Presentation on theme: "It is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles."— Presentation transcript:
What is done in Chain Surveying? In Chain Surveying, only linear measurements are made i.e. no angular measurements are made. Triangle is the only figure that can be plotted with only measurement of sides to enclose an area. Hence, in chain survey area to be plotted should be covered with a network of triangles. Therefore, chain surveying is also known as Triangulation. 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.
Suitable cases Area to be surveyed is comparatively small. Ground is fairly level. Area is open. Details to be filled up are simple and less. Non-Suitable cases Area to be surveyed is large. Ground is quite uneven. Area is crowded. Details to be shown are too many.
Requirements for a Chain Survey Chain Measuring tape Ranging rod Arrows Cross Staff Pegs Field book Plumb bob
Chain A chain is a unit of length; it measures 66 feet or 22 yards or 100 links. There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in Britain and in some other countries influenced by British practice. The ends of the chain are provided with handles for dragging the chain on the ground, each wire with a swivel joint so that the chain can be turned without twisting.
The length of the chain is measured from the outside of one handle to the outside of another handle. Following are the various types of chain in common use: Metric Chains Gunters Chains Engineers Chains Etc.
METRIC CHAINS Metric chains are made in lengths 20m and 30m. Tallies are fixed at every five-meter length and brass rings are provided at every meter length except where tallies are attached.
GUNTERS CHAIN One of the first chains used in the U.S. was the Gunters chain. The Gunters chain was a series of links attached to a handle which included an adjustment for wear. The chain was 22 yards (66 ft) long.
MEASURING TAPE A tape measure or measuring tape is a flexible form of ruler. It consists of a ribbon of cloth, plastic, fiber glass, or metal strip with linear- measurement markings. It is a common measuring tool. Its flexibility allows for a measure of great length to be easily carried in pocket or toolkit and permits one to measure around curves or corners.
The following are the various types of tapes Cloth tape Metallic tape Steel tape Invar tape Among the above, metallic tapes are widely used in surveying. A metallic tape is made of varnished strip of waterproof line interwoven with small brass, copper or bronze wires. These are light in weight and flexible and are made 2m, 5m 10m, 20m, 30m, and 50m.
RANGING ROD Ranging rods are used to range some intermediate points in the survey line. The length of the ranging rod is either 2m or 3m. They are shod at bottom with a heavy iron point. Ranging rods are divided into equal parts 0.2m long and they are painted alternately black and white or red and white or red, white and black. When they are at considerable distance, red and white or white and yellow flags about 25 cm square should be fastened at the top.
ARROWS Arrows are made of good quality hardened steel wire of 4 mm diameter. The arrows are made 400 mm in length, are pointed at one and the other end is bent into a loop or circle
CROSS STAFF The simplest instrument used for setting out a right angle.
OFFSET RODS The offset rod is used for measuring the off set of short lengths. It is similar to a ranging rod and is usually of 3m lengths.
PEGS These are rods made from hard timber and tapered at one end, generally 25mm or 30mm square and 150mm long wooden pegs are used to mark the position of the station on.
PLUMB BOB While chaining along sloping ground, a plumb bob is required to transfer the points to the ground.
Technique of unfolding and folding of a metric chain. UNFOLDING: Remove the strap of the folded chain and take both the handles in the left hand and hold the remaining portion of the chain in the right hand. Holding both the handles in the left hand, throw the remaining portion o f the chain in the forward direction on the ground. Now the follower stands at the starting station by holding one handle and directs the leader to move forward by holding the other handle until the chain is fully stretched.
FOLDING: Bring the two handles together on the ground by pulling the chain at the center. Commencing from the center two pairs of links are taken at a time with the right hand and placed alternatively in both directions in the left hand. When the chain is completely folded the two brass handles will appear at the top. Now tie the chain with leather strap.
Methods of Measuring distance DIRECT MEASUREMENT Counting of paces: Passometer Pedometer Use of wheel-based instruments: Perambulator Odometer Speedometer INDIRECT MEASUREMENT It involves methods such as tachometry, electromagnetic distance measurement, and trigonometric leveling. Tachometry uses a theodolite fitted with a stadia diaphragm or a tachometer to compute distances from intercepts of cross hairs on a staff or stadia rod.
Technical terms used STATION- It is a point of importance at the beginning or at the end of a survey line. MAIN STATION- These are the stations at the beginning or at the end of lines forming main skeleton. SUBSIDIARY OR TIE STATIONS- These are the stations selected on main lines to run auxiliary/secondary lines for the purpose of locating interior details.
BASE LINE- It is the most important line and is the longest line. Main framework of survey lines are built on it. DETAIL LINE- If the important objects are far away from the main lines, the offset readings are too large, which results into inaccuracies and time-consuming in the field work. In such cases the secondary lines are run by selecting stations on main lines. CHECK LINES- These are the lines connecting main station to a subsidiary station on the opposite side or connecting two subsidiary stations on the sides of main lines. These lines are also known as PROOF LINES.
Selection of Stations A station selected should be visible from at least two more stations. If possible should have one or two base lines which run on level ground and through the middle of the area. Main frame should have as few lines as possible. All triangles should be well-conditioned. Each triangle should have at least one check line. Subsidiary stations should be selected such that offsets to main objects from subsidiary lines are as short as possible.
Avoid obstacles to ranging and chaining. As far as possible survey lines should be on the level ground. Sides of the larger triangles pass as close to boundary lines as possible. They should be almost parallel to the boundary. Trespassing and frequent crossing of the roads should be avoided.
Offsets Lateral measurements to chain line for locating ground features are known as Offsets. There are two types of offsets used in chain surveying viz. PERPENDICULAR OFFSET and OBLIQUE OFFSET. In case of perpendicular offset, foot of the perpendicular on chain line is found from the object and the surveyor notes down offset distance and the chainage of foot of the perpendicular. In case of oblique offset, the distance of the object from two convenient points on the chain lines are measured and noted down.
Setting out Perpendicular Offsets Perpendicular offsets may be set by Swinging Using cross staffs Using optical square Using prism square
Perpendicular offset by Swinging The leader takes the end of the tape and holds it on the object. The follower swings the tape on the chain line and finds the shortest distance of the object from the chain line. Since the perpendicular distance is the shortest distance of a point from a line, it is noted as the perpendicular distance. The follower reads the corresponding chainage and the offset length. The recorder records it in the field book.
Prism Square It works on the principle as the optical square. In this case, a prism with angle between reflecting surfaces of 45 degrees is used as shown.
Errors Incorrect length of chain. Incorrect ranging. Loose chain. Temperature change. Variation in pull. Errors in slope measurements. Incorrect marking. Personal mistakes. Cumulative errors. Compensating errors.
Field Work Reconnaissance- Reconnaissance of the area to be surveyed has to be undertaken first. This identifies key features of the area the survey are to be located and determines the kind of equipment needed to be carried to complete the survey. Equipment- Generally the following equipment will be required. A chain with at least 10 arrows, a metallic or steel tape, a dozen of ranging rods, an offset rod, pegs, a plumb bob, etc. Marking stations- Survey stations should be marked on the ground as per the plan prepared. Chaining and locating details- The survey lines are then measured accurately, starting from the base line. The details are located by taking offset at right angle or oblique offsets.
Plotting a Chain Survey (Office work) After carrying field work next step in surveying is plotting to get plan of the area surveyed. It is carried out by the surveyor himself or the assistance of the draftsmen may be obtained. Steps involved in plotting are as follows: SCALE: Depending upon the are in the field and area of drawing sheet scale is decided. Normally, it is decided before the commencement of survey itself
ORIENTATION: Skeleton of the network of triangles should be drawn to a scale on a tracing sheet and the orientation of the plan on drawing sheet be decided. After the orientation is decided using the skeleton diagram on tracing sheet base line and stations are pricked. DRAWING NETWORK OF TRIANGLES: First base line is drawn to the scale. By intersection other stations are fixed and main triangles are drawn. The network of triangles is checked using check lines.
PLOTTING OFFSETS: Any one of the following methods may be used- Mark points along chain line and draw perpendicular with set square. If oblique offsets are taken, arcs are drawn from the respective positions on chain line and the position of the objects fixed up. Main scale and offset scale may be used. Main scale is kept along direction of chain line and offset scale gives the perpendiculars to it. OTHER DETAILS: If readings are taken to only one or two faces of buildings, using overall dimension outline of the building may be completed. Graphical scale should be plotted so that even if the paper shrinks, correct measurements may be obtained without calculations.
Traversing with chain and tape only Though triangulation is the basic principle of chain surveying, it is possible to go for traversing using only chain and tape. Traversing is the survey which is conducted along desired lines by measuring the length and the direction of survey lines. CLOSED TRAVERSE- When the lines form a circuit which starts from a line and after covering an area ends at starting point. OPEN TRAVERSE- If the starting point of survey and ending point are different. Closed traverse- lakes and building surveys. Open traverse- road and canal surveys.
Bibliography Books- Surveying and Leveling, S.S. Bhavikatti Surveying and Leveling, T.P. Kanetkar and S.V. Kulkarni Websites- mcs.nust.edu.pk www.google.com