Angelo Filomeno

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Laurel Snow
5 years ago
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1 Angelo Filomeno

2 In these lectures we will cover : The role of linear measurement Equipment Procedures and rules Errors Applications of linear measurement

3 27.9 metres Simply the measurement of distance : 50.5 metres New building site - how big is it? 27.9 metres 50.5 metres

4 Pacing (1:100) Optical range finder (1:300) Trundle wheel (1:500) Stadia tacheometry (1:1000) Taping or chaining (1:10,000) Electronic distance measurement (1:50,000)

5 Equipment is fairly cheap (except EDM) Equipment is easy to maintain and adjust (!) Distances are easy to measure Very accurate results can be achieved (with care) Measurement line needs to be unobstructed Errors occur and need to be managed or minimised

6 Tape must always be straight Tape must not be twisted Use chaining arrows for intermediate points Tape horizontally if possible Tape on the ground if possible Slope taping needs to be reduced Catenary taping requires correction Step taping suits some applications

7 obstruction measured distance required distance

8 measured distance required distance

9 DH q horizontal distance = h To calculate the horizontal distance : Or. For example : s = s = m m DH q = o m DH = m then then h = ( ) 1/2 h = h = m cos(2.5) h = m h = s cosq or h = (s 2 - DH 2 ) 1/2

10 required (horizontal) distance

11 required distance measured distance required distance

12 Blunders mistakes and gross errors Systematic errors repeated size and sign affect accuracy Random errors small and usually undetectable (noise) affect precision These error types apply to any measurement technique used in surveying

14 Temperature correction L = L +L.c.DT where : L L c = DT = is the corrected distance is the measured distance 1.15 x 10-5 m/ o C (for a steel band) T actual - T standard

15 Standardisation The tape is not of true length L L x measured length assumed length

A standardised tape is normally kept in reserve and the working tape often compared to this to ensure that measurements taken in the field are correct.

16 A standardised tape is normally kept in reserve and the working tape often compared to this to ensure that measurements taken in the field are correct. Comparison must be made at the standard temperature, pressure and tension. Any discrepancy between the working tape and the standard is recorded and all measurements taken with working tape are corrected to take account of this discrepancy. Standard tape/band Stretched Working tape/band Discrepancy

17 True Distance = Example s tan dard _ tape _ length Recorded _ Dis tan ce s tan dard _ tape _ length Discrepancy A 30m steel band used for recording a survey is compared to a standard 30m band held at the appropriate standard temperature and tension. With the two 30m marks held together it is found that the working tape shows a reading of 55mm in line with the zero mark on the standard. If one of the survey lines was recorded as m what is the true length of the surveyed line? Apply logic: When the working tape reads m (30m -55mm) then the true distance is 30m. If we cut the tape at this length of m we could use this as an accurate representation of 30m. We could see how many times this cut length of tape needed to be stretched out to measure the recorded distance of ( = ). Now every time we stretch out this cut length we are really measuring out a true distance of 30m. So the true overall distance is equal to times 30m = m. or Apply Formula: True Distance = x 30m / (30m - 55mm) = x 30/(29.945) = x = m

18 Catenary (sag) A suspended tape will measure too long L 2 3 (Mg) L 2 L cos 2 24T where : M is the mass per unit length (0.011 kg/metre) g is gravity (9.8 metre/sec 2 ) T is the tension (50 Newton) is the slope angle

19 Tension Tape length will depend on applied tension Slope Distances must always be reduced to horizontal

20 Dimensions of building features Block dimensions Location and size of site features Setting out for construction Clearances and tolerances

21 road width side boundary clearances block dimensions building setbacks building dimensions underground services

e.g. Buildings, Spoil Heaps, Hills PROBLEM: Survey Line is AB. Cannot take straight measurement through obstruction. SOLUTION: Set up point D and point E intervisible from A and B.

22 e.g. Buildings, Spoil Heaps, Hills PROBLEM: Survey Line is AB. Cannot take straight measurement through obstruction. SOLUTION: Set up point D and point E intervisible from A and B. Locate point C as the point which is on line AD and line EB. (Set up ranging rods at A, B, E, and D. Line up another ranging rod between A and D and move it along line AD until it also lines up with points E and B.) Record the distances AC,CB, EC, CD and ED. Use Similar Triangles to calculate the distance AB. i.e. DABC DECD Hence AB ED and thus AB ED AC CD CD AC Note that the following is also true:- AB BC ED CE

23 A OBSTRUCTION C B E D A linear survey for the above situation gave the following information:- AC =48.00m CD = 16.00m BC = 51.00m CE = 17.00m ED = m Find the length of the survey line AB.

24 e.g. River, Motorway PROBLEM: Survey Line is AB. Cannot take straight measurement across obstruction. SOLUTION: Set up point D as offset from AB at point A. (Offset means a line at right angle to the main survey line) Set up an intermediate point E between D and B but accessible from the survey area. Locate point C as an offset from survey line AB. (Easiest method is to stretch tape from E to line AB and swing arc with tape. The lowest reading on the tape will occur when line CE is at right angle to line AB.) Record the distances AC, AD and CE. Use Similar Triangles to calculate the distance AB. i.e. DABD DDEF

25 river Hence AB AD FE FD and thus AB FE FD AD but remember FE =AC and FD =AD CE, so AB AC AD CE AD B C A F E A linear survey for the above situation gave the following information:- AC = 20.00m AD = 18.00m CE = 12.00m D Find the length of the survey line AB

Measurement of Horizontal Distances. the distance between two points means the horizontal distance

Measurement of Horizontal Distances the distance between two points means the horizontal distance Pacing Mechanical Devices Taping * Tachymetric Photogrammetric * Electronic Distance Measurement (EDM)