Homework ..
Ssult89
1/19/2020
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Distance Measurement-Chaining
CSMT 345
LECTURE 5
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Fieldwork Practices cont..
• Chaining equipment
• Basics of chaining
• Proper care for chains
• Chaining a distance
• Chaining a traverse
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Chaining Equipment
• Distance measurement can be simple with chains along a line
• But note that the process must be error free in reading
– Needs skills and good understanding of procedures involved
– A successful field engineer must be consistent in chaining procedure
– Chaining and taping is sometimes used interchangeably
• Common equipment
– Field book
– Pencil-hard tipped (2h, 3h or 4h)
– Two range pole
– Hammer, two plumb bobs
– Steel tape
– Wood stakes
– Chaining pins 3
Chaining Basics
• Familiarity with the chain/tape measure used
– To avoid errors; examine chain closely before use to note point ‘0’
• Know the difference in chains/tapes
– The metric chains vs engineering chains
• Use two people who communicate well with each other
– Two people communicating verbally, by signs; loudly and clearly
– Good long term working relationships is key to success of chaining
• Measure horizontally and maintain good alignment
• Breaking the chain
– Applied in measuring slopes for lengths shorter than full chain length
• Apply proper tension; for precision
• Measure all distances twice; forward and back 4
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Care of Chains and Related Equipment
• Keeping the chains in reel when not in use
• Unwinding the loops/knots on chains
• Drying the chains when wet
• Cleaning off dirt and sand from the chain
• Wiping the chain with oily cloth
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Chaining a Distance
• Success at measuring distances accurately and precisely requires consistency in chaining procedure
• Procedure for measuring distance – You would want the distances to be measured horizontally
– Take measurements by incorporating head chain and rare chain
– The forward and back distances are averaged by adding them together and diving by 2 to obtain a mean distance. A discrepancy ratio can be calculated for each line measured and compared to a standard to determine if the distance is acceptable or needs a repeat • Forward – back = discrepancy
Discrepancy ÷ mean = 1/x
Example:
A horizontal distance measurement using a steel chain/tape recorded a forward distance of 168.29’ and back distance of 168.25’. What is the discrepancy ratio (relative error)? (168.29-168.25)/168.27 = 1/4200
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Chaining a Traverse
• Traverse chaining is a step to determine the location of points
– The interior angles can also measured after chaining and thus determine the precision of work
– Total Station (TS) distance measurement limit the use of traverse chaining;
• Traverse chaining in used in absence of TS
– Traverse chaining starts at one point , goes around and ends at the same point
– Always have the following
• Designated person for head chain and rear chain
• Necessary equipment for chaining
• Prepare sketches in the field book of the traverse to measure 7
Chaining Fundamentals
• Know your equipment ; its graduations/markings/scaling and use
• Pull hard to ensure accuracy and precision
• Maintaining good alignment
• Chain horizontally
– Ensure horizontal distant measurement by using plumb bob. Very sloppy terrain may necessitate breaking chain
• Measure both ways
– Distances must be measured forward and back as a check
• Set solid points using stakes
• Protect points using laths with information describing the use of points
• Maintain equipment by drying, cleaning, oiling regularly
• Record complete concise and properly arranged notes
• Maintain precision
– Forward and back readings to agree at 0.01 per 100 feet measured
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Chaining Calculations
• Forward – Back = Discrepancy
Discrepancy/mean = 1/x
• Example
A traverse chaining field notes had the following data;
Forward distance =146.80’
Back distance = 146.84’
Calculate the mean distance (ft) , the discrepancy and the relative error
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Errors in Chaining
• Sources of errors
– Instrument error
• Resulting from manufacturer’s defect; Calibrating can reduce it
– Natural error
• Effects of temperature, wind and weight of the tape
– Personal error
• Carelessness in reading the tape or in manipulating the equipment
• Types of errors
– Systematic • Occur over and over again and are predictable; can be eliminated
• Every time a distance is measured 1’ is added to attain correct distance
– Random • Occur due to human limitations such as in reading; always exist 10