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Modern SurveyingEquipmentUNIT 5 MODERN SURVEYING EQUIPMENT

Structure

5.1 Introduction

Objectives

5.2 Micro-optic and Electronic Theodolites

5.2.1 Micro-optic Theodolites

5.2.2 Electronic Theodolites

5.2.3 Working of Micro-optic and Electronic Theodolites

5.3 Electronic Distance Measurement (EDM)

5.3.1 Principle of EDM

5.3.2 Working of EDM

5.3.3 Accuracy Considerations

5.4

Total Station5.4.1 Concept of Total Station

5.4.2 Working of Total Station


5.5 Automatic Levels

5.6 Global Positioning System (GPS)

5.6.1 Navstar GPS

5.6.2 GPS Equipment

5.6.3 Principle of GPS

5.6.4

Surveying with GPS5.6.5 Accuracy Considerations

5.7 Summary

5.8 Answers to SAQs

5.1 INTRODUCTION

The measurement of angles and distances is the focus of all land surveying jobs.In your earlier courses, you have been introduced to the use of a number of field

equipment for a variety of surveying works such as control establishment, routesurveying, construction and mapping surveys. Over the years, due to theadvancement in electronics and computer technologies, a range of electronicequipment have been developed in the field of surveying and levelling. With theintroduction of these equipment, not only the efficiency of the work has increasedbut the jobs can now be performed with more precision and accuracy within muchlesser time than before. Further, with the inclusion of data recording facilities inthese equipment, a large amount of data can be stored in proper format which canthen be analysed with the computer. Some of the modern equipment areElectronic Distance Measuring (EDM) equipment, Optical and ElectronicTheodolites, Auto and Digital Levels, Total Stations and Global PositioningSystem (GPS). These equipment can provide accurate data in no time that can berecorded in suitable media which can then be connected to a computer to generatequality map products.

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Advanced Survey In this unit, an introduction to some important modern surveying equipment andtheir use has been explained. The first section deals with the angle measuringequipment such as micro-optic and electronic theodolites. In the next section, theEDM has been discussed. This is followed by a discussion on electronic and autolevels. The penultimate section provides details on the Total Station that can beused for angle, distance and height measurements in one go. In the last section, an

introduction to the latest technology, namely GPS, has been provided.

Objectives

After studying this unit, you should be able to

get an overview of some commonly used modern surveyingequipment and their uses, and

understand the working of these new generation equipment for fieldsurveying jobs.

5.2 MICRO-OPTIC AND ELECTRONICTHEODOLITES

As you know that the survey field measurements include distances (horizontaland sloping) and angles (horizontal and vertical) measurement. The latter can bemeasured with a transit, or theodolite. You have already studied the use of verniertheodolites that are designed to read angles to the closest minute, 20 seconds or10 seconds. Over the years, the vernier theodolites have been in practice forconducting surveys of ordinary precision. For very precise surveys, these havebeen superseded by modern theodolites. The modern theodolites can becategorised as micro-optic and electronic theodolites. Unlike vernier theodolite,the observations are taken through an auxillary eyepiece (i.e. through optics) inthe micro-optic theodolites and hence the name. In electronic theodolites, theobservations are taken from the visual displays. These can read, record, displayand store horizontal and vertical angles in the electronic recorder attached tothem.

5.2.1 Micro-optic Theodolites

The design of these instruments is such that these become compact andlight-weight. These are generally characterised by a three-foot screw levellinghead and an optical plummet. There is a circular level for approximate levelling

and a plate level for precise levelling. Optical plummet is provided for accuratecentering particularly in windy climatic conditions. The plummet consists of asmall eyepiece generally built into the tribach. The graduations are marked onhorizontal and vertical circles made up of glass. The observations are readthrough an optical reading system that consists of a series of prisms. The verticalcircle is normally graduated such that 0o corresponds to the telescope pointingupwards towards the zenith. The graduations increase clockwise with 90o and270o marked on the horizontal line and 180o on the vertical line pointingdownwards towards the nadir. The glass circles are read with the aid of an

eyepiece adjacent to the telescope. The angles can be read to a least count of 1.

Many manufacturers have developed a variety of micro-optic theodolites eachhaving a particular optical system such as circle microscope system, optical scalesystem, single reading optical micrometer and double reading optical micrometeretc.

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Modern SurveyingEquipment

A list showing the performance of some of the direction measuring equipment isgiven in Table 5.1.

Table 5.1 : Some Micro-optic Theodolites for Angle Measurement

Least CountSl.No.

Name of Instrument Make

Direct

(Seconds)

Estimation

(Seconds)1. T2 (Universal) Leica, Switzerland 1.0 0.5

2. T3 (Precision) Leica, Switzerland 0.2 0.1

3. T4 (Astronomy) Leica, Switzerland 0.1 0.05

4. Theo 010 Zeiss, Germany 1.0 0.1

Wild T3 theodolite is used for geodetic triangulation and all other precise surveyswhereas Wild T4 theodolite is commonly used for astronomical determination ofco-ordinates and azimuth. Wild T2 and Zeiss Theo 010 are commonly used forengineering surveys.

5.2.2 Electronic TheodolitesA major change in the design of theodolites has occurred in recent years with theintroduction of electronic circle reading systems to their design. The electronictheodolites are similar to micro-optic theodolites in their design and operation.However, the difference lies in the system of taking reading. Here, theobservations are taken through digital readouts or displays. The commonly useddisplays are Light-Emitting Diodes (LED) and Liquid Crystal Displays (LCD).The direct display of angular readings eliminates the guessing and interpolationsassociated with the vernier scale and micrometer readings in other theodolites.

The angles can be measured to a least count of 1 with precision ranging from

0.5 to 10. One of the significant characteristics of these theodolites is that thedata can be recorded in a data collector attached with the theodolite. The data canthen be processed in a computer for subsequent analyses. The theodolites have azero set button for initial setting of the readings. Once attached with EDM, it canthen be used as a Total Station (to be discussed in Section 5.4 of this unit). Atypical electronic theodolite is shown in Figure 5.1.

Figure 5.1 : An Electronic Theodolite with Data Collector

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Advanced Survey 5.2.3 Working of Micro-optic and Electronic Theodolites

The working of these theodolites is more or less similar to that of a verniertheodolite. The major difference is in the centering procedure, which is throughoptical plummet instead of the conventional plumb bob centering. For an easy andquick set up, following steps may be followed :

(a) Place the instrument over the point with the tripod plate as horizontalas possible.

(b) From a distance of 1 to 2 meter, check if the instrument appears to beset over the station. If not, adjust the location and check again. Movein the direction 90o to the original setting and repeat the steps.

(c) Through the optical plummet, look the station mark and then firmlypush in the tripod legs into the ground.

(d) Manipulate the levelling screws while simultaneously looking throughthe optical plummet until its cross hair is exactly over the station mark.

(e) Level the theodolite with the circular bubble in the usual fashion.

(f) Look into the optical plummet to confirm that its cross hair is quiteclose to the station mark.

(g) The circular bubble can now be brought into centre by turning one ormore levelling screws.

(h) The tripod clamp is now loosened to slide the instrument on the flattripod top till the optical plummet cross hair is exactly centered overthe station mark.

(i) The instrument can now be precisely levelled using longitudinalbubble in the usual fashion as we do in vernier theodolite.

(j) Start measuring the horizontal and vertical angles.

The instrument can be used for various surveying operations such as laying offangles, prolonging a straight line, balancing in, intersection of two lines etc.

5.3 ELECTRONIC DISTANCE MEASUREMENT(EDM)

For providing precise horizontal control using trilateration (Section 6.3.1 of this

block), it is necessary that the distances be measured as accurately as possible.The advent of EDM has made this possible. The EDM was first introduced in thelate 1950. Since then, many refinements to these equipment have been made. Theearlier EDMs were very big, heavy and expensive. With the advancements inelectronic and computer technologies, these have become smaller, simpler andless expensive. The EDMs come in two parts : the instrument and the reflector.

The Instrument

The EDMs are generally of two types : electro-optical systems andelectronic systems. The electro-optical systems use either light and laserwaves or infrared waves whereas electronic systems use microwaves. Themicrowave systems require transmitter/receiver at both ends of the line tobe measured. The infrared system requires a transmitter at one end and areflector at the other end. The microwave systems are capable of measuringdistances up to a limit of 100 kms whereas the infrared EDMs come in three

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different ranges, long range (10-20 km), medium range (3-10 km) and shortrange (0.5-3 km) equipment. A typical EDM is shown in Figure 5.2(a).

The Reflector

The reflector is usually a prism or a set of prisms (Figure 5.2(b)). Generally,a cube corner prism is used that has the characteristic of reflecting light raysprecisely back in the same direction as they are received. This means that

even if the prism is somewhat misaligned with respect to the EDM, it canstill be effective. These prisms can be mounted on a tripod or a pole heldvertical on the point. For higher accuracy, the prisms should be mounted ona tripod. The height of the prism is normally set equal to the height of theinstrument.

(a) An EDM Fitted on a (b) A Set of Reflectors UsedDigital Theodolite with EDM or Total Station

Figure 5.2

Recently, some EDMs have been introduced that can measure the distanceswithout reflectors. In these situations, the surface itself behaves as areflector. However, the EDMs without reflectors can only be used for themeasurement of shorter distances within 1 km and also with reducedaccuracy.

The EDM when mounted on a precise theodolite can be used to determineboth slope and vertical distances. This arrangement has given rise to

another category of surveying instrument known as Total Station or FieldStation.

5.3.1 Principle of EDM

The EDM systems are based on the principle of distance travelled between thetransmitted wave from one end and its reception at the other end. Thus, the basicrelationship between time, speed and distance is applied. The instrumenttransmitting the infrared or microwaves is kept at one end whereas the reflector iskept at the other end. The instrument sends the waves, which are reflected by thereflector to be received by the instrument. Figure 5.3 shows a wave of wavelength

travelling along thex-axis with a velocityv. The relationship betweenwavelength (), frequency (f) and velocity (v) can be given as,

v

f = . . . (5.1)

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R

2

E

1 3

n n-1

L

S Station

Z Target

Modulationwave length

Fraction to bemeasured of awhole wave lengthof modulation ( )

E Reference plane within thedistance meter for phase comparisonbetween transmitted and received wave

R Reference plane for the reflection of thewave transmitted by the distance meter

Advanced Survey

Figure 5.3 : Principle of EDM Measurement(Source : K avanagh and Bird, 1996)

In Figure 5.3, a modulated wave transmitted by the instrument and its reflectionback to it is shown. It can be seen that the double distance 2L can be determinedby knowing the total number of wavelengths plus the fraction of wavelengthreaching the EDM. Thus,

( )

2

nL

+ = . . . (5.2)

The fraction wavelength can be determined in the instrument by noting thephase delay required to precisely match the transmitted and reflected waves.The instruments are designed to determine the number of wavelengths (n) withinseconds and compute the distance in no time.

Corrections

Since the wave travels through the atmosphere, the velocity of the wavemay be affected by temperature, pressure and water vapour content.Therefore, the appropriate corrections for these must be applied. Normally,the provision for these corrections is made in the instruments themselves bysupplying the required values of the prevailing atmospheric quantities onthe day of measurement.

Alternatively, these corrections can be applied manually by looking at thecharts and graphs (showing the relationships between the quantities and thecorrections) provided by the manufacturers of the instrument. It may,however, be mentioned that the effect of atmosphere is more pronounced inlong distances of the order of kilometers. For short distances, less than akilometer, the atmospheric corrections are less significant and may not berequired.

5.3.2 Working of EDM

Before using EDM in the field, these are normally checked for their accuracy and

proper adjustment. EDM instruments are calibrated against the known distances.The zero error (distances between electronic and physical centre), if any, isdetermined. This activity requires several measurements on different knownlengths.

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Once zero error is found out, the measurements can be taken. The typicaloperation of any EDM involves four basic steps of setting up, bisection,observing and recording.

Setting Up

The EDM instrument is first inserted into the tribrach on the tripod, whichis centered exactly over the station mark through optical plummet. Reflector

is set over the other point of the line whose distance is to be measured. Thepower of the instrument is turned on and certain initial checks are made.For example, to examine proper working of the battery and the display.

Bisection

The instrument is unclamped to bisect the reflector through the built-insighting device. There are horizontal and vertical tangent motion screws forexact bisection of the reflector.

Observing

The distances are measured by simply pressing the measurement key andwaiting for a few seconds. The result appears on the LCD panels. If there isno display, the user should check the previous steps. Repeatedmeasurements are often made to observe the distances with more precisionby pressing the repeat mode key. Some of the corrections normally appliedon the distances measured by EDM instruments are atmospheric and zeroerror correction, slope to horizontal distance conversion etc. Since themeasurements obtained are slope distances, some EDM have built-incalculators to compute horizontal and vertical distances if the verticalangles are fed manually through the keypad.

Recording

These days, all the EDMs are supplied with an electronic field bookwherein the measurements can be recorded directly or by manual entry. Theobservations must be accompanied with all relevant atmospheric andinstrumental correction data.


In general, the accuracy of an EDM is expressed in terms of a constantinstrumental error and a measuring error proportional to the distance being

measured. Thus, an accuracy value of (5 mm +5 parts per million (ppm))signifies that 5 mm is the constant instrument error (independent of the length ofthe measurement), whereas the 5 ppm (5 mm/km) represents the distance related

error. For example, if the distance to be measured is 10 km then the total error inthe measurement shall be 5mm +(5 10) mm which works out to be 55 mm or5.5 cm. This is equivalent to an accuracy of 55 in 1,00,00,000 (or 1 in 181818).

Now-a-days, EDM equipment are being manufactured by various companiesthroughout the world. The specifications of these vary in terms of the distancerange and accuracy. A list of some EDMs manufactured by Leica Geosystems(earlier Wild) with their salient features is given in Table 5.2.

Table 5.2 : Some Models of EDM

Sl. No. Name Distance Range Accuracy

1. DI1001 800 m with 1 prism (5 mm +5 ppm)



4. DI300S 19 km (3 mm +1 ppm)

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The criterion for the selection of an EDM depends upon the range and accuracyachievable. The instrument capable of taking measurements to an error of 1 to2 ppm is the best suited for geodetic control establishment. For civil engineeringworks, where accuracy requirement may not be high, short range EDM with5 ppm error can be used.

Advanced Survey

5.4 TOTAL STATION

In the previous sections, you have been introduced to electronic theodolites andEDMs. When these instruments are combined into one assembly, these give riseto another category of surveying instruments known as Electronic Tacheometers.These are also referred to with other names such as Total Stations and FieldStations.

5.4.1 Concept of Total Station

The basic idea behind the development of Total Station is the fact that theequipment can be used to perform all surveying operations in one go from astation (or point) and hence the name. Thus, a total station is an equipment thatcan electronically measure both angles and distances and perform limitedcomputational tasks using an internal micro-processor such as reduction of slopeto horizontal distance, computations of coordinates from a bearing and distanceetc. Often, these are provided with built-in facility for atmospheric andinstrumental corrections. The data are recorded by the instrument in internalmemory or in external memory cards. The advantage with these cards is that thesecan be directly inserted into the computer for easy data transfer. Moreover, thesecards come in different memory sizes and, thus, the data for many days andmonths can be recorded.

There are two basic designs of a Total Station : integrated design; and modulardesign. In integrated design (Figure 5.4), both the electronic theodolite and theEDM are assembled in a single unit, whereas in the modular design these act asseparate units. The latter arrangement is more flexible, since the theodolites andEDM units with varying precision can be combined to form a suitable design asper the requirement of the project.

One important feature of any total station is the provision of data recorder orcollector in it. A data recorder is basically a hand-held computer. It can record allthe measurements in suitable format and can perform some basic computationssuch as figure closures and adjustments. Also, many total stations can record all

measurements (i.e., slope distance, horizontal and vertical angles) of a point byjust pressing a button. The point number and its description may also be recorded.

Figure 5.4 : A Total Station (Courtesy: Elcome Technologies Pvt. L td., New Delhi)

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5.4.2 Working of Total Station

There are many surveying tasks where Total Station can be used effectively.These include preliminary control and construction surveys etc. However, thesehave mostly been used for topographic surveys where the three coordinates of apoint (i.e., Northings, Eastings and Heights above msl) are required. Typical stepsin the operation of a Total Station for a traverse computation can be listed as

below.

Entry of Initial Data

After switching on the equipment, at first instance, some initial data are fedto it through the controller. These data include the description of theproject, date and survey team, atmospheric pressure and temperature values,prism constant, sea level, curvature and refraction corrections, choice ofmeasurement units etc. It is likely that you may bypass feeding of certaindata as the default values may themselves be sufficient.

Entry of Traverse Station (Occupied Point) and Feature (Sighted Point) Code

All the traverse stations and features to be plotted must be given a suitablecoding system for their recognition. The coding system varies from onemodel of Total Station to the other. These codes may be entered through thekeypad on most of the equipment. Some models now have the provision ofbar codes to enter the codes. For the traverse station, in addition to thestation codes, the data such as height of instrument, station name andnumber, coordinates of traverse station (forward and backward), azimuth ofreference line etc. may also be entered. Similarly, for the sighted point,besides its code, the other data to be supplied are height of prism orreflector, point name and number etc.

Measurement of Angles and Distances

After entering the required data, an observer may start taking measurementsusing the following steps (refer Figure 5.5) :

(a) Centre the Total Station over the traverse station 11.

(b) Sight at station 14, zero the horizontal circle.

Control Traverse

Road

Instrument Station

(IS) 104

(IS) 103

(IS) 102

(IS) 10111

13

12 (FS)

14 (BS)

Stations 101 to 104 are Control Monuments

Figure 5.5 : Sketch Showing Intermediate Road Ties to a Control Traverse

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Advanced Survey (c) Enter code of sighted station 14.

(d) Measure and enter the height of prism/reflector.

(e) Press appropriate measure key as there may be different keys fordifferent measurements such as horizontal and vertical angles,horizontal and vertical distances etc.

(f) Press record button.

(g) From this traverse station, any number of points signifying thetopographical features such as 101, 102, 103 are sighted andtheir measurements recorded. For doing this, the prism mountedon a pole has to be moved to the respective points.

(h) Once measurement and recording of all the points is completed,the Total Station is moved to the next traverse station (i.e., 12)and the procedure is repeated till all the stations are covered.

Transfer of Data and Its Processing

All the models of the Total Station are supplied with software forprocessing the data stored in the data collector or electronic field book. Theprocessing may require operations such as preliminary analysis,adjustments and coordinate computations. For example, to process the datafrom Leica models, the software LISCAD may be used. However, thesoftware supplied with other model may also be used to process the datacaptured by Leica model through some manipulations. For any dataprocessing, first the data have to be downloaded from the electronic fieldbook to computer where the software is installed. It is possible to connectthe field book directly to the computer through a cable. Otherwise, the datastored in the memory card of the field book can be inserted into appropriateslot in the computer for its transfer. The data transfer is followed by desiredprocessing operation for the computation of coordinates of points andfeatures.

Plotting of Details

After processing the field data in the desired form (i.e., the coordinates), thedata required for plotting may be assembled and the survey can be quicklyplotted at any scale on a printer or a plotter. The symbols necessary for

plotting different topographical features can be extracted from the symbollibrary provided in the software. Some software have the provision ofgenerating your own symbols, if these are not available in the software.


The accuracy of a Total Station is generally referred in terms of distancemeasuring accuracy and angle measurement accuracy. Since the distancemeasurement is through EDM, all the accuracy standards of EDMs apply to TotalStation. Similarly, all the accuracy standards of digital theodolites apply to theangle measurement accuracy of the Total Station. A number of Total Stations are

available in the market these days. Some of them (e.g., manufactured by Nikonand Leica) along with their accuracy standards are mentioned in Table 5.3.

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Table 5.3 : A List of Some Total Stations

Sl.No.

Name Distance Rangewith 1 Prism

Distance Accuracy AngularAccuracy

1. DTM850 (Nikon) 2400 m (2 mm +2 ppm) 1

2. DTM550 (Nikon) 2400 m (4 mm +2 ppm) 1

3. DTM310 (Nikon) 1000 m (5 mm +5 ppm) 5

4. TCA1101 (Leica) 1000 m (3 mm +1 ppm) 1.5

5. TC303 (Leica) 3000 m (2 mm +2 ppm) 3

6. TC905 (Leica) 2500 m (2 mm +2 ppm) 2

7. TCA2003 (Leica) 2500 m (1 mm +1 ppm) 0.5

SAQ 1

(a) Give the full form of abbreviations, EDM, LCD, LED, ppm.

(b) Differentiate between micro-optic and electronic theodolites.

(c) What is the function of an optical plummet?

(d) Describe the reading system of a typical micro-optic theodolite.

(e) Write down the steps required for setting up of an eletronic ormicro-optic theodolite.

(f) What are two different types of EDM?

(g) What is a reflector?

(h) On what principle the working of an EDM is based?

(i) Write four basic steps of working with an EDM.

(j) How will you signify the accuracy of an EDM?

(k) Define Total Station.

(l) What are the two basic designs of a total station? Explain thedifference.

(m) Describe the steps for the operation of an EDM.

5.5 AUTOMATIC LEVELS

Levelling is the process of determining the vertical position of different featuresbelow, on or above the surface of the earth. The vertical position is normallyreferred to as elevation (or height) above mean sea level (msl). The elevations canbe determined by direct and indirect means. In direct method, the elevations aredetermined by direct observations to measuring rods or staffs using an equipmentcalled level. You have already studied spirit levels (Dumpy and Tilting levels) inyour earlier courses. The focus here will be on the understanding of a newgeneration of levels known as Automatic levels. In indirect levelling, the

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elevations are determined indirectly by taking measurements of horizontal andvertical angles. The trigonometric levelling and tacheometric surveying are theexamples of indirect levelling. These two procedures are described in detail inUnits 1 and 2 of this Block.

Advanced Survey

The automatic levels differ from other forms of spirit levels in the sense that thesehave a compensating device that maintains the horizontal line of sight when theinstrument is approximately levelled (Figure 5.6). This increases the efficiency ofthe levelling work. In fact, automatic levels have become very popular these daysand are available from most of the surveying manufacturers. They are quick to setup and easy to use.

These levels are similar in design to any other level as these also have a threelevelling screws and a circular bubble. The instrument is quickly levelled usingthe circular bubble and these screws. After this, the compensator takes over andautomatically makes the line of sight horizontal even if the telescope is slightlytilted. Once the line of slight is horizontal, same levelling operations can beperformed as with any other level used in spirit levelling.

Figure 5.6 : An Auto Level (Courtesy: Elcome Technologies Pvt. L td., New Delhi)

These days, auto levels have arrangements for digital displays and data collectorsand are thus named as Digital levels. These are supplied with a bar coded staff.As soon as the staff is bisected, the readings are automatically recorded in thedata collector that can then be connected with a computer for data reduction andanalysis. These bar coded staves can read to a least count of 0.001 mm.

5.6 GLOBAL POSITIONING SYSTEM (GPS)

The GPS is an emerging technology in the field of geodesy, geography, surveyingand spatial analysis. In particular, the technology overcomes the limitations of theconventional field surveying methods, such as the requirement of intervisibility ofsurvey stations, dependability on weather, difficulties in night observations etc.Advantages over the conventional techniques, economy in operation and timemakes the GPS most promising surveying technique of the future.

5.6.1 Navstar GPS

The NAVSTAR (Navigational Satellite Timing and Ranging) GPS, developed byUnited States Department of Defense, is a satellite-based radio navigation systemthat can provide three-dimensional position and time information in one go. Thesystem can be successfully used for many civil engineering and other applicationssuch as

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(a) Provision of geodetic control.

(b) Alignment surveys.

(c) Large Scale Mapping.

(d) Navigation of ships and aircrafts.

(e) Crustal movement studies.

(f) Photogrammetry and Remote Sensing

GPS has three segments

(a) Space segment

(b) Control segment

(c) User segment

Space Segment

The space segment consists of 24 satellites and 5 additional satellites. These

satellites are placed in six orbital plane at a height of 26,200 km semi majoraxis. Each orbit is inclined at 55 degrees to the equator and each satellitecompletes one rotation in 12 hours of sidereal time. This provides a repeatsatellite configuration every day four minutes earlier in respect to universaltime.

All these satellites carry very precise atomic clock with an accuracy ranging

from 1 10 12 to 1 10 13 seconds. Each satellite transmits signals on twocarrier wave frequencies,L1 andL2, derived from fundamental frequency10.23 MHz.

L1 =154 10.23 MHz =1575.42 MHz (=19.05 cm)L2 =120 10.23 MHz =1227.60 MHz (=24.45 cm)

The GPS signals must provide some means to determine the position on realtime basis. To achieve this, the carrier phase is modulated with PseudoRandom Noise (PRN) codes. There are two types of codes in use, theP-code (Precision or Protected code) and the C/A code (Clear/Acquisitioncode).

Control Segment

There are five control stations around the globe that continuously track the

satellites and feed the information to the Master Control station atColorado, USA. At control stations, the pseudoranges (to be explainedlater) are determined to all the visible satellites. This information, alongwith local meteorological data, is sent to Master Control station. From thesedata, satellite ephemeris and the behaviour of the satellite clocks arecomputed which are then transmitted in the form of navigation (message)data to the ground antennas.

User Segment

A user segment consists of a GPS receiver with antenna and power supplyunit. A GPS receiver must have enough channels with low noise level tocollect data from all the available satellites. A minimum of eight channels isrecommended for the determination of accurate position. The antenna is oftwo types Chock Ring and Microstrip antenna.

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Advanced Survey 5.6.2 GPS Equipment

A complete GPS set has three major parts as given below :

(a) Hardware

(b) Software

(c) AccessoriesThe hardware part of a GPS unit consists of two components which are thereceiver and the antenna. The design of both receiver and antenna varies from onemanufacturer to other. For civilian uses, most GPS receivers are single frequency,C/A code receivers. These receivers have access to both code and satellitemessage. The satellite message contains the broadcast ephemeris, clockcorrection coefficients, and the age of ephemeris data. It also providesinformation about the health status of the satellites.

There are generally two software that are obtained with a GPS. These areinstrument specific software and scientific software. The former is supplied with

the GPS unit and is expected to perform the following jobs(a) To transfer data from GPS hardware to the computer.

(b) To provide baseline solutions.

(c) To perform datum transformation from WGS 84* to the desiredprojection.

(*World Geodetic System of 1984 is a 3-D, Earth-centered officialGPS reference system developed by US Defense Mapping agency.)

(d) To determine Geoid heights.

(e) Network adjustments.Some of the instrument specific software are Ski, GPS Survey etc. The instrumentspecific software are generally suitable for processing short baselines of the orderof 10 to 20 km. For processing of larger baselines of several hundred kilometers,it is necessary to consider all kinds of errors (to be discussed later) for obtaininghigh precision results. This requires complex mathematical algorithms. Therefore,many scientific software have been developed. Some of them are BERNESE,GAMIT etc. Besides performing the above mentioned jobs, these software areexpected to carry out all sorts of error modelling tasks and their adjustment.

In addition to the standard GPS equipment, some auxiliary equipment are also

necessary. These include

(a) Tripod and Tribrach

(b) Antenna Cable

(c) Field computer

(d) Spare batteries

5.6.3 Principle of GPS

The basic principle of GPS is to determine the position of points in three-dimensional space. The determination of position is based on measurement of

distances from the point of observations to the GPS satellite. The distances arecomputed by observing the travel time of the signals from the satellite to thepoint. The travel time has a systematic bias because the satellite and the receiverclocks are of different precisions. The satellite has atomic clock whereas the

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receiver has quartz clock. Thus, the computed distances (also referred to as range)shall be biased and, therefore, these are called pseudoranges. To compute theposition based on this pseudorange, the error due to time bias has to be corrected.It is because of this reason that time is also taken as unknown and determinedbefore deriving the true range. The range can be determined from

2 2( ) ( ) ( 2)R Xs X Ys Y Zs Z= + + . . . (5.3)

whereX,Y andZare the co-ordinates of the point, on the ground andXs,YsandZsdenote the position of the satellite broadcast by the Master Control station.

To find the true range, the time biasthas also to be considered. Thus,

2 2 2( ) ( ) ( )R Xs X Ys Y Zs Z tc= + + + . . . (5.4)

wherec is the velocity of light.

From Eq. (5.4), it can be seen that there are four unknowns (i.e.,X,Y, Zandt).Therefore, the data from at least 4 satellites have to be collected for the solution

of this equation.

An alternative way to determine the pseudorange is the phase measurementtechnique. The technique is based on a simple principle that if the wave length,the full number of cycles elapsed between signal travelling from satellite antennato receiver antenna and the length of part wavelength are known, then

Range =N+ . . .(5.5)

whereN is the number of full cycle of wave length and is the length of partwave.

The integer, number of cycles between the antenna and the satellite at the firstphase measurement is called ambiguity. The initial ambiguity has to bedetermined with appropriate techniques to exploit the full accuracy potential ofGPS carrier phase measurements. Ambiguity determination is one of the mostdemanding problems. Many ambiguity resolution algorithms are available that areimplemented in the software component of the GPS unit.

The position obtained by GPS is in the form of geographical coordinates(latitudes and longitudes) and in WGS 84 (World Geodetic System 84)coordinates. However, in India, the polyconic projection system on Everest

spheroid is used for all thegeodetic computations. Therefore, the coordinatesobtained from GPS need to be appropriately transformed to polyconic mapprojection system. This can be achieved by finding out transformation parametersfrom known position of at least three points in both WGS-84 and polyconicprojection system.

Moreover, heights or elevations of points obtained from WGS-84 are ellipsoidheights. The height measured in point positioning mode can have errors up to150 m. However, the ellipsoidal height difference can be measured with very highprecision. For using GPS for determining heights, one receiver is kept at a pointwhose ellipsoidal height is known very accurately. Now, assuming that Everest

and WGS-84 ellipsoids are parallel within a small region the heights of desiredpoints can be determined by adding/subtracting the observed difference betweenthe two ellipsoids. To get the orthometric height, a Geoidal separation correctionis added at each point.

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Advanced Survey 5.6.4 Surveying with GPS

There are two main ways in which position of a point can be determined usingGPS. These are point positioning and differential positioning. In pointpositioning, a single GPS receiver is kept at the point whose coordinates are to bedetermined. The receiver records the observations for many hours. These

observations are then processed as single-point mode using appropriate GPSsoftware. The accuracy achieved in point positioning mode is low (i.e., of theorder of meters) unless the data are post processed with a scientific software suchas Bernese. In differential positioning, minimum of two GPS receivers arerequired. One receiver (called the reference receiver) is kept at the reference pointwhose coordinates are known to a high accuracy from other surveys. The otherreceiver (called the rover receiver) is kept at the unknown point whosecoordinates are to be determined. The observations by both the receivers arecollected for a common period of time but for a drastically shorter period thanthat required in point positioning. The position of the unknown point isdetermined relative to the reference point by computing the length of the line

joining the two points by processing the observations in baseline mode. Theaccuracy, thus, achieved is of the order of centimeters and millimeters.

Thus, in point positioning, the accuracy of the coordinates is within 100 m.Differential positioning have no effect of Selective Availability (SA) (explainedlater) and coordinates of the station can be calculated on the basis of fixedstations coordinate system or any arbitrary system. Thus, accurate positioning ispossible after post processing the observed GPS data. Therefore, this technique isgenerally used in surveying operations. Normally, three differential positioningtechniques are used when observing GPS. These are (a) Static, (b) Rapid Static,and (c) Kinematic.

Static Positioning

In this technique, at least two receivers placed at two points collectcarrier-phase observations in static mode for a longer period of time. Thesoftware analyzes all data simultaneously to obtain the differential positionbetween two receivers. Since the long observation sessions allow a carefultreatment of systematic errors, static differential positioning yields moreaccurate results than any other technique. Therefore, this procedure is usedextensively for a variety of high precision surveys such as establishment ofcontrol networks and monitoring of earths crustal deformations. Typicaldistances between the receivers range from several tens of kilometers to

thousands of kilometers. Observation sessions of several hours may berequired to achieve high accuracy over such long distances.

The results of the observations taken in static mode are found to be accuratewithin 5 ppm or better, which are 3-4 times better than the results obtainedthrough other surveying methods. It is also true that GPS is moreeconomical and at least 3 times faster than the other methods.

Rapid Static Positioning

It is essentially similar to conventional static surveying but features a vastlyshortened point occupation time. The reduction in observation time

primarily results from faster ambiguity resolution which is achieved eitherby combining pseudorange measurement technology with carrier phasemeasurements or by making use of redundant carrier phase measurements.

Kinematic Positioning

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When surveying is conducted for a local area and, thus, all baseline lengthsare within several kilometers, then some of the systematic errors in carrierphase measurements will be negligible and will have no effect on thedifferential positioning result. In that case, one may resort to kinematicpositioning. A very reduced length of point occupancy is the mainadvantage of this technique. Kinematic positioning can be carried out intwo ways such as

(a) Pseudo Kinematic Surveying, and

(b) Stop and Go Surveying.

Thepseudo-kinematic methodcalls for one receiver to remain static at thereference point while other receiver occupies all remote points in sequence.At each point, the roving receiver collects measurements for a few minutes.After at least one hour, the whole procedure is repeated and all remotepoints are reoccupied. The procedure is useful when there are a largenumber of points so that waiting time between point reoccupations may beavoided. The data collected in the first and second occupancy are combined

in a processing scheme similar to the one used in static surveying.InStop and Go surveying(also referred as semi-kinematic surveying), thecarrier phase ambiguities are resolved before the actual survey begins. Oncethe ambiguities are resolved, surveyor moves one of the receivers throughall the remote points in sequence. In this method, surveyors can accuratelydetermine the differential position of remote points with observationperiods as small as few seconds. The limitation with this method is thatwhen roving receiver is moving between the remote points, it must maintainphase lock to at least four satellites for a successful survey. Accuracy atsub-centimeter level can be achieved with this method.

The relative performance of different observation techniques is given inTable 5.4. On comparison, it can be stated that static positioning demandsmore observation time resulting in fewer base line measurements, althoughwith greater accuracy. The truly kinematic positioning outputs the results ina preset time interval, resulting in greater turnouts and accurate positioningbut not at the required ground points. For large-scale surveying, we need atechnique that is in-between the static and kinematic. Therefore,pseudo-kinematic and Stop and Go techniques can be considered ideal forlarge scale surveying purposes. Pseudo kinematic can be usedadvantageously in areas where there is fear of signal shading due tovegetation, built areas, tall buildings and obstructions, as there is no

requirement for the receiver to maintain its lock to the satellite during themovement of rover receiver. But in open areas, Stop and Go technique mayprove useful.

Table 5.4 :Relative Performance of GPS Surveying Methods

Method Accuracy Remarks

Navigational Solution (i) 10 - 20 m

(ii) 100 - 200m

-----------------

SA and AS on

Static

(i) 1.00 ppm

(ii) 0.10 ppm

(iii) 0.01 ppm

Observations

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In GPS solutions, varying levels of accuracy are associated with different surveyand position techniques. The position is also effected by a system error known asSelective Availability (SA). Intentional degradation of quality of broadcastinformation is called Selective Availability. This has been introduced by USA todeny accurate positioning on real time basis. This can be done by deliberatelydegrading the stability of the satellite clock or by degrading the navigation

message, transmitted by satellites. In May 2000, the SA has, however, beenremoved and, therefore, higher levels of accuracy can be expected from GPS.

Advanced Survey

In Section 5.6.1, it was mentioned that there are two codes, i.e. C/A code andP-code (precision code). The P-code is available to certain selected group of usersand is not available to all. The denial of P-code by USA is known as Anti-Spoofing (AS). As per the announcement by Department of Defence, USA, theAS will remain on till the satellite constellation is complete.

Thus, from Table 5.4, it can be reckoned that the GPS can provide an accuracy of10 to 20 meter in point positioning mode provided neither SA nor AS is on.

However, such accuracy is not sufficient for geodetic purpose. Therefore, thesurveyors use the system in differential mode where most of the errors due to SA,Tropospheric and Ionospheric get cancelled out and the distance between twopoints with very high accuracy can be obtained instead of position. Thus, theequipment can be used for first order survey.

A precision term commonly used while collecting GPS observations is called theGeometric Dilution of Precision (GDOP). It is a measure of strength of figure ofthe satellites being observed for finding out the position. The tracked satellitesclustered at one place shall have large GDOP whereas well-distributed satellitesshall have small. Smaller the GDOP, greater are the chances of achieving

accurate position.

SAQ 2

(a) Give the full form of following abbreviations

NAVSTAR, GPS, SA, AS, GDOP, WGS-84.

(b) What do you mean by levelling?

(c) How do automatic levels differ from conventional spirit levels?

(d) Enumerate some applications of GPS.(e) What are three segments of a GPS?

(f) What do you mean by a P-code and C/A code?

(g) What is the purpose of control segment?

(h) Describe major parts of a GPS equipment.

(i) What should be the requirements of a GPS software?

(j) Give basic principle of a GPS.

(k) Define pseudo-range.

(l) Differentiate between

(i) pseudo-range and carrier phase measurements

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(ii) point and differential positioning

(iii) static and kinematic positioning

(m) Describe two methods of kinematic positioning.

(n) Define selective availability, anti-spoofing, geometric dillution ofprecision.

5.7 SUMMARY

Although a range of new surveying equipment has been developed by severalmanufacturers, the working principles of a given type of equipment remain thesame. In this unit, you were exposed to some modern surveying equipment and

their operation in the field. After reading this unit, you shall be able to handleelectronic surveying instruments of different makes supplemented with theiroperation manual.

5.8 ANSWERS TO SAQs

Refer the relevant preceding text in the unit or other useful books on the topiclisted in Further Reading given at the end to get the answers of SAQs.

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