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Distance Measuring

Distance Measuring. Two principles of measuring distance. 1) It takes two points to form a line. 2) The shortest distance between two points is a straight line. distance. B. length. A. Selecting The “Best” Method.

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Distance Measuring

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  1. Distance Measuring

  2. Two principles of measuring distance 1) It takes two points to form a line. 2) The shortest distance between two points is a straight line.

  3. distance B length A

  4. Selecting The “Best” Method The best distance measuring method/device to use is influenced by many factors. • Environment • Use of the data • Equipment available • Expertise of individuals • Personal preference • Topography • Client specifications • Regulations • Standard practice The most important factor is the intended use of the data.

  5. Methods of Measuring Distance Direct Methods of Measuring Lengths Pacing Car Odometer Measuring Wheel Chain Tape

  6. Distance Measuring Methods--Pacing • Measuring distance by counting steps (paces). • Distance is calculated by multiplying the number of paces by the individuals pace factor(usually every step one meter ). • Advantages • Simple • Low tech • No specialized equipment • Disadvantages • Topography affects accuracy • Requires practice to take a consistent pace. • Must be able to traverse the distance. • Only measures slope distance. • With practice, accuracy is about 2% of distance paced

  7. Car Odometer

  8. Distance Measuring Methods--Odometer Wheel • An odometer wheel is a wheel which uses an odometer to count the rotations of the wheel. • The gear ratio of the odometer is usually designed to provide a measurement of the distance in standard units--feet, decimal feet, feet and inches and/or meters. • Disadvantages • Accuracy is influenced by surface conditions. • Must be able to traverse distance. • Only measures slope distance. • Advantages • Easy to use • Low tech Error 1% of distance

  9. Equipment Used in Chain Surveying 1- Equipments used for the measurements of lines : A- The chain

  10. MEASURING TAPE The following are the various types of tapes • Cloth tape • Metallic tape • Steel tape • Invar 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.

  11. Invar Tapes: accurate tape made of steel (65%) & Nickel (35%) but they are very expensive .

  12. Distance Measuring Methods--Chaining • Advantages • High precision • 0.001 foot accuracy • Can be used to measure horizontal distances. • Disadvantages • Multiple people • Must have a clear, travelable route. • High precision requires temperature and tension correction. Error 0.1% of distance.

  13. Equipment Used in Chain Surveying 3- Other equipments: a- Ranging Rods: 2m, 2.5m, 3m long & painted red and white each 0.5mwith a pointed steel shoe. .

  14. b- Arrows: 40cm long & 3- 4 mm in diameter c- Pegs: wooden (square or circular cross-section) & steel

  15. Equipment Used in Chain Surveying d- Plumb Bob: a metallic cone object used to erect vertical lines. e- Clinometer & Abney level: used to measure the inclination angle of slope lines.

  16. Taping: Corrections STANDERD CONDITIONS Metric system Tempreture 20 c Tape fully supported Tention 50 Once a line is being measured, it is necessary to convert the measured length into a horizontal length. Series corrections have to be applied. Five possible corrections have to be considered. These are • Erroneous Tape Length • Slope • Tension • Temperature • Sag

  17. Taping: Corrections • For synthetic tapes, only Erroneous Tape Length and slope corrections will be applied • The best accuracy that can be achieved is the order of 1:1000 • When using steel tapes, if only Erroneous Tape Length and slope corrections are considered, the best possible accuracy that can be obtained in the range 1:5000 If tension and temperature are added into consideration, accuracy can be increased to better than 1:10000 ~ 1: 20000 • Sag only applies if tape is supported only at ends

  18. standardisation length (actual tape length) nominal length (assumed tape length) Erroneous Tape Length • tape has a nominal length under certain conditions, a tape stretches with time. • standardisation needs to be carried out frequently by using reference tape or baseline.

  19. For a 30m Nominal Length Tape Tape used When comparing to a standard tape, the tape used indeed has a length 30 m + ∆l For every 30m measurement, the small elongated amount should be added for correction.

  20. Slope Correction • All plan distances are always quoted as horizontal distances L, therefore any distance not measured on the horizontal will need to be corrected for slope. Slope correction must ALWAYS be considered, and either eliminated in the field or mathematically compensated. Angle may be measured by Theodolites

  21. Standardisation tension Cross section Area Modulus of Elasticity of tape material For steel, E = 200,000 N/mm2 Tension Correction • A tape is a given length when pulled with a certain tension. If the tension changes then so does the tape length. Tension applied

  22. Length error due to Temperature change Temperature change Measured length Coefficient of linear expansion Temperature Correction • Most materials expand and contract with temperature change, and this effects taped distances. If a tape has stretched due to heat it will read shorter than it would at its normal (or standard) temperature.

  23. Weight of tape per unit length Angle of slope Tension applied to the ends Sag Correction • If the tape cannot be supported for its length then it will hang freely under the influence of gravity. The shape of the tape will take is known as (sag) and can be determined mathematically.

  24. Actual length is: Steel Taping: Example Combined Errors

  25. horizontal length AB = 29.872-0.0050+0.0139-0.0022-0.0611+0.0052 = 29.823m

  26. A steel tape of nominal length 30 m was used to measurea line AB by suspending it between supports. The following measurements were recorded Line Length Measured Slope Angle Mean Temp. Tension AB 29.872 m 3o 40’ 5oC 120 N The standardisation length of the tape against a reference tape was known to be 30.014 m at 20oC and 50 N. If the tape weighs 0.17 N/m and has a cross sectional area of 2 mm2, calculate the horizontal length of AB.

  27. Processes in Chain Surveying • Two Measurement are performed • ranging and measurement of lines • setting out of right angles • Take the following points in mind when doing measurements: • the measurement should be in a straight line • the tape should be reasonably pulled • systematic way should be followed

  28. Processes in Chain Surveying • Ranging and Measurement of Lines • Level Ground • Uniformly Sloping Ground • Uneven Ground

  29. Level Ground Rod Rod A A1 A2 A3 A4 B

  30. Uniformly Sloping Ground

  31. Horizontal Distance-cont.Chaining • To measure horizontal distance with a chain, a level and plumb bob must be used. • The chain is held level, horizontal, and the measurement at the elevated end is transferred using a plumb bob. • This method is limited to slopes of 5% or less. • When horizontal distances are measured by chaining on slopes > 5%, the technique called “breaking chain” must be used.

  32. Uneven Ground

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