instrument can also be used to eliminate the effect of systematic errors.
3. Random or Compensating Errors
Although every precaution may be taken certain unavoidable errors always exist in any measurement caused usually by human limitation in reading/handling of instruments.
Random errors cannot be removed from observation but methods can be adopted to ensure that they are kept within acceptable limits.
In order to analyze random errors or variable, statistical principles must be used and in surveying their effects may be reduced by increasing the number of observations and finding their mean. It is therefore important to assume those random variables are normally distributed.
Corrections to Linear Measurement and their Application:-
The following corrections are to be applied to the linear measurements with a chain or a tape where such accuracy is required.
(i) Pull correction,
(ii) Temperature correction
(iii)Sag correction
Pull Correction:-
A chain or tape of nominal length ‘L’ having cross sectional area of the link or that of a tape, as the case may be, equal to A and standardized under a pull Ps is employed to measure a length at a pull PF. If Young’s modulus of elasticity of the material is E the extension of its length is
The recorded length is less than the actual by this extension. The error is here, -ve, the actual length is obtained by adding the extension to L. the correction is +ve. If PF is less than PS the error will be +ve and correction –ve.
Temperature Correction:-
A chain or a tape of nominal length ‘L’ standardized at temperature TS and having cross sectional area A is employed to measured length at temperature TF being the coefficient of linear expansion of the material of the chain or tape per unit rise of temperature , the extension
(TF – TS)L
If TF is more than TS, recorded length is less than the actual by the amount of extension. The error is –ve and the correction to the length L is +ve by the amount of extension. If the field temperature TF is less than TS the error is = +ve and the corrections is –ve.
Sag Correction:-
In case of suspended measurement across a span L the chain or tape sag to take the form of curve known as catenary.
Where w= weight of the tape per meter length
W= Total weight of the tape
P=pull applied (in N)
l1 =the length of tape suspended between two supports
l= length of the tape = nl1 (in m)
Sag correction is always negative.
Obstacle while chaining:
Obstacles which obstruct chaining but not ranging:
Water bodies like lakes, ponds and rivers are typical examples of obstacles in this category. It is possible to chain around these obstacles by using the following methods.
By constructing rectangles: Chaining had reached A and encountered an obstacle. To get to B, mark A and B with an arrow. Set of perpendiculars AC and BD high enough to clear the obstacles. Join and measure DC which now equals AB. This allows chaining to continue from B.
ii. By constructing similar triangles:
To continue chaining from B, fix a point C away from the obstacle. Range a pole at D to align with AC hence AC = CD. In line with BC range another point E in line with BC. Hence BC= CE. Measure ED which equals AB hence chaining can continue from B.
Obstacle which obstruct both ranging and chaining
GD = (FC x GA) / FA
HE = (FC x HA) / FA
COMPASS SURVEYING
Another type of survey instrument that forms the subject of this section is the compass. Here, we will explain the meaning, types of compass survey and also introduce and discus the concept of
bearing.
Compass surveying:-
In compass survey, the direction of the survey line is measured by the use of a magnetic compass while the lengths are by chaining or taping. Where the area to be surveyed is comparatively large, the compass survey is preferred, whereas if the area is small in extent and a high degree of accuracy is desired, then chain survey is adopted. However, where the compass survey is used, care must be taken to make sure that magnetic disturbances are not present. The two major primary types of survey compass are: the prismatic compass and surveyors compass. Compass surveys are mainly used for the rapid filling of the detail in larger surveys and for explanatory works. It does not provide a very accurate determination of the bearing of a line as the compass needle aligns itself to the earth’s magnetic field which does not provide a constant reference point.
THE PRISMATIC COMPASS
This is an instrument used for the measurement of magnetic bearings. It is small and portable usually carried on the hand. This Prismatic Compass is one of the two main kinds of magnetic compasses included in the collection for the purpose of measuring magnetic bearings, with the other being the Surveyor's Compass. The main difference between the two instruments is that the surveyor's compass is usually larger and more accurate instrument, and is generally used on a stand or tripod.
• The prismatic compass on the other hand is often a small instrument which is held in the hand for observing, and is therefore employed on the rougher classes of work.
Temporary adjustment of prismatic compass
The following procedure should be adopted after fixing the prismatic compass on the tripod for measuring the bearing of a line.
Centering: Centering is the operation in which compass is kept exactly over the station from where the bearing is to be determined. The centering is checked by dropping a small pebble from the underside of the compass. If the pebble falls on the top of the peg then the centering is correct, if not then the centering is corrected by adjusting the legs of the tripod.
Leveling: Leveling of the compass is done with the aim to freely swing the graduated circular ring of the prismatic compass. The ball and socket arrangement on the tripod will help to achieve a proper level of the compass. This can be checked by rolling round pencil on glass cover.
Focusing: the prism is moved up or down in its slide till the graduations on the aluminum ring are seen clear, sharp and perfect focus. The position of the prism will depend upon the vision of the observer.
OPERATION PROCEDURE
• Remove the corner and open out the prism and window, holding the compass as level as possible.
• Then focus the prism by raising or lowering its case until the divisions appear sharp and clear. If necessary with the needle on to its pivot
• Holding the compass box with the thumb under the prism and the forefinger near the stud, sight through the objector station lowering the eye to read the required bearing as soon as the needle comes to rest naturally.
• The bearing read will be a forward bearing and normally a “whole circle” bearing clockwise angle between 0o to 360o.
Bearing
The bearing is the angular direction measured clockwise starting from North with reference to the observer. The reference North may be true or magnetic. While the true bearing is the angular direction measured in a place with the direction of true or geographical north; the magnetic bearing is the angle which it makes with the direction of Magnetic North measured in the clockwise direction.
Back and Fore bearing:
Fore bearing is the compass bearing of a place taken from a status to the other in the direction that the survey is being carried out. The back bearing in the other hand is the bearing in the opposite direction i.e. the bearing taken backwards from the next station to its preceding station that the fore bearing was taken. The difference between BB and FB is always 180°.
If B is sighted from an observer at A, and the NS and N1S1 are the magnetic NS lines, then Forward bearing (FB) = < N A S + < S A B
Back bearing BA = < N1 B A
Therefore Back Bearing BA = Forward Bearing AB - 180°
If the observer relocates to B and observers B, then forward bearing (FB) BA = < N1 BA and back bearing (AB) = < NAS + SAB. Hence, we can conclude that Forward Bearing = < N1 B A + 180°. As a general rule, if the Fore Bearing is less than 180°, add 180° to get the Back. Bearing, and if the Fore Bearing is greater than 180°, then subtract 180° to get the Back Bearing.
Correction of error in Chain
The error while using a chain can be corrected by following methods
Where: = Incorrect length of a chain or tape
L = normal correct length of chain or tape
Use Plus sign when chain is too long and minus sign when chain is too short.
Where: = Excess length or shortage in chain or tape
Problem: The length of a chain is observed to be 180 m when measured with a 20 meter chain. If the chain is 7 cm too long, calculate the correct length of the chain.
Solution: Correct Length =
Here, = 20 + E = 20+0.07 = 20.07 m
And L= 20 m
Thus Correct Length = 20.07/20 x 180 = 180.63 m
