3. field measurement method

8/3/2019 3. Field Measurement Method

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Field Measurement Method

Dr. Amin Akhavan Tabassi

REG 232- Land Surveying


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Contents

Introduction to the Field Measurement Methods1

Coordinate Systems2

Surveying System3

Instruments and Techniques of Measurement 4

Measurement of Distance5

Errors and Corrections in Measurement 6


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Introduction

Survey field measurements include of measuring the distances andangles.

Distance Measurement:Distances can be horizontal, slope, or vertical.

Measured with tapes, or more commonly with Electronic DistanceMeasuring (EDM).

Horizontal distances are always required for plan plotting purposes.

Vertical distances are most often measured with a surveyorslevel.

Angle Measurement:Horizontal and vertical angles are measured with a transit, orTheodolite. Theodolites are typically more accurate than transits.


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Coordinate Systems

Coordinates are unique identifiers, which locate points of interest in space with respect to a reference frame.The point of interest may be a survey instrument stationor a point of detail such as a land parcel corner on a map

or a building feature in a CAD model.Once a point has coordinates associated with it, thelocation can always be recovered with respect to thereference frame.


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The reference frame, also known as thecoordinate system, may be a simplerectilinear system or a more complex non-linear system such as latitude and longitudeon the surface of the Earth.Coordinate systems may be 2D, describingonly the position of a point in a plane, likethe location of grid intersections on a pieceof graph paper.

Alternatively, the reference frame may be3D, defining the location of a point interms of 2D position and height, such asengineering CAD model of a bridgedesign.

Coordinate Systems (Cont.)

Rectilinear System

Latitude and Longitude


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The final products of plane surveying activities aregenerally maps or CAD models of the natural and builtenvironment.

Coordinate systems are a fundamental part in the creationof these final products as coordinates are used to describethe relationships between survey measurements andinterest point locations.

Coordinate Systems (Cont.)


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Rectangular Cartesian System

N

E

Origin ofsystem

Rectangular coordinate system

A

B

EA

EB

NA

NB

E

N


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Polar Coordinate System

r

r cos

r sin r


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FOR SURVEYING we use a slightly different form of notation.instead of r, we use :D (Horizontal Distance), (Whole Circle Bearing).

A(D, ( N

D

Surveying System


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is always measured in a CLOCKWISE direction from North. is known as the WHOLE CIRCLE BEARING (WCB).

Any line has two bearings:Forward bearing (

PQ), and Backward Bearing (

Qp)

The difference between back & fore bearing is always = 180

True north: Through the geographic poles about which the Earth rotates.Magnetic north: Through which lines of magnetic flux pass.

Arbitrary north: One adopted for a particular project.

NN

P

Q

PQ QP


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The whole circle bearing (W.C.B) of a line is the horizontalangle measured clockwise from the North limb of the meridian.It varies from 0 to 360 .In the figure, The whole circle bearing (W.C.B) of the line OA is

52 and that of line OB is 208 .

WHOLE CIRCLE BEARING (WCB)


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For More Information

True north (geodetic north) is the direction along the earth'ssurface towards the geographic North Pole.True geodetic north usually differs from magnetic north (thedirection a compass points toward the magnetic north pole), andfrom grid north (the direction northwards along the grid lines of amap projection). Geodetic true north also differs very slightlyfrom Astronomical true north (typically by a few arc seconds)because the local gravity may not point at the exact rotationalaxis of the earth.The North Magnetic Pole moves slowly over time due tomagnetic changes in the Earth's core. In 2001, it was determinedby the Geological Survey of Canada to lie near Ellesmere Islandin northern Canada at 81.3 N 110.8 W. It was estimated to be at82.7 N 114.4 W in 2005. In 2009, it was moving toward Russiaat between 34 and 37 mi (55-60 km) per year.


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Anatomy of the Earth


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Given the co-ordinates of two points, calculate thedistance between them and the bearing of the line

joining them.

Northing (m)Easting (m)point

0885.801435.40P

1215.480935.02Q

Example


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solution

Q

P

N

PQ

The distance PQ = [(E Q-E P)2+(N Q-N P)2]1/2

= 599.22 m

N

E

E

N


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Forward bearingPQ = tan -1(EPQ / NPQ )

= - 56 37 15 Since

( PQ ) is negative value we must add 360 ,

Since

( PQ ) = 303 23 45


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Instruments and Techniques

In surveying, measurements may be made directly,electronically, by the use of optical instruments, bycomputations from known lines and angles, or by

combination methods.Instruments used for direct linear measurementsinclude the Gunterschain (which is 20 m long and

divided into 100 links); the tape, usually of steel.


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Gunter's Chain

Gunter's chain is a measuring device used for land survey. It wasdesigned and introduced in 1620 by English mathematicianEdmund Gunter (1581 1626) long before the development of thetheodolite and other more sophisticated equipment, enabling plots

of land to be accurately surveyed and plotted, for legal andcommercial purposes.


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Taping involves measurement of thedistance with tapes (steel/linen), eitherby placing it on the ground or sometimesby getting it suspended between points.

Tapes which made of Invar metal (analloy of steel and nickel) are used forvery precise work because of their low

coefficient of thermal expansion.

* Invar is an alloy of iron and nickel having a low coefficient of thermalexpansion; used in tuning forks and measuring tapes and other instruments

Tapes

Linen Tape

Metal Tape


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In many situations electronic instruments, such as thegeodimeter, which uses light waves, and thetellurometer, which uses microwaves, provide a moreconvenient and more accurate means of determining

distance than do tapes.


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Measurement of Distance

Linear measurement is the basis of all surveying and even though anglesmay be read precisely, the length of at least one line in a tract must bemeasured to supplement the angles in locating points.

Methods of measuring a horizontal distance:Rough Measuring: Pacing, Odometer readings, Tacheometry* (Stadia),

Taping, EDM, and Global Positioning System (GPS)Only the last three meet survey accuracy requirementsDistance from stadia: (High wire-Low wire) * 100 = Distance (ft)

More accurate measuring: taping, EDM (1966), GPS EDM and GPS are most common in todays surveys In pacing, one establishes the # of paces/100 by counting the # of

paces over a pre-measured 300 line*is a system of rapid surveying, by which the positions, both horizontal and vertical, of points on the earth surface relativelyto one another are determined without using a chain or tape or a separate leveling instrument.


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Types of Chains and Tapes

Before the ability to make steel rods and bands, sticks were cut intolengths of 16.5 (Rod) and they were laid end to end to measure.

Gunters Chain 66 long with100 link w/each link being 7.92 inches or 66 feet

longDeveloped by Edmund Gunter in 1600 s in England and madewith individual wires with a loop at each end connectedChain had between 600-800 wearing surfaces which with hard usewould wear and cause chain to elongate

Measurements were recorded in chains and links7ch 94.5lk = 7.945 ch = 7.945 X 66 /ch = 524.37 1 chain = 4 rods; 80 chains = 1 mile


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Types of Chains and Tapes

Engineers Chain Same construction as Gunters Chain, but each link is 1.0 loand was used for engineering projects

Surveyors and Engineers Tapes Made of to 3/8 wide steel tapes in 100; 200; 300 leng Multiple types of marking and graduation: Available in chains, feet, and metric

Graduated: Throughout feet and tenths marked the entire length Extra foot feet marked the length of the tape with additional foot at the 0 end

graduated in tenths and hundreds of the foot


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The table below gives a brief summary of relevant techniques andtheir respective accuracies.

UseRelative

Accuracy

Method

Rough Identification Surveys1:100Pacing

Engineering Survey, Control1:10000Chainage

Site Investigation,

Mapping Plans

1:1000,

1:5000

Tacheometry

Engineering Survey, Geodeticsurvey,

Monitoring, Control

1:10000,1:100000

Electronic Distance(EDM)


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Surveying Metric Conversion

1 foot = 0.3048 meter 1 Meter = 3.2808 feet


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Pacing- Accuracy: 1:100

Method : Don't try to pace out one meter with every step.Walk casually over 100m counting the number of steps.Work out the length of a casual step and use this instead.

ExampleIt takes 125 steps to walk 100(m) so 100/125 = 0.8(m) for thecasual step.If the number of steps measured is 89 steps then the distance is 89x 0.8 = 71.2(m).


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Used for rough identification surveys.Factors Which Influence The Use Of Pacing Low accuracyNo equipment neededUnimpeded paths of travel required between surveypoints

Examples Of Usage


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Field Problems in Taping

During measurement of distance, various obstacles may be encounteredin the field. Depending upon the type of obstacle, a suitablegeometrical figure has to be framed and an equivalent distance has tobe measured or computed. Obstacles encountered in the field can be

divided into three broad categories.Type I : Ranging along obstacle is possible but not measurement such as

pond, river etc.


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To carry measurement along the type of obstacles wheremeasurement round the obstacle is possible, perpendicularoffsets are drawn from the line one at each side of the obstacle,as shown in Figure 1. Then, a parallel distance equivalent todistance along the obstacle is measured.

Figure 1


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In some cases, the distance is being calculatedeither adopting basic principle of geometryand/or trigonometric relations Figure 2.

Figure 2


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When measurement round an obstacle is not possible,similar triangles are established as shown in Figures 3and 4.The unknown distance is calculated by solvingsimilar triangles.

Solution :

Figure 3

PQ= 900 +500 -2 500 900 cos (C)


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Solution :

Example: A survey line AB crosses a river. A line APis run perpendicular to the survey line having length200m. Another line PB is set at right angle to QPsuch that the point Q lies on the survey line at theextension of BA and at a distance of 50m from A.Compute the distance AB.

Figure 4


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Type II

Type II : Measurement along obstacle is possible but notranging such as bush etc.

In this case, random line method is being adopted to range a linebetween stations and subsequently measurement of distance isbeing carried out.


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Type III

Type III : Both ranging and measurement along the obstacle is notpossible such as building.In this case, two perpendicular offsets of equal length are erectedfrom the line before the obstacle and a parallel line is ranged, asshown in Figure 5. The parallel line is extended and two

perpendicular offsets, of same length as before, beyond the obstacle,are dropped from the line beyond the obstacle. Equivalent distancealong a line parallel to ranged line is then measured to get thedistance. However, depending on field condition surveyor can apply

suitable geometrical /trigonometric concepts to find the unknowndistance.


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Figure 5


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Assignment 1

A survey line AB crosses a river obliquely. P and Q are two pointsselected on the line one at each end of the river. Another line EPFis run parallel to the centre line of the river and point E is such thatangle QEP is right angle and EP = PF = 200 m. A third point G is

set at a distance of 280 m from F such that angle GFP is also rightangle. Compute the distance PQ.

Referring to Figure 3 and the Solution Answer=344.093 m


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5 Steps of Taping

1. Shortest distance between two points is a straightline.2.Horizontal distance requires tape to be horizontal.3.Marking tape lengths each application of the taperequires marking using chaining pins to obtain totallength.

4.Reading the tape the graduated tape must be readcorrectly.

5.Recording the distance the total length must bereported and recorded correctly.

C i f T i


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Corrections for Taping

Listed below are several corrections that can beapplied to taped distances to minimize systematic errorsand to correct for the effects of the physical environmentof the measurement process. Temperature Standardization Catenary (Sag) Slope Tension
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L- true length , l - length as measuredto- temperature of standardizationt - temperature at observation , t= (t - t o)M- mass per unit lengthTo -standard tensionT - Field tension , T=(T To)w weight of tape per unit lengthc - coefficient of linear expansionE Young s modulus,q - angle of slope in degreesL' - actual standardized length , L= correction to lengthA is the cross sectional area of the tape

T t ti
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Temperature correction

Most materials expand and contract with temperaturechange, and this effects taped distances.If a tape has stretched due to heat it will read shorter thanit would at its normal (or standard) temperature.The corrected length is computed as follows:

L = l + L = l + l c t L- true lengthl - length as measuredc - coefficient of linear expansiont = (t - t o)to- temperature of standardizationt - temperature at observation

Example
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Example

to =20 C (usual) , t =37 C (hot) , t= 17 Cl =79.984m , c =1.15 x 10 -5 m / C then:L =79.984 + 79.984 x 1.15 x 10 -5 x 17

= 80.00 m

So, if a distance of 79.984m was measured at this temperature,the actual distance would have been 80.000m.Conversely, if it was needed to lay out a line of 80.000m then atape distance of 79.984 would be used.
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Example 2

Tapes in U.S. are standardized at 68 F (20 C); thetemperature difference above or below that will change thelength of the tape

Tapes have a relatively constant coefficient of expansion

of 0.00000645 per unit length per FCT = 0.00000645(Temp ( F)-68) LengthExample: Assume a distance was measured whentemperature was 30 F using a 100 tape was872.54 (68 30) X 0.00000645 X 872.54 =0.21 error tape is short, thus distance is long, error must besubtracted and thus 872.54 0.21 =872.33
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Standardization

The chain or tape being used may not be the correct length soit may always over or under read (a systematic error).

This can be corrected by:

L= l Actual tape length

Assumed tape length
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Example

l = 226.20 m ,assumed tape length = 30.00m ,actual tape length = 30.005 m

Then:L= 226.20 = 226.238 m

A tape is standardized by comparison with a"standard" tape or by measuring a definite"known" distance.

30.00530.00

C t ( ) ti
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Catenary (sag) correction

If the tape cannot be supported for its length then itwill hang freely under the influence of gravity.

The shape of the tape will take is known as acatenary can be determined mathematically.

( )
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The correction for sag is:L= -

ExampleA steel tape 30 m long ,field pull (tension) of 16 kg , weight of the tape 0.8 kg, find thecorrection for sag ?

L= - = - 3.125 mm

W2 l324 T 2

(0.8/30) 2 30 3 24 (16) 2

W: weight of tape per meter T: Tensionl: Measured Length

Catenary (sag) correction (Cont.)
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Slope correction

All plan distances are always quoted ashorizontal distances, therefore any distance notmeasured on the horizontal will need to becorrected for slope.Slope correction must always be considered,and either eliminated in the field ormathematically compensated.
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L

Hl

q

The correction for slope is :

L = ( l 2 - H 2) ,

L = l cos q

Slope correction (Cont.)

Tension correction
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Tension correction

A tape is a given length when pulled with a certaintension.If the tension changes then so does the tape length.

The correction is as follows:

L= l TE A

l= Measured Length T = Tension differences A = cross-sectional area of the steel, E= young's modulous of the tape

l
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Example

A line measured in the field and found to be1242.823 m, with a steel tape with length of 30m, and weight of 0.70 kg, the cross sectionalarea of the tape 0.03 cm 2, the tape standardizedat a temp 20 0C and under tension of 5 kg. Findthe correction of the line due to tension, tempand sag - then find the corrected length of the

line, if the distance measured in the field undertension force 10 kg and temperature 20 0C -modulus of elasticity E = 2.10 10 7 kg/cm 2.

l
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Solution

Correction for tension:

C T =

C T = = +0.986cm or + 0.00986 m

Correction for temperature:

C t = c l t

C t = c 1242.823 (20-20) = 0 m

1242.823(10 -5) 100

0.03 2.1 10 7

l TE A
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Correction for sag:w = 0.70 / 30 = 0.023 kg/m C S =1242.823/30 =41.4274

CS = -{41 + 1 }= -.2445

Corrected length= 1242.823 + 0.00986 + 0 0.2445= 1242.588 m

(0.023) 2 30 324 (10 ) 2

W2 l324 T 2

(0.023) 2 12.823 324 (10 ) 2

A i 2
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Assignment 2

A 3000 m line measured at three steps, after measuringa distance of 800 m the tape was found to be 2cm tooshort, after measuring the total distance of 1800 m thetape was found to be 4cm too long, and after

measuring the total distance of 3000m the tape wastested again and was found to be 8-cm too long.Find the correct distance measured.
http://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.html


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A

800 m 1000 m 1200 m

+ 8 cm

+ 4 cm

- 2 cm

0

3000 m

+

-

BC D
http://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.htmlhttp://webraft.its.unimelb.edu.au/451100/pub/topic/top03-03-02.html


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3. field measurement method

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