survey engineering

8/13/2019 survey engineering

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Geodesy, GPS and GIS

Assist. Prof. Dr. Himmet KARAMAN


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Chaining

Electronic Distance Measurement

GPS and other space techniques

Distance Measurement

2 Istanbul Technical University


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Chaining

In many instances, it is easiest to simply measure the horizontaldistance by keeping both ends of the chain (steel tape) at the sameelevation. This is not difficult if there is not a big elevation change

between points.

When the difference in elevation along the measurement becomes toogreat for level chaining, other methods are called for. One option,

break chaining, involves simply breaking the measurement intotwo or more measurements that can be chained level. This works wellfor measurements along a gentle slope where a reasonable distancecan be measured between break chaining points.



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Electronic Distance

Measurement (EDM)INTRODUCTIONElectronic distance measurement instruments (EDMI) determine

lengths using phase changes that occur as electromagnetic energy

of known wavelength travels from one end of a line to the other

end and returns.

The first EDM instrument was developed in Sweeden in 1948,

which was called geodimeter (geodetic distance meter) based on amodulated light beam. The second one was designed in South

Africa in 1957, called tellurometer employs modulated

microwaves.4 Istanbul Technical University


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Modern EDMs display distances in digital form, and many have

microcomputers which calculate horizontal (DX & DY) and vertical

components (DH) of measured slope distances.

EDMs are now being incorporated with theodolites having automaticangle readout capabilities to create, so called total station

(electronic tacheometers). These systems are also called field-to-

inish systems. They can simultaneously and automatically measure

both distances and angles. They record field notes electronically and

transmit them to computers, plotters and other office equipment for

processing.



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CLASSIFICATION OF EDM INSTRUMENTS

They are mainly two types of EDM instruments:

Electro-optical instruments : They transmit light havingwavelengths in the range of 0.7 to 1.2 micrometers within or

slightly beyond the visible region of the spectrum.

Microwave instruments : They transmit microwaves with

frequencies in the range of 3 to 35 GHz corresponding to

wavelengths of about 1.0 to 8.6 millimeters.



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FUNDAMENTAL PRINCIPLE OF EDMI OPERATION

Electromagnetic energy propogates through to atmosphere in

accordance with the following equation:

V = f.

Where V is the velocity of electromagnetic energy, in meters per

second; f the modulated frequency of the energy, in hertz; and

the wavelength, in meters. This propogation can be represented by

the sinusoidal curve illustrated in the following figure, which

shows one wavelength or cycle . Portions of wavelengths or the

positions of points along the wavelength are given by phase

angles .



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FIG. A wavelength of electromagnetic energyillustrating phase angles



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The generalised procedure of measuring distance electronically is

shown in the following figure. An EDM device, centered by means oa plumb bob or optical plummet over station A, transmits to station

B a carrier signal of electromagnetic energy on which a reference

frequency has been superimposed or modulated . The signal isreturned from B to the receiver, so its travel path is double the slope

distance AB . In the following figure, the modulated electromagnetic

energy is represented by a series of sine waves, each having

wavelength .

EDM devices in surveying are operated by phase shift measurement.9 Istanbul Technical University


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A

B

SA,B, HORZ

SA,B, SLOPE

Z

Reflector

EDM

Instrument

Electromagnetic wave

Generalized EDM procedure10 Istanbul Technical University


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Prisms

The reflector, or prism, is a corner cube of glass in which thesides are perpendicular to a very close tolerance. It has the

characteristic that incident light is reflected parallel to itself,thus returning the beam to the source. This is called aretrodirective prism or retro reflector .



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These reflectors have a so-calledeffective center . The location of thecenter is not geometrically obvious

because light travels slower throughglass than air. The effective center will

be behind the prism itself and isgenerally not over the stationoccupied. Thus there is a reflector constant or prism constant to besubtracted from the measurement.Some manufacturers shift the center of the EDM forward the same amount asthe prism offset to yield a zeroconstant.



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ERRORS IN EDMsTotal error = Constant (+5 mm) + 5 ppm ppm = part per million ppb = part per billion

Constant error is negligible for long baselines, but is significant for short baselines. The proportional part varies depending on thedistance measured.

The errors in EDM can be summarised as follows:ersonal errors

inaccurate setups of EDMs and reflectors over stations.

faulty measurements of instrument and reflector heights. errors in determining atmospheric pressures andtemperatures (and humidity if microwaveinstruments are used).



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nstrumental errors

calibration.they become maladjusted from time to time, andgenerate errors in frequencies.

errors in reflectors (especially corner cube reflectors).constant offsets between electrical center and effectivecenter in both instruments and reflectors.

atural erros

atmospheric variations in temperature, pressure andhumidity.multiple refraction of signals ( ground swing ).



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FIG Reduction of EDM slope distance to horizontal

COMPUTATIONS HORIZONTAL LENGTHS FROMSLOPE DISTANCES

hr

he

elev A

elev BDatum

d

hr -he

S

L

B

A

t



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FIG. Correction for vertical offset between theodolite and EDMImounted on standards

v

Horizontal

Lv

m



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1. Reduction of short lines by elevation differences

d = (elev A + h e) - (elev B + h r ) S L d 2 2

2. Reduction of short lines by vertical angles

rad sec/ * Lcos )hh(

" t er 265206

3. Considerations for different theodolite mounts

rad sec/ * Lcosv m"

v 265206



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4. Reduction of long lines to horizontal

755

)(5.0

10

359474.0)068.08864.4

604.287(

2.27310..5026.1

2.27310..

1

''

)6609.03.237

57'

42

56

'

'

P t t E e

E

N

t e

t P N

n

t

t .(

GR

GR



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where

no : refractive index of light or microwaves(given bymanifacturer)P : pressure (measured)t : dry temperature (measured)t' : wet temperature (measured)e : partial pressure of water vapour (computed)E' : saturation pressure of water vapour (computed)

N GR : group refractive index (computed)n : refractive index in other conditions (computed)

Other parameters

D' : distance (measured)he : height of instrument (measured)ht : height of reflector (measured)HA : height of point A (known)HB : height of point B (known)R : earth radius of curvature (6373394 m)



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Computations

H 1 = H A+h e H 2 = H B+h t DH = H 2 - H 1

Velocity correction (K 1) K 1 = D' (n o-n)Corrected slope distance (D) D = D'+K o+K 1 (K o is

the zero constant)Distance on mean sea level (S)

= D / (R+H 1)

S R DH

D DH

22 1

22( cos( ))

Projection correction ( ds ) dsS

RY Y Y Y

A B A b 6 2

2 2( )

Distance on projection s = S+ds20 Istanbul Technical University


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EXAMPLE

Refractive index of the instrument n = 1.0003108Group refractive index N GR = 107.925Zero constant K o = 0.124 m.Height of instrument (tripot) h e = 0.320 m.Height of reflector h t = 0.450 m

R = 6373394 m.

Meteorological ObservationsInstrument Reflector Mean

Wet temperature 22 o.2 21 o.5 21 o.85Dry temperature 16 o.9 15 o.5 16 o.20Pressure 773.5 757.4 765.45

E' = 13.81589692, e = 13.81215662, n = 1.000279287Velocity correction (K 1) = D' (no-n)=0.232 m.Corrected slope distance (D) = D'+K o+K 1=7357.823

MeasuredLengths7357.473 467

465 474468 470462 470

472 467469 459468 463462



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Electro-optical EDM instruments



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Total Stations



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Total stations with data collectors



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Single Reflectors



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Multiple reflectors


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