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Lecture5-DistanceMeasurement-Chaining.pdf

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