Electronic Distance Measurement (EDM) Ms Siti Kamariah Md Sa’at PPK Bioprocess Universiti Malaysia...

Electronic Distance Measurement (EDM)

Ms Siti Kamariah Md Sa’atPPK Bioprocess

Universiti Malaysia Perlis

ERT 247-GEOMATICS ENGINEERING

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D irect(len gth m easu rem en t)

eg m easu rin g tap e

G eo m etrical(O p tical)

E lectro n ic(W ave P h ys ics )

In d irect(d is tan ce m easu rem en t)

D is tan ce M easu rem en t

Principle of operation:

Velocity = distance / time


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Theodolite & TapeStadia

EDM & Theodolite

EDM, Theodolite & Data Collector

Prior to the total station, Theodolite with EDMs and data collectors were used to record large numbers of points, and for measuring long distances. The systems were heavy, prone to failure, and many times the parts incompatible. Prior to these systems, optical (stadia) and manual (tape) systems were used to measure distances.

History


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History

First introduced in the late 1950’s At first they were complicated, large, heavy, and

suited primarily for long distances Current EDM’s use either infrared (light waves) or

microwaves (radio waves) Microwaves require transmitters/receivers at both

ends Infrared use a transmitter at one end and a

reflecting prism at the other and are generally used more frequently.


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EDM Properties

They come in long (10-20 km), medium (3-10 km), and short range (.5-3 km). Range limits up to 50 km

They are typically mounted on top of a theodolite, but can be mounted directly to a tribrach.

Total station Total station

==Theodolite with built in EDM Theodolite with built in EDM

++

MicroprocessorMicroprocessor


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EDM Classifications

Described by form of electromagnetic energy. First instruments were primarily microwave (1947) Present instruments are some form of light, i.e. laser or

near-infrared lights. Described by range of operation.

Generally microwave are 30 - 50 km range. (med) Developed in the early 70’s, and were used for control

surveys. Light EDM’s generally 3 - 5 km range. (short)

Used in engineering and construction


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EDM is very useful in measuring distances that are difficult to access or long distances.

It measures the time required for a wave to sent to a target and reflect back.


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Operation:A wave is transmitted and the returning wave is measured to

find the distance traveled.

Principles of EDM


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Transmitted Energy

Returned Energy

Principles of EDM


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General Principle of EDM

Electromagnetic energy Travels based on following relation:

f

Vλ so fλV

• Intensity modulate EM energy to specific frequency

ncV


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Distances determined by calculating the number of wavelengths traveled.

Errors are generally small and insignificant for short distances. For longer distances they can be more important. Errors can be accounted for manually, or by the EDM if it has the

capability.

Velocity of light can be affected by:Velocity of light can be affected by:TemperatureTemperature

Atmospheric pressureAtmospheric pressureWater vapor contentWater vapor content

Principles of EDM

Measures and Records:Horizontal Angles

Vertical Anglesand

Slope Distances

CalculatesCalculates::Horizontal Distance

Vertical DistanceAzimuths of LinesX,Y,Z Coordinates

LayoutEtc.


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EDM CharacteristicsEDM Characteristics

750-1000 meters range Accurate to ±5mm + 5 ppm Operating temperature between -20 to +50 degrees centigrade 1.5 seconds typical for computing a distanc, 1 second when

tracking. Slope reduction either manual or automatic. Some average repeated measurements. Signal attenuation. battery operated and can perform between 350 and 1400

measurements.


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PrismsPrisms

Made from cube corners Have the property of reflecting rays back precisely in the

same direction. They can be tribrach-mounted and centered with an

optical plummet, or they can be attached to a range pole and held vertical on a point with the aid

of a bulls-eye level.


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PrismsPrisms

Prisms are used with electro-optical EDM instruments to reflect the transmitted signal

A single reflector is a cube corner prism that has the characteristic to reflecting light rays precisely back to the emitting EDM instrument

The quality of the prism is determined by the flatness of the surface and the perpendicularity of the 90˚ surface


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Accuracy

Distance is computed by (no. of wavelengths generated + partial wavelength)/2.

Standard or Random errors are described in the form of +(Constant + parts per million). Constant is the accuracy of converting partial

wavelength to a distance. ppm is a function of the accuracy of the length of

each wavelength, and the number of wavelengths.


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EDM Accuracy


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Error & Accuracy

o-------------------------------o-------------o

A B C

Typical accuracy ± 5 mm + 5 ppmTypical accuracy ± 5 mm + 5 ppm

Both the prism and EDM should be corrected for off-center characteristics.The prism/instrument constant (about 30 to 40 mm) can be measured by measure AC, AB, andBC and then constant = AC-AB-BC

Blunders:• Incorrect ‘met’ settings• Incorrect scale settings• Prism constants ignored• Incorrect recording settings (e.g. horizontal vs. slope)


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Sources of Error in EDM:

Personal:• Careless centering of instrument and/or reflector• Faulty temperature and pressure measurements• Incorrect input of T and p

Instrumental• Instrument not calibrated• Electrical center • Prism Constant (see next slide)

Natural• Varying ‘met’ along line• Turbulence in air


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A B C

Determination of System Measuring Constant

1. Measure AB, BC and AC2. AC + K = (AB + K) + (BC + K)3. K = AC- (AB + BC)4. If electrical center is calibrated, K rep- resents the prism constant. Good Practice:

Never mix prism types and brands on same project!!!

Calibrate regularly !!!


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Systematic Errors/Instrumentation Error Microwave

Atmospheric conditions Temperature Pressure Humidity - must have wet bulb and dry bulb temperature.

Multi-path Reflected signals can give longer distances

Light Atmospheric conditions

Temperature Pressure

Prism offset Point of measurement is generally behind the plumb line. Today usually standardized as 30mm.


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EDM instrument operation

1.Set up EDM instruments are inserted in to the tribrach Set over the point by means of the optical plummet Prisms are set over the remote station point The EDM turned on The height of the prism and the EDM should me

measured


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2.Aim The EDM is aimed at the prism by using either the built-

in sighting devices on the EDM Telescope (yoke-mount EDMs) will have the optical line

of sight a bit lower than the electronic signal When the cross hair is sight on target the electronic

signal will be maximized at the center of the prism Set the electronic signal precisely on the prism center


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3. Measure The slope measurement is accomplished by simply

pressing the measure button The displays are either liquid crystal (LCD) or light

emitting diode (LED) The measurements is shown in two decimals of a foot or

three decimals of a meter EDM with built in calculators can now be used to

compute horizontal and vertical distances, coordinate, atmosphiric,curveture and prism constant corrections


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4. Record The measured data can be recorded in the field note

format Can be entered manually into electronic data collector The distance data must be accompanied by all relevant

atmospheric and instrumental correction factors


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TopographicTopographic&&

As BuiltsAs Builts

MonitoringMonitoring&&

ControlControl

Construction LayoutConstruction Layout

Uses


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Uses

Total stations are ideal for collecting large numbers of points.

They are commonly used for all aspects of modern surveying. Only when harsh conditions, exist or distances are short will a transit and tape be used.


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Problems

Total stations are dependant on batteries and electronics. The LCD screen does not work well when it is cold .

Battery life is also short, batteries and electronics both do not work well when wet.

Total stations are typically heavier that a transit and tape

Loss of data is an important consideration


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“The Flat Earth Society”

Plane geometry vs. Spherical geometry

Angleserror ≈ 1” within 200 km2 area

DistancesError ≈ 0.009 mm per km

Plane Geometry


30xA

xO

A

OTan

xH

xA

H

ACos

xH

xO

H

OSin

CosABBAC

C

Sin

A

Sin

B

Sin

2222

A C

B

γ α

β

A

O

H

β

Plane Geometry


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Geometry of EDM Measurement

Relatively simple if hi = HR

More complicated when the EDM is on top of the theodolite and the prism is higher than the target

(delta HR not equal to delta hi).


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R

Pt. A

BS

α

Pt. 1 X

YR

Pt. A

BS

α

Pt. 1

N

Plane Coordinates


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ΔX

ΔYHD

Pt. A

BS

α

Pt. 1

N

β α+β

HD

XArcSin

Y

XArcTan

YXHD

CosHDY

SinHDX

YX

][

][

Y)X,( From )][(R,

)(

)(

),(HD, From ),(

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β = BS Bearingα = HAα+β = Bearing of 1toA

Horizontal Coordinates


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HD

VDSD

Pt. A

Zenith

Pt. 1

VA

SD

HDArcSinVA

VD

HDArcTanVA

VDHDSD

S

VACosSDVD

VASinSDHD

VVDHD

][

VD)(HD, From )VAD,(

)(

)(

A)(SD, From ),(

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Zenith = UpNadir = DownHorizon = 90°

Vertical Coordinates


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HD

VDSD

Pt. A

Zenith

Pt. 1

VA

HI

HT

HI

ΔZ

Trigonometry Levelling

Electronic Distance Measurement (EDM) Ms Siti Kamariah Md Sa’at PPK Bioprocess Universiti Malaysia...

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