Chain surveying is the easiest type of survey in which area to be surveyed is divided into a number of triangles. Because all the geometrical figures only whose shape and size are determined when the length of the sides are known. The perpendicular distance, called offsets, of various objects in the field from the line, are measured and recorded in a book called field book. From theses records in the field book, the whole area can be plotted on a drawing sheet to a reduced variance scale.
- 1 Chain surveying instruments:
- 2 Procedure of Chain surveying:
- 3 Advantages and disadvantages of chain Surveying:
- 5 Types of obstacles in chain Surveying and and Solutions of obstacles in chain Surveying:
- 6 Types of Errors in chain surveying:
- 7 Correction of Errors in chain surveying:
- 8 Record keeping in Chain Surveying or Field book in chain surveying:
- 9 Plotting details in Chain Surveying:
- 10 Chain surveying Notes or chain surveying exercise:
- 11 Some questions about Chain surveying or chain surveying Notes:
- 12 Some mathematical problems about Chain surveying or chain surveying Notes for practice:
- 14 Chain surveying pdf Download:
- 15 Chain surveying ppt download:
- 16 Chain surveying video for clear overview:
- 17 Conclusion:
Chain surveying instruments:
The following instruments are used in Chain surveying.
- Ranging Rods
- Offset Staff
- Optical Square
Different types of Chain in Chain surveying:
It is a steel wire with links connected by steel rings. It has brass handles at both ends. There are many types of chains of which the Engineer’s Chain and Gunter’s chain are commonly used in most of the country. The Engineers chain is 100 ft, long and Gunter’s chain 66 ft. The Engineer’s chain consists of 100 links each one a foot long and at every 10 links a tally is attached to facilitate reading. The Gunter’s chain is also divided into 100 links each link is 7.92 inches.In a metric, a small brass ring is given in every one-meter interval. Brass tallies are also given at each 5.0 m length of the chain. Every tally has a complex shape which indicates 5, 10, 15m from any one side of the chain. However, metric chains are prepared in 20 m and 30 m length. Revenue chain is 33 ft long chain.It consists of 16 links. This chain is used for length measurements in feet & inches for small areas. Steel bands are preferred than chains as they are more accurate and easy to use. The disadvantages are they get broken down easily, and it’s difficult to service in the field. The length of the tape is 20 and 30 m, 12 to 16 mm wide and 0.3 to 0.6 mm thick. They are numbered at every meter. Brass studs divide them at every 20 cm.
Testing and Adjustment of Chain:
Due to continuous use of chain gradually the length of the chin gets modified. Its length is shortened mainly due to warping of links. Its length is elongated either for stretching of the links or joints and opening out of the small rings. So, For actual work, it is important to test the chain time to time. We can test the chain with a steel tape or by a standard chain. Sometimes, it is useful to have a permanent test gauge established where the chain is tested. When the length of a chain is estimated at a pull of 8 kg at 20 0C, the length of the string should contain 20 m ± 5 mm or 30 m ± 8 mm, and for 20m and 30m long chain, the length should be close to within 2 mm. Following measures can be taken to accommodate the length of a chain. If chain is found to be too long it can be adjusted by………
- By Locking up the joints of the rings if it found to be opened out
- Removing one or more small rings
- Reshaping damaged rings
- Adjusting the links at the end Sample
- Opening the joints of the rings
- Straightening the bent links
- Adjusting the links at the end
- Inserting new rings where necessary
- By replacing one or more small circular rings by bigger ones
Measuring Tapes used in Chain Surveying:
They may be either of steel or linen to measure short lengths. They are generally of 100 ft,50 ft, 66 ft or 33 ft, and are graduated in feet arid inches.
Cloth or linen Tape:
Linen tapes are closely woven linen. This types of tapes surfaced to resist moisture. They are usually 10 m, 20 m, 25 m and 30 m long and 12 to 15 mm wide. They are normally used for offset measures. These tapes are light and flexible.
Fibre Glass Tape:
These tapes are alike to linen and plastic coated tapes. These are made of glass fibre. They are relatively flexible, strong and nonconductive. These can be used in the proximity of electrical equipment. These tapes do not stretch or shrink due to changes in temperature or moisture or other stress. They are available in length of 20 m, 30m and 50 m length.
These are wooden blocks of conical shape %sect in fixing stations.
They are of steel wire 15 inches long pointed at one end, and the other end is looped for a convenience of handling. They are used for making chain length on the ground.
They are about 10 ft. Long, 1.5 inches diameter round or hexagonal wooden poles painted with black and white alternative bands Each band is of one-foot length.
They are wooden rods 10 feet long. Each foot is painted black and white alternately. They are used for measuring short lengths.
It is used to find the foot of the perpendicular from a given object in the field to a given chain line to take the offset. It consists of a wedge-shaped hollow brass box of about 2 inches sides and 1.25 inches depth with a brass handle about 3 inches long fixed at the bottom. Two plane mirrors set at 45 degrees are fixed to the inclined sides of the box. There are two slits above these mirrors. In using it, a ranging rod is held at an object for. Which an offset is to be taken. A man is holding the optical square in his right-hand stands on the chain line. He looks towards the front ranging rod on the chain line with the open face of the optical square towards the ranging rod at the object. Now the man looks through one mirror while the other mirror is turned towards the object. Then he walks along the chain line forwards and backward till he sights the image of the ranging rod at the object in the mirror and the front ranging led in the slit in the same line. The position of the man on the chain line gives the exact point at which the perpendicular from the object meets the chain line.
Procedure of Chain surveying:
The entire operation of chain survey can be divided into three major groups namely, field work, keeping of records in the field book and plotting of data to prepare maps.
Field work in Chain surveying:
In includes reconnaissance. Selection of the station, measurement of lines and taking offsets of different objects in the field.
This is the preliminary survey in which to the survey party will examine the plot to be surveyed in order to know as to how the works can be executed in the best possible ways. The party will note all details like roads, buildings, canals, ditches, culverts and the difficulties and obstacles that may arise during the carrying out of the work. The party should locate the suitable points for stations by driving pegs. Sometimes a small triangle or a circle is made around the stations and the pegs are inserted into the centers. The party should then make a rough sketch of the plot showing the possible stations and from there the arrangement of different lines. It is important to give a north line on the rough sketch and though the sketch is not prepared according to the scale, it should represent the approximate positions of the different things in the plot and hence to be a good guide for further work.
These are points on the ground fixed by driving pegs. Every station should be located with respect to three permanent objects i.e. the distances from these objects to the stations should be measured very accurately and recorded in the field book. The advantage of taking this measurement is that if in future the peg at the station is lost, then it can be located again by knowing descriptions and distances of these objects. The selection of a particular station depends upon the fallowing important considerations:
- The triangle should be a well-defined one, i.e. nearly equilateral triangle.
- Every main station should be visible from the other two
- There should be a minimum number of obstacles in Ranging and chaining
- The chain line should run near the boundary of the plot
- The chain line should be as few as possible
- The chain line should be over an approximately leveled ground
- In the case of chaining along the road, it is always better to run chains on one side of the road to avoid interruptions by vehicles. It is better not to cross the road frequently
- Offsets should not exceed one chain
- Check and tie lines should be provided in sufficient number so that all the main lines, of sets and other details, can be checked thoroughly
Tie lines and check lines:
A tie line is one which connects two points on the two main lines of the triangle. It helps in taking offsets of the objects falling within the triangle and which are too far away from the main line. A checking line is also a tie line which helps in checking the accuracy of the work after plotting in a drawing sheet. A checking line or tie line is never extended beyond the main lines.
Measurement of lines and taking offsets:
In Fig the main station A is located with respect to three permanent objects and a ranging rod is fixed to the station. One ranging rod is fixed at main station B and another at an intermediate point in between A & B. The three rods will be in a straight line when only the intermediate rod is visible if a man looks from A to B. Now measurement of line AB is taken by the chain. The chain should be properly stretched so that there is no sag in it. As the measurement proceeds, offsets are taken on both side’s of the line AB and recorded in the field book. In this way, all the lines including tie and check lines are measured and offset taken and recorded in the field book.
Advantages and disadvantages of chain Surveying:
- This type of survey work is suited for a small plain ground
- It requires simple instruments
- Plotting of maps is very simple and easy
Types of obstacles in chain Surveying and and Solutions of obstacles in chain Surveying:
Sometimes it will be observed that many obstructions like rivers, canals, ponds. thick jungles, ditches, buildings, etc. lie on the chain line. These obstacles can be avoided in chaining operation by applying some fundamental geometric rules.
Drawing a perpendicular from a point on the chain line:
AC is taken 4 units on the chain line XY.AB and BC 3 and 5 units respectively. Then 〈BAC will be 90° at point A on the chain line because if the sum of the squares on two sides of the triangle is equal to the third. The included angle between the two sides is a right angle (BC²=AB²+AC²)
Drawing a perpendicular from an external point of chain line:
XY is the chain line and P is the external point. Keeping the zero end of the tape at P and swinging the tape along the chain line the point of minimum tape length on the chain line is noted which should be the foot of the perpendicular. Because the perpendicular is the shortest distance.
Drawing a line parallel to the chain line:
Let XY be the chain line and Q is a point through which a line parallel to the chain line is to be drawn. From Q, perpendicular QP is drawn on XY at P. Point R is now selected on XY and RS is drawn the perpendicular to XY at R in such a way that RS=PQ; QS is joined which is now parallel to XY. In Fig (b), point R is selected on XY. QR is joined and bisected at O. Another point P on XY is selected and PO is joined. Now PO is extended to S so that PO=OS. QS is joined. QS is parallel to XY. The following are the geometrical figures by which. chaining can be done in spite of obstacles lying on the chain line. How chaining operation can be done when it is obstructed by a bend of a canal, has been shown in above fig (a) which is self-explanatory. Fig (b) shows the procedure of chaining operation when it is obstructed by a river. A and P are the two points close to the bank on opposite sides of the river. At P a perpendicular PR is drawn. Q is the midpoint of PR, At R again a perpendicular RS is drawn. Point S is fixed by extending AQ. From two similar triangles APO and QRS, RS=AP. The first figure shows the method of chaining when it is obstructed by a hill or ridge. A and D are the foot-hill points, each hidden from view of the other on the either side of the hill. Points B and C are chosen in such a way that a man at B can easily see ranging rods at C and D, while at C, can see the ranging rods at B and A. Now C puts B in the line with A, and B puts C in the line with D. Hence, A, B, C, D are in the same line. If the hill is wide enough, then it can be chained in the usual manner. The method is also known as reciprocal ranging. The second figure shows chaining through a thick wood. The figure is self-explanatory. From the Fig we found that AC²= √(AB²+BC²). The above figure shows the chaining across a building. In fig (a) two points A and B are taken on the chain line and two perpendiculars AC and BD of equal length are erected. The diagonals AD and BC which should be equal, are checked to have the correct result. The line CD is produced past the building and two points G and H are taken on it. Two perpendiculars GE and HF equal in length to Ac or BD are drawn. In this case also diagonals GF and HE are checked. Now ABEF is a straight line and DG=BE. Another method has been shown in Figure (b) Where e point B has been taken on the chain line and Perpendicular BC erected. A is another Point on the chain line so that BC=BA. This makes the angle BAC=45°. AC is joined and extended to D which is roughly opposite the middle point in the length of the building. At D, a perpendicular DF is set to AD, Making DF=DA, On DF, Point G is taken in such a way that DG=DC. By the procedure, explained above ∠GFE is made 45°. Points E and F lie on the straight line AB produced. Now CG=BE.
Chaining along the sloping ground:
First method: During chaining along a sloping surface, the horizontal projection or a chain line is found by the process shown in above Figure. In this method, a portion of the chain, 15 ft to 30 ft is generally used. The length of the chain, of course, depends upon the steepness of the sloping surface. The chain is held horizontally with zero ends of it at P on the ground, while the point Q1 vertically below the other end of the Chain at Q is found by means of a drop-arrow. The next step is commenced from point Q1 and the process is continued until the whole horizontal distance is measured. This method is also known as stepping. Second Method: In this method, the sloping length and the angle of inclination are measured and the horizontally projected length is calculated mathematically. From Figure The xy=L1=measured distance along the slope, ZX=h and θ=angle of inclination, which is measured by instruments such as Clinometer. Abney level, etc. Cosθ= L ⁄ L1 so, L = L1 Cosθ Third Method: If the difference of height h between the points X and Y are known (Using leveling instrument), L can be calculated. L=√(L1²-h²) Example: The distance between two points X and Y measured along a sloping surface is 12.4 chains. Calculate the horizontal projected distance when the angle of inclination is 10°30’. Also find the same when elevations of X and Y above mean sea-level are 740 and 840 respectively. L=L1, Cosθ=1240 Cos 10°30’= 1220 ft. Again, L=√(L1²-h²) =√((1240)²-(840-740)²) =1237 ft.
Types of Errors in chain surveying:
It is always very difficult practically to measure length accurately. The permissible-error with a steel tape is 1 in 2000 in a flat country and 1 in 3000 for a rough undulating country. The error in chaining may happen in various ways. Sometimes there may be mistakes or confusion in reading the tallies such as 30 and 70. There may be also omission of chain lengths due to miscounting or when chaining is interupted by buildings, canals, etc. The error may also be either cumulative or compensa-ting. Cumulative errors are these which may either go on increasing or decreasing when a chain is shorter or longer than its standard length. When the chain is too short, the measured length of the line is too great. i.e., greater than its true length and therefore, the error is positive and the correction is negative. Again when the chain is too long the measured length is too short, i.e., less than its true length. So the error is negative and the correction is positive. Compensating errors are those which cancel one an other and finally their total effect remains approximately same. While stretching a chain one may pull it less than the standard pull of that chain. Again one may stretch it with a greater pull than the standard one. As a result the measured length in the former case will be less and in the latter case will be higher. But when these two lengths are added, the two errors will compensate each other.
Correction of Errors in chain surveying:
(a) Correction in Length, Area and Volume :
Let Le be the incorrect length of the chain and Lc the correct length of the chain. The correct distance, L= Le ⁄ Lc X measured length by the the incorrect chain or tape. The correct area, A =(Le ⁄ Lc )² X Calculated incorrect area The correct volume, V= (Le ⁄ Lc )³ X Calculated incorrect volume Example: The road from Dhaka to Mirpur is actually 25320 ft long. This distance was measured by an Engineer’s defective chain and was found to be 25273 ft. How much correction does the chain need ? L= (Le ⁄ Lc) X measured incorrect length Le = (L * Lc) / Measured length =(25320 * 100)/ 25270 = 1000.197 ft So the chain should be shortened by 0.197 ft Example : The length and breadth of a plot of land were measured by an Engineer’s chain exactly 100 ft. in length at the beginning. But it was found to be 100.3 ft. long at the end of the survey work. The area of the plot drawn to a scale 1 inch=100 ft. was 25.60 sq. inches. What was the true area of the plot 7 True area A =(Le ⁄ Lc )² X Calculated incorrect area =(100.3/100)² * 25.6 sq. in =25.70 sq.in From the scale on the map 1 in²= 100²=10,000 sq. ft. Area of the plot= (25.70 x 10,000) / 43560 (1 acre=43560 sq ft.) =5.89 acres Example : The length, breadth and depth of a pond were measured by an incorrect Gunter’s chain. The volume of the pond was calculated to be 1,60,000 cft. The chain was tested at the end of the measurement of the tank. True volume,= (Le ⁄ Lc )³ × incorrect volume =(65.8/66)² × 160000 =159200 cft
(b) Correction for Pull :
Sometimes, a steel tape is pulled in excess of the pull at standardization, then the correction to be made is as follows : Correction. Cp = L(Fi − Fs)/ AE Where L=length of tape, A=cross-sectional area of tape. Fi=pull applied in the field, Fs = pull at standardization, and E= Young’s Modulus of Elasticity ( for steel, E=30 x 106 p.s i. ) Since the effect of pull on tape is to make the measured length too short the correction is always positive. Example : A steel tape of 100 ft. length, standardized at 25 lb. pull, was used in the field with a pull of 35 lbs. The cross-sectional area of the tape is 0.025 sq. inch. Take the value of Young’s Modulus of Elasticity for steel, 30 x 106 p.s i. Calculate the correction for excess pull.
Correction = 100(35−25)/(0.025×30×106 )
= 0.00134 ft (positive)
(C) Correction for sag:
Correction, C3 = W²L/ 24 Fr²
Where, W=wt. of the tape in lb., L=length of the tape in ft. and Fr =pull applied in the field in lb. Since the effec of sag on tape is to make the measured length too large the correction is always negative. Example : A steel tape of 100 ft. length weighing L lbs. As pulled with a force of 20 lbs. in the field to measure a certain distance. Calculate the correction for sag.
Correction, C3 = W²L/ 24 Fr²
=0.15 ft (negative)
Since the length of the tape is increased as temperature is raised, where measured distance in too small, it is therefore. essential to apply this correction.
Correction, L= a (tf- ts) × L
Where, L=measured length in ft., tf=temperature at which the tape was standardized, ts=temperature at which the tape is used in the field, and a =coefficient of thermal expansion of the tape per degree °F per foot length. The co-efficient of thermal expansion of steel varies from 5.5 x 10-6 to 6.85 x 10-6 per degree °F. The sign of the correction is greater or ‘less than ts. The steel tapes are generally standardized at 65°F. Example : A distance of 1840 ft. was measured with a steel tape which was exactly 100 ft. long at 65°F. The temperature during measurement in the field vials 85°F.
Record keeping in Chain Surveying or Field book in chain surveying:
All the details including a rough sketch of different types of stations, offsets, etc. in the field are recorded in a book called Field Book. It is an Important book or document which should be maintained carefully. It is 9″x 5″ in size with two parallel lines ruled longitudinally in the center of every page. These two parallel lines are imaginary lines representing the chain line and the space in between has no existence in the field. The record keeping starts from the bottom of the end page of the field book. A rough sketch of the plot is drown beforehand on the last page for reference. Neat figures and sketches with clearness in representing points to which offsets are taken should be properly maintained.
Plotting details in Chain Surveying:
Before plotting the details of, chain survey on a drawing paper a suitable scale Should be chosen first. Because drawings are prepared to a reduced scale. A scale is the ratio between ground the actual length on the ground and the corresponding length of the line on the map. For an example, when the scale of a map is 1 inch=100 ft it indicates a length of 100 ft. on ground being equal to 1 inch on the map. A scale may also be donated by term representative fraction which is the ratio of 1 inch to the distance on the ground both reduced to inches. As for an example, the representative fraction of 1 inch=100 ft. scale is 1/1200 i.e. 1 inch to 1200 inches. The triangle is the first plotted from its known according to a suitable reduce scale. Then the lines and check lines are drawn and checked the accuracy of the work. New to the accuracy of the work. New to plot offsets like railroads, trees, building, electric posts etc. lines are taken up one by one the the offsets are laid down on either side of the lines. When the offsets are plotted the conventional signs for offsets are put in.
Chain surveying Notes or chain surveying exercise:
Examine the following statements very carefully and identify whether they are true or false: (a) Chain survey is a type of survey in which the survey is done with the help of a chain and tape only. b)Chain lines can be measured with the help of a chain only. c) Tie lines are generally extended outside the main triangle. d)In an optical square the mirrors are placed at an angle of 60° to each other. e) Right angles can also be set with the help of a chain and a tape. f) The space between two parallel lines in the field book has no existence in the field. g) Check lines are also tie lines. h) Chain surveying is suitable for crowded cities i) Gunter chain is most suitable for measuring areas j) A chain survey is a class of survey in which a triangle is formed in the field whose sides are used as reference lines. k)In Chain Survey angles are measured with considerable accuracy L) The station are the points from which measurements are taken up to any object in the field m) If the triangle is too big then it may be necessary to run few more straight lines within the triangle n) It is not necessary for the tie Lines to touch any other tie line or main line o) The main lines and the tie lines have common name known as chain lines p) The offsets are also chain line measured up to different objects in the field q) Comparing the plotted length of the tie lines on the map with the measured length in the field the accuracy of the survey may be determined r) The start with as big a triangle as possible is formed and subsequently the area inside is subdivided by tie lines s) Chain is used for measuring the chain lines and tape is used for measuring the offsets t) The offsets should not be more than the full length of the tape measuring them u) The ranging rods which are painted in red and white are used for fixing stations also v) Arrows are used for aligning chain lines
Some questions about Chain surveying or chain surveying Notes:
- What is a ranging rod ? Why it is painted in different colors ?
- What is an optical square ? Explain its working principle with neat sketch. What are the sources of error in an optical square ? How can you detect them ?
- Discuss the procedures of chaining when it is obstructed by a bend of a canal, a building, a pond and a bush. Show with sketches.
- What is reconnaissance survey ? Why a sketch map of the plot to be surveyed is prepared ?
- Write notes on : (a) Offset (b) Field Book (c) Cross-staff (d) Reciprocal Ranging (e) Stepping Method of Chaining (f) Well-conditioned Triangle (g) Sag Correction
Some mathematical problems about Chain surveying or chain surveying Notes for practice:
- The true length of a line was known to be 1500 ft. The line was measured with a 100 ft. tape and found to 1505 ft. Calculate the correct length of the tape. Ans: 99.66 ft.
- The distance between b two sub-stations was found to be 5305 ft. when measured by an engineers chain and 7946 links by Gunter’s chain. Both the chains were incorrect. What correction is needed in the Engineers’ chain if the Gunter’s chain is 0.4 link too long Ans: 0.748 too short
- A draftsman measured a line on a map prepared by cbain survey by 1 inch= 80 ft. scale and found it to be 3200 ft. long on the ground. Later he discovered that he made a mistake and me scale was 1 inch=60 ft, Find the correct distance in his calculation. Ans: 2400.0 ft
- On a map covering an area of 2.5 sq. miles, the scale is by mistake recorded to be 12 inch=1 mile with the result that the area comes to be 4.sq. miles and 2844 sq. chains. Determine the correct scale of the map. Ans: 16 inches=1 mile
- A 103 ft. tape 13 suspended between its ends under a pull of 20 lbs. The wt. of the tape is 1.5 lbs. Calculate thds connected length cf the tape between its ends. Ans: 99.976 ft
- A survey was commenced at 7:30 A.m. when the temperature was 65°F with a 100 ft steel tape which was correct. After chaining a distance of 12650 ft. the work was suspended at 1:30 P.M. when the temperature was 102°F. Find the correct distance. Take the coefficient of expansion of steel =6.25 x 10-6″ per deg. °F. Ans:12,652.92 ft. 16.
- A line was measured with a 20 m chain and found to be 98.4 m long. The chain was subsequently found to be 0.02 m too short. What was the correct measurement of the line ? Ans: 98.3 m
- Chaining along a slope the first 30 ft. were on a slope of 8° and the next 20 ft were on a slope of 5° What is the true horizontal distance ? Ans: 49.633 ft
- A chain line terminates at an inaccessible point D. Chaining is continued up to point B and a perpendicular BC is laid out 20 ft. in length. At C a right angle DCA is erected; A being on the chain Line. BA is measured and found to bo 8.62 ft. What is the distance from B to the inaccessible point D? Ans:46.4 ft.
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Chain surveying is the easiest survey. Hope by reading this article you can understand all about chain surveying. If you have any question about chain surveying please comment below here. happy Chain surveying.