CHAIN SURVEYING


 











CHAIN SURVEYING
Specific Objectives:
Students should be able to:
  1. Describe chain and tape survey
  2. Explain the different types of equipment used in chain and tape surveying
  3. Practice chain and tape surveying activities at a school level
  4. Explain the importance of chain  surveying
Meaning:
It is a method of Surveying in which no angles are measured but only linear measurement is taken in the field by using a chain or tape measure.
It measures a series of straight lines on the ground with a chain or tape measure and all fixed points relative to the line of traverse either by right angles (offsets) or tie lines.
 Equipment used in chain surveying
i. Chain 
  • The chain is made up of steel wire which is divided into links and togs (rings) to facilitate folding.
  •  It is sometimes used as a unit of measurement
  • It has brass handles at both ends for easy handling. The link is 0.2m or 200mm in diameter.
  • The length is 20m or 30m.
ii.Tape
  • Steel tape
  • Linear tape
A tape is made from fiberglass or a steel strip and is 10m, 20m or 30m in length graduated in 10mm divisions and numbered at each 100mm (10) divisions
It is used for measuring short distances
 iii. Ranging poles
Ranging poles are made up of wood or light metal and measure about 2m long at the top. The equipment has steel shades on its legs so it can be stuck into the ground. Ranging poles are painted red and white so that they can be easily seen even from a distance. They are used for making stations.
 iv. Arrows
Arrows are made of steel wire of diameter 4mm and their ends are bent into a circle where red cloth is tied to facilitate visibility. They are used for showing points on the ground.
They are also used for counting the number of chains while measuring a chain line.
 v. Pegs
Pegs are made of wood 40mm square by 50cm long and are used for permanently marking positions during survey
 vi. Surveyors' band
The surveyor’s band is made of a steel strip which is rolled into a metal frame with a winding handle. It is 30m, 50m or 100m long. Is used in projects where more accuracy measurement is required.
 vii. Cross staff
The cross staff is made of metal or wood with eye slips at right angles and is used to measure right angles from the line of traverse
 viii. Notebook
Notebooks are used during field work to record data obtained. The notebook should be of good quality and 150mm x100mm in size
 ix.A hard pencil and a rubber
Hard pencils are used for drawing in the field and a rubber is used to erase mistakes or errors which are made. A pencil should be HB or HHB.
Methods and procedures involved in chain survey
  • A survey team involves three people, the leading chainman or leader, the follower and the booker.
  • The chain is thrown to extend it and disentangle any knots
  • The leader takes ten arrows and a ranging rod, and the follower takes a ranging rod
  • The follower erects his ranging rod/pole at the first base point and places a brass handle of the chain against the ranging rod.
  • A leader straightens the chain and inserts an arrow at end of the brass handle. Offsets and tie lines can now be taken.
  • The leader drags the chain so that the follower’s end is on the leader’s arrow; the follower moves to another point and places his ranging pole behind the arrow. This procedure is then repeated.
 The importance and usefulness of chain surveying
  1. It is suitable for small areas of fairly open ground.
  2. It is used to fill in details on a map whose large features have been surveyed by other methods.
  3. It is used in mapping small areas of flat or near-flat ground and associated objects, for example paths, roads and railways.
  4. It is used in adding detail to existing plans or large maps.
 Advantages of chain surveying:
  1. It is the simplest method of surveying through the old method.
  2. It is suitable for surveying clear areas.
  3. It tends not to attract attention.
  4. It is suitable for surveying a flat surface on the Earth’s surface, for example a school compound.
  5. It can be read easily and quickly.
  6. It can withstand wear and tear.
  7. It can be easily repaired or rectified in the field.
 Disadvantages of chain survey:
  1. It is a slow method of surveying.
  2. It is the oldest method of surveying
  3. It is not suitable for surveying large areas.
  4. More difficult areas cannot be chain surveyed.
  5. Errors may be encountered due to the use of many chains and other reasons.
  6. It is time consuming.
  7. They are heavy and take too much time to open or fold.
  8. They become longer or shorter due to continuous use.
  9. When the measurement is taken in suspension, the chain sags excessively
 THE ERRORS THAT OCCUR IN CHAIN SURVEYING
What is an error?
An error is a mistake or shortcoming that happen during the survey process leading to wrong measurements. It is sometimes called discrepancies. The following are the type of errors or discrepancies in chain surveying.
 Sources of Errors in Chain Survey and Their Correction
The errors can be divided into three groups:
  • Cumulative (systematic) errors
  • Compensating (accidental) errors
  • Gross Errors
 Cumulative Errors:
Cumulative errors are said to be systematic errors as they are one-directional hence keep on accumulating as the survey progresses. If not checked they have serious implications to the accuracy of the survey. Errors in this class include incorrect length of the tape, page of the tape or the tape not being in line. Since the sources of these errors are known, they can be eliminated.
 They can either be positive or negative errors. While positive errors shortens the measurement (e.g. where the tape length is shorter than what it should be) while negative errors elongates the measurements (e.g. where the tape is longer than what it should be). Checking the equipment can eliminate these errors.
 Compensating Errors:
Compensating errors are said to be accidental errors hence cancel out and does not pose serious problem to the accuracy of the survey. They arise as a result of not being perfect in the use of the equipment or in the whole survey process. For example, if the pull exerted on the tape in either more than or less than what should be the case, faulty results be gotten. The effect can either be positive or negative.
 Gross Errors :
These are mistakes that can be attributed to the inexperience of the team leaders. These are very serious errors which although are random in accordance may lead to faulty plans and maps if not checked. They include discontinuing the chain length (e..g where some arrows are cost or misplaced); misreading of the tape; reading tape upside down (e.g. taking 6 to be 9), etc. By taking the necessary precautions, these errors can be corrected.
Overcoming obstacles during chaining
 TYPES OF OBSTACLES
  1. Visual obstacles: Is an obstacle that prevents an object but the chain remains free. An example of a visual obstacle is a forest.
  2. Chain obstacles: This is the obstacle where by the chain is obstructed but visually it is free. Examples are rivers and ponds
  3. Neither visual nor chain obstacles for example buildings
 HOW TO OVERCOME THOSE OBSTACLES:
  1. By rectangular method
  2. By triangle method
  3. By using similar triangle method
 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.

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.

 
Obstacles which obstruct both ranging and chaining
Chaining has reached B from A where an obstacle like a building has been reached. Erect equal perpendiculars AC and BD from A and B along the chain line. Along CD, range E and F beyond the obstacle. Set off perpendiculars EG and FH from E and F equal to AC. AS G and H are in line with AB, then CE equals AG.
 By constructing similar triangles
Chaining had reached A and there is the need to overcome the obstacle created by the stream to really B. Set out a perpendicular AC and mark the midpoint E. Set out another perpendicular CD so that D, E and B are in a straight line. The 2 triangles created are congruent hence CD = AB which is the required length hence chaining can now proceed from B.
 Importance of surveying
1. It help to prepare a topographical map which shows the hills, valleys, rivers, villages, towns, forests, etc. of a country.
2. It helps to prepare a cadastral map showing the boundaries of the fields, houses and other properties.
3. It help to prepare an engineering map which shows the details of engineering works such as roads, railways, reservoirs, irrigation canals, etc.
4. It help to prepare a military map showing the road and railway communications with different parts of a country.
5. It helps to prepare a contour map to determine the capacity of a reservoir and to find the best possible route for roads, railways, etc.
6. It helps to prepare a geological map showing areas including underground resources.
7. It helps to prepare an archaeological map including places ancient relics exist.
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