Unit 1 Adv Sur

8/13/2019 Unit 1 Adv Sur

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Unit IUnit IModern Surveying EquipmentsModern Surveying Equipments

Introduction:

Revolutionary changes have taken place in last few years in surveying instruments that areused for measuring level differences, distances and angles. This has become possible becauseof introduction of electronics in these measurements. With rapid advancements in the technologyand availability of cheaper and innovative electronic components, these instruments havebecome affordable and easy to use.

Digital Level:

Traditionally various types of levels have been used for measurement of elevation

differences such as dumpy level, tilting level, automatic level etc. Recently electronic digital levelshave evolved as a result of development in electronics and digital image processing.

Digital levels use electronic image processing to evaluate the special bar-coded staffreading. The observer is in effect replaced by a detector diode array, which derives a signapattern from the bar-coded leveling staff. This bar-coded pattern is converted into elevation anddistance values using a digital image matching procedure within the instrument. Automatic dataconversion eliminates personal errors in reading the staff and the field data is stored by theinstrument on its recording medium, thus further eliminating booking errors

Digital levels electronically read a bar-coded scale on the staff. These instruments usually

include data recording capability. The automation removes the reuirement for the operator toread a scale and write down the value, and so reduces blunders. !t may also compute and applyrefraction and curvature corrections.

A digital level offers the following advantagescompared to the conventional leveling andrecording procedures"

#. $atigue-free observation as visual staff reading by the observer is not reuired.

%. &ser friendly menus with easy to read, digital display of results.'. (easurement of consistent precision and reliability due to automation.). Automatic data storage eliminates booking and its associated errors.

*. Automatic reduction of data to produce ground levels, eliminating arithmetical errors.+. $ast, economic surveys resulting in saving in time.. Data on the storage medium of the level can be downloaded to a computer enabling uick

data reduction for various purposes.. Digital levels can also be used as conventional levels with the help of dual marked staff.

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Components of digital levels:

(ain components of digital level consist of two parts" ardware /Digital level and levelingstaff0 and 1oftware.

#. 2oth digital level and associated staff are manufactured so that they can be used for bothconventional and digital operations.

%. As mentioned earlier, digital leveling staves have dual marking. 3ne side is binary bar-coded for digital recording. $or e4ample, 1okkia 1D5'6 uses a RA2 /RAndom 2i-directional 7ode0 staff. The other side is marked as the conventional staff for conventionastaff reading. The staff is made from a glass-fiber-strengthened synthetic material with lowcoefficient of thermal e4pansion for high accuracy. $or highest precision work, !nvar barcoded staves are also available.

Typically digital level has the same optical and mechanical components as a normaautomatic level. owever, for the purpose of electronic staff reading a beam splitter isincorporated which transfers the bar code image to a detector diode array. $igure #.# showscomponents of a typical digital level /1choffield, %66%0. The light, reflected from the white

elements only of the bar code, is divided into infrared and visible light components by the beamsplitter. The visible light passes on to the observer, the infrared to diode array. The acuired barcode image is converted into an analogous video signal, which is then compared with a storedreference code within the instrument. The image correlation procedure then obtains the heightrelationship by displacement of codes, while the distance from instrument to staff is dependent onthe scale of code.

The data processing is carried out on a microprocessor and the results are displayed onmatri4 display. The measurement process is initiated by an interactive keypad and data can bestored onboard.

Data from digital levels is stored onboard /e.g. R87 module 9R(#6, 9:7# with;A%66%


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Electronic Theodolite:

Theodolites or transits are used to measure hori?ontal angles. These have evolved asfollows"

#. =ernier theodolite /open face and =ernier euipped instruments0

%. 3ptical theodolite /enclosed with optical readouts with direct digital readouts ormicrometer euipped readouts0

'. 8lectronic theodolites /enclosed with electronic readouts0

8lectronic theodolites operate like any optical theodolite with one ma>or difference thatthese instruments have only one motion /upper0 and hence have only one hori?ontal clamp andslow motion screws.

Characteristics of electronic theodolites:

#. Angle least count can be #@ with precision ranging from 6.*@ to %6@

%. Digital readouts eliminate the personal error associated with reading and interpolation ofscale and micrometer settings.

'. Display window


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#. (agnification" %+ to '6

%. $ield of view /$3=0 #.*6.'. 1hortest viewing distance #.6 m). Angle readouts, direct *@ to %6@*. 5evel sensitivity" plate level vial )6@


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8D(!s were first introduced in #*6Cs by 9eodimeter !nc. 8arly instruments were largeheavy, complicated and e4pensive. !mprovements in electronics have given lighter, simpler, andless e4pensive instruments. 8D(!s can be manufactured for use with theodolites /both digital andoptical0 or as an independent unit. These can be mounted on standard units or theodolites or canalso be tribrach mounted.

The electronic methods depend on the value of velocity of 8lectromagnetic radiation/8(R0, which itself is dependent upon measurement of distance and time. ence, there is no

inherent improvement in absolute accuracy by these methods. The advantage is mainly functional- precise linear measurement can now be used for longer base lines, field operations can besimplified and trilateration can replace or augment triangulation.

Principle of EDMI

The general principle involves sending a modulated 8lectro-magnetic /8(0 beam from onetransmitter at the master station to a reflector at the remote station and receiving it back at themaster station. The instrument measures slope distance between transmitter and receiver bymodulating the continuous carrier wave at different freuencies, and then measuring the phase

difference at the master station between the outgoing and the incoming signals. This establishesthe following relationship for a double distance /%D0"

Where m is unknown integer number of complete wavelengths contained within doubledistance, FG is the measured phase difference and H is modulation wavelength, and kis constant.(ultiple modulation freuencies are used to evaluate m , the ambiguity.

=arious 8D(!s in use are based on two methods"

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&sing timed pulse techniues such as those used in variety of radar instruments.

&sing measurements of a phase difference which may be euated to one part of a cycle

e4pressed in units of time or length.

:ulse methods have advantages over the phase difference methods but their weight andpower reuirement is such that they cannot be classed lightweight portable instruments.

(i) Pulse techniues

All such measurements incorporate a very precise measurement of time usually e4pressedin units of nanoseconds /#4#6 -s0, which a 8( wave takes to travel from one station to another!n this method, a short, intensive pulse radiation is transmitted to a reflector target, which isimmediately transmitted back to the receiver. As shown in $igure #.), the distance /D0 iscomputed as the velocity of light /=0 multiplied by half the time /It


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Classification of EDMI

8D(! can be classified on the basis of three parameters "/i0 wavelength used

/ii0 working range

/iii0 achievable accuracy

Classification on the basis of wavelength

:resent generation 8D(!s use the following types of wavelengths"

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/a0 infrared

/b0 laser

/c0 microwaves

The first two types of systems are also known as electro-optical whereas the third category

is also called the electronic system.

8lectro-optical 1ystems

#. !nfrared 1ystems employing these freuencies allow use of optical cornereflectors /special type of reflectors to return the signal, e4plained later0 but need opticallyclear path between two stations. These systems use transmitter at one end of line and areflecting prism or target at the other end.

2. 5aser: These systems also use transmitter at one end of line and may or may not use areflecting prism or target at the other end. owever, the reflector-less laser instruments areused for short distances /#66 m to '*6 m0. These use light reflected off the feature to be

measured /say a wall0.

8lectronic 1ystem

icrowave

These systems have receiver


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8D(!s are also available as"

#. long range radio wave euipment for ranges up to #66 km

%. medium range microwave euipment with freuency modulation for ranges up to %* km'. short range electro-optical euipment using amplitude modulated infra-red or visible light

for ranges up to * km

Classification on the basis of accuracy

Accuracy of 8D(! is generally stated in terms of constant instruments error and measuringerror proportional to the distance being measured" B /a mm L ! ppm0.

#. The first part in this e4pression indicates a constant instrument error that isindependent of the length of the line measured.

%. The second component is the distance related error.

ere, a is a result of errors in phase measurements /M0 and ?ero error /"0whereas ! results from error in modulation freuency /f0 and the group refractive inde4 /ng0. Theterm group inde4 pertains to the refractive inde4 for a combination of waves- carrier wave andmultiple modulated waves in 8D(!. M and " are independent of distance but fand ngarefunctions of distance and are e4pressed as

!n above euations, N indicates the standard error. (ost 8D(! have accuracy levels fromB /' mm L # ppm0 to B /#6 mm L #6 ppm0. $or short distances, part a is more significantG for longdistances ! will have large contribution.

!elected electronic distance measuring instruments

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("nderson and Mi#hail$ %&&')

Instrument Manufacturer Emission

sourceange (m)

(!Ingle Prism)"ccuracy

(mean suare error)

!hort ange

D!#66# 5eica !nfrared #-66 B /* mm L * ppm0

R8D ( ini % sokkia !nfrared 66 B /* mm L ' ppm0

D(-! Topcan !nfrared 6.#*-66 B /# mm L % ppm0

D(-A* Topcan !nfrared 6.#*-66 B /* mm L ' ppm0

;D %6


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Total !tation:

This is an electronic instrument. !n this instrument, all the parameters reuired to beobserved during surveying can be obtained. The value of observation gets displayed in a viewingpanel. The precision of this type of instrument varies in the order of 6.#@ to #6@.

These instruments can record hori?ontal and vertical angles together with slope distanceand can be considered as combined 8D( plus electronic theodolite. The microprocessorin T1 can perform various mathematical operations such as averaging, multiple angle anddistance measurements, hori?ontal and vertical distances, , Q, P coordinates, distancebetween observed points and corrections for atmospheric and instrumental corrections.

Due to the versatility and the lower cost of electronic components, future field instruments

will be more like total stations that measure angle and distance simultaneously having"o all capabilities of theodolites

o electronic recording of hori?ontal and vertical angles

o storage capabilities of all relevant measurements /spatial and non-spatial attribute

data0 for manipulation with computer. ;owadays surveying systems are available which can be use in an integrated manner with

9lobal :ositioning 1ystem /9:10. ence, future theodolites


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instrument or directional for longer distances. During stakeout, the control unit is used tomove to point of interest. !t improves the time efficiency by up to 6.

!alient features of modern T!

T1 is a fully integrated euipment that captures all the spatial data necessary for a three-dimensional position fi4. The angles and distances are displayed on a digital readout and can be

recorded at the press of a button. =arious components of a typical T1 are shown in $igure '.'and are described below"

A typical T1 has the following characteristics"

9raphic display" All commands for survey operation as well as results are displayed on

graphic 57D using alphanumeric keyboard. &sing built in software with menu and editfacilities, they automatically reduce angular and linear observations to three dimensionacoordinates of the vector observed. Detachable control units are available on particular

instruments. Dual a4is compensation " The dual a4is tilt sensor monitors any inclination of the standing

a4is in both - and Q-directions. These tilt sensors generally have range of'* 7onseuently hori?ontal and vertical angle readings are free from error due to anydeviation of the standing a4is from the perpendicular /$igure '.)0. The hori?ontal andvertical angles are automatically corrected, thus permitting single-face observationswithout loss of accuracy.

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5eveling and centering" A few T1 have electronic display for leveling operation enablingrapid and precise leveling. The electronic leveling also eliminates errors caused by direct sunlighton plate bubbles. 5aser plummet are replacing the optical plummet. A clearly visible laser dot ispro>ected on to the ground that helps in uick and convenient centering of the instrument.

1torage " (ost T1 have on-board storage of records using :7(7!A memory cards of differentcapacity. The card memory unit can be connected to any e4ternal computer or to a special cardreader for data transfer /$igure '.*0. The observations can also be downloaded directly intointelligent electronic data loggers. 2oth systems can be used in reverse to load information intothe instruments. 1ome instruments andectivelens and enables the target operator to maintain alignment when setting-out points. This systememits two visible beams of coherent red light, one steady and one blinking, enabling the rod-manto locate the correct line uickly and easily by finding the position where both are visible /$igure'.+0. This light changes colour when the operator moves off-line. With the instrument in thetracking mode, taking measurements every 6.' s, the guide light speeds up the setting-out

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process. !t can also be used as a convenient signal to the rod-man, assists in one-man clearing oflines and work as a prism illuminator in night surveying.

(easurement modes " =ariety of measurement modes are available with T1 such

asprecise, acc%rate, and fast tracking, etc. These modes are result of a combination ofaccuracy and speed. Depending up on accuracy levels reuired and measurement times, thesurveyor can choose an appropriate measurement mode.

Automatic target recognition /ATR0" This facility ensures that the instrument will lock on to theactive target /by using R(T" remote measurement target0. The instrument receives codedsignal by !R diode on the R(T. !n this mode, the instrument automatically follows the reflectorafter the first measurement. The telescope is pointed in the general direction of the target, andthe ATR module completes the fine pointing with e4cellent precision and minimum measuringtime as there is no need to focus. !t can also be used on a moving reflector. A single key touchrecords all data without interrupting the tracking process. 3mni-direction /'+6o0 prismsreflector are used for short distances which are always aligned automatically ensuring highaccuracy /$igure '.0. $or longer distances directional active targets are available. The ATRmode also allows operation in darkness.

Reflectorless or direct refle4 measurement" Distance measurement without prism is alsoavailable on many instruments, typically using two different coa4ial red laser systems. 3nelaser is invisible and is used to measure long distances /+ km to a single reflector0, the other isvisible, does not reuire a reflector, and has a limited range of about %66 m. A single keystroke allows one to alternate between the visible or invisible laser. With Trimble *+66 DR%66L, distances of up to *66 m have been recorded /$igure '.0. The reflector-lessmeasurements are useful for surveying the facades of buildings, tunnel profiling , coolingtower profiling, bridge components, and dam faces - indeed any situation which is difficult orimpossible to access directly. The e4tremely narrow laser used clearly defines the targetpoints.

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Remote control systems " This arrangement allows truly one-person surveying capability. !t isparticularly useful for mass point surveys, cadastral surveys, staking out and machineguidance. 7ontrol of operation is transferred to the surveyor at the survey point where alfunctions can be called up. The unit generally employs a radio communication between T1and the prism. The control unit, battery, antenna and radio modem are integrated to allow fullcontrol over instrument and its operation. $igure '. shows a Trimble system with one radio

antenna for communication with R(T.

$ield techniues with T1"

=arious field operations in T1 are in the form of wide variety of programs integrated withmicroprocessor and implemented with the help of data collector. All these programs need that theinstrument station and at least one reference station be identified so that all subseuent stationscan be identified in terms of /, Q, P0. Typical programs include the following functions"

:oint location

1lope reduction

(issing line measurement /(5(0

Resection

A?imuth calculation

Remote distance and elevation measurement

3ffset measurements

5ayout or setting out operation

Area computation

Tracking

1takeout

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+P! signal structure:

1atellites have highly precise oscillators with a fundamental freuency of #6.%' (?. 1atellitesignals basically consists of ' components /$igure +.%0"

#. Two micro wave 5-band /also called 7arrier0 waveso 5# carrier" #**.)% (?

o 5% carrier" #%%.+6 (?

%. Ranging codes modulated on the carrier waveso 7


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$or purposes of imposing these binary data onto the carriers, all of the codes aretransferred from the 6 and # states to the -# and # status respectively. $igure +.' shows how bothcode and carrier are combined in 5# signal /1ickle, %66#0. The techniue is called binary bi-phasemodulation /alternatively binary phase shift keying 2:1O0. $or more details on signal structureone can refer to 1eeber /%66'0, 1ickle /%66#0 and :arkinson et al. /#+0.

+P! Carrier /aves

The two carrier waves 5# and 5% are pure right handed circularly polari?ed sinusoidalwaves. Two freuencies are useful to eliminate ionospheric effects /discussed later in 5ecture#60. $igure +.' provides a schematic diagram for carriers and codes for 9:1 signal.

+P! Codes

(a) C0" Code !t is a binary seuence of information. !t is also called pseudo random noise /:R;0 code

/states of 6 and #0 consisting of #,6%' elements, or chipsor bits, that repeats itself everymillisecond giving rise to a chipping rate /the rate at which each chip is modulated onto thecarrier0 of #.6%' (bps /mega bits per second0. The term pseudo random indicates that thecode is apparently random although it has been generated by means of a known process,providing the repeatability.

:R; codes allow range measurements, accesses to underlying carrier signals, satellite

message, and time markers. The chip length /distance corresponding to one chip or bit0 corresponds to %' m in length.

Due to the code length, the ambiguity in measurement with 7


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;avigational message includes information on

o 1atellite time of transmission

o :recise satellite position /ephemeris0

o 1atellite health

o 1atellite clock correction

o :ropagation delay effects /due to signal propagation in ionosphere andtroposphere0

o

Time transfer to &T7 /7oordinated &niversal Time0o 9:1 satellite 7onstellation status

5ength of navigation message is #*66 bits which is modulated onto both 5# and 5%

carriers. (essage takes about '6 seconds, each second contains *6 bits. Data is modulated at a much slower rate of *6 bps and thus it takes #%.* minutes to

transmit all of the information. !n order to reduce the time it takes to obtain an initialposition, the ephemeris and clock data is repeated every '6 seconds.

8ach satellite sends /:arkinson and 1pilker, #+0"

o a full and precise description of its own orbit and clock data /within the ephemeris

information0 and an appro4imate guide to the orbits of other satellites /almanacinformation0.

o parameters representing the delay caused by signal propagation through theionosphere /called the ionospheric propagation delay parameters0.

Accuracy of some aspects included in navigation message deteriorate with time which areupdated by certain renewal mechanisms from the ground monitoring stations.

Contents of su12frames

;avigational message contains * sub-frames, each of #6 words with each word of '6 bits/$igure +.)0.

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,igure -*3 ,rame and su1frames in 4avigational message (Pa5ares et al*$ .663)

8ach sub-frame contains a telemetry /T5(0 and handover /3W0 word which is spaced +

seconds uniformly and contains system time. 3W is part of 9:1 message containing timesynchroni?ation information for transfer from 7


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+eneration of +P! signals

The structure of 9:1 carrier signals and codes is uite comple4 in order to satisfy the severareuirements as given below"

(ulti-user system"

o 9:1 is used for one-way measurements /a listen-only system0

Real-time positioning"

o 1ince there is simultaneous measurements to many satellites, there is a need to

identify different signalso &nambiguous range measurements - need to determine signal delay

o

1atellite positions needed hence one needs broadcast ephemerides igh accuracy positioning"

o (icrowave carrier freuency - #.% to #.+ 9?G use of dual-freuency to minimi?eionospheric delay

o igh freuency modulation

o Anti->amming reuirement" 9:1 uses a special techniue called the 1pread

spectrum techniue for this purpose (ilitary and civilian users"

9:1 needs two different codes and restriction on dual-freuency use in order to providedifferential access to civilian and military


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modulation /alternatively binary phase shift keying 2:1O0. $or more details on signal structureone can refer to 1eeber /%66'0, 1ickle /%66#0 and :arkinson and 1pilker /#+0.

,igure -*- Code modulation of the L% carrier (!ic#le$ .66%)

The structure of 5# and 5% signals is given below where epochs /times0 of both codes andcarriers are synchroni?ed /1eeber, %66'0"

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1ince the codes /:R; or pseudo random noise codes0 and message are binary data

streams, only two states of phase modulations are possible. The state L# or -# leaves thecarrier unchangedG a code transition from L# to -# or from -# to L# involves a phase shift of#6 degrees.

The 5# channel has to carry both : and 7


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,igure -*7 +eneration of +P! signal (Pa5ares et al*$ .6638 !ee1er$ .669)

+P! !ervices:

Two types of services are available"

1:1 /1tandard :ositioning 1ervice0

:ositioning accuracy that is provided by 9:1 measurements based on the single 5#freuency 7

Unit 1 Adv Sur

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Transcript of Unit 1 Adv Sur