Reduced levels of monitoring network stations Level of network...Reduced Levels (Monitoring) 1 1 Introduction This note focuses on the geographical co-ordinates and Reduced Levels

REDUCED LEVELS

OF

MONITORING NETWORK STATIONS

December 1998

Reduced Levels (Monitoring) i

REDUCED LEVELS OF MONITORING NETWORK STATIONS

Table of Contents

ABBREVIATIONS.................................................................................................................II

1 INTRODUCTION...........................................................................................................1

2 BACKGROUND .............................................................................................................2

3 SURVEYING NEEDS ....................................................................................................3

4 REQUIRED ACCURACY.............................................................................................4

4.1 CO-ORDINATE ACCURACY.........................................................................................44.2 LEVEL ACCURACY.....................................................................................................4

5 REFERENCE SURFACE FOR LEVELLING............................................................6

6 CONVENTIONAL SURVEYING TECHNIQUES.....................................................7

6.1 LEVELLING METHODS ...............................................................................................76.2 REFERENCE SURFACE................................................................................................76.3 ACCURACY ...............................................................................................................76.4 PRACTICAL ASPECTS .................................................................................................86.5 STAFF, DURATION AND COST ....................................................................................86.6 TRAINING................................................................................................................10

7 GLOBAL POSITIONING SYSTEM BASED SURVEYING TECHNIQUES.......11

7.1 LEVELLING TECHNIQUES.........................................................................................117.2 REFERENCE SURFACE..............................................................................................127.3 OVERALL ACCURACY..............................................................................................137.4 PRACTICAL ASPECTS ...............................................................................................137.5 STAFF, DURATION AND COST ..................................................................................147.6 TRAINING................................................................................................................16

8 COMPARISON OF SURVEYING TECHNIQUES..................................................17

8.1 AVAILABLE OPTIONS...............................................................................................178.2 COMPARISON OF TECHNIQUES.................................................................................17

9 CONCLUSIONS ...........................................................................................................20

10 RECOMMENDATIONS..............................................................................................21

MANUFACTURERS AND REPRESENTATIVES ...........................................................22

Reduced Levels (Monitoring) ii

List of tables

Table 1: Status of RL survey, as per MIS 30 September 1998 ................................................3Table 2: RL accuracy requirement per type of terrain .............................................................5Table 3: Basic reference point accuracy ..................................................................................6Table 4: Indicative instrument accuracy and range..................................................................8Table 5: Estimation of remaining work for optical instruments ..............................................9Table 6: Estimation of operational and instrument cost for optical instruments .....................9Table 7: Indicative accuracy and operational range...............................................................13Table 8: Estimation of remaining work for GPS instruments................................................15Table 9: Estimation of operational and instrument cost for GPS instruments.......................15Table 10: Tabular summary of total costs ...............................................................................18Table 11: RL survey costs and duration for digital level and total station..............................19Table 12: RL survey costs and duration for GPS L1 and GPS L1/L2 receivers .....................19Table 13: Known GPS equipment manufacturers represented in India...................................22

AbbreviationsCGWB Central Ground Water BoardE EastingEGM96 Earth Geopotential Model 1996, a spherical harmonic model of the Earth's

gravitational potentialGPS Global Positioning SystemGTS Great Triangular SurveyHIS Hydrological Information SystemHP Hydrology ProjectMIS Management Information SystemMSL Mean Sea LevelN NorthingNIMA National Imagery and Mapping Agency, USARL Reduced LevelSGWD State Ground Water BoardSoI Survey of IndiaUTM Universal Transverse MercatorVP Very high precision GTS benchmarkVVP Very very high precision GTS benchmarkWGS84 World Geodetic System 1984

Reduced Levels (Monitoring) 1

1 IntroductionThis note focuses on the geographical co-ordinates and Reduced Levels for observationwells. For proper assessment of groundwater flows and interactions between aquifers,groundwater elevation data are required. The Hydrological Information System covers a largenumber of wells. At present only half of the observation wells have been levelled, whichhampers a thorough analysis of the geo-hydrological conditions of the aquifers. There is needfor a speedy elimination of the backlog in levelling and surveying. For this a number ofoptions are available, with differences in accuracy, staff requirements, duration and cost. Theadvantages and disadvantages of conventional surveying techniques and GPS-basedtechniques are being compared. Finally, conclusions are drawn and recommendations aregiven for preferable options.

After an Introduction in Chapter 1, the current procedures, mostly based on map and compassreadings, are touched upon in Chapter 2. Next, Chapter 3 defines the co-ordinate and leveldata requirements. Chapter 4 touches upon the accuracy requirements, both for position andfor level. Some basic aspects of the reference surface for levels are covered in Chapter 5. Theconventional surveying techniques based on levelling instruments and theodolites arediscussed in Chapter 6, whereas the new technology of GPS surveying is covered in Chapter7. These surveying techniques are compared in Chapter 8. Chapter 9 summarises the mainconclusions. Chapter 10 gives recommendations on improvement of the levelling output.

It proved extremely difficult to obtain consistent accuracy data retaining to the referencebenchmarks. The figures given in this Note should only be regarded as best guesses.

Throughout the note, a distinction is made between survey with a high accuracy and surveywith a moderate accuracy. High accuracy is mostly associated with coastal areas andmoderate accuracy refers to hilly and upland terrain.


2 BackgroundUnder the Hydrology Project (HP) a computerised Hydrological Information System (HIS)will be established by the Central Ground Water Board (CGWB) and the State GroundwaterDepartments (SGWDs) in the eight participating states inducting Gujarat, Andhra Pradesh,Madhya Pradesh, Orissa, Tamil Nadu, Maharashtra, Karnataka and Kerala. The networkscontrolled by the HIS comprise 25,000 manually monitored wells and 5,900 wells equippedwith digital water level recorders and many additional exploration wells.

Geographical co-ordinate and elevation values of observation wells are some of theparameters that have to be accurately measured for enabling correct interpretation of thevariables measured in the field. For the present procedure to obtain the well the co-ordinates acompass and 1:50,000 topo-sheet are required. At the well site, compass bearings are taken toconspicuous features, e.g. buildings, in the well surroundings. The bearings are then plottedon the topo-sheet. On the topo-sheet, the intersection of the bearings is determined andsubsequently the co-ordinates are read.

For each well, the accurate elevation of ground level and the Top of Casing (in m aboveMSL) are to be measured. The measured values need to be verified with the elevationcontours represented in topo-sheets.

The geographical co-ordinate values are essential inputs for GIS application.

The water level elevations will form the basis for production of water level contour maps.Based on these maps major inferences can be drawn on the hydro-geological regime in anyarea such as groundwater flow paths and gradients, recharge and discharge areas andgroundwater basin boundaries, etc.


3 Surveying needsAt present, about half of the existing monitoring wells have been levelled. Moreover, thenewly constructed piezometers also need to be connected to MSL. The level survey work hasbeen slow due to shortage of survey staff and equipment. The position on level survey work,as per MIS, 30 September 1998, is given in Table 1. Note that the numbers given in the tableare slightly higher than the actual size of the HIS network as it also includes those wellswhich will be replaced or have been abandoned. The inclusion of these wells in the levellingcount is necessary as in the final processing the historical data have to be linked to the dataunder HIS as well.

Table 1: Status of RL survey, as per MIS 30 September 1998

well type observation piezometer total RL pendingCGWB 8450 2176 10626 6769Andhra Pradesh 3149 625 3774 1281Gujarat 2068 366 2434 121Karnataka 1540 500 2040 2010Kerala 300 359 659 649Madhya Pradesh 4450 665 5115 665Maharashtra 3920 700 4620 2388Orissa 647 305 952 853Tamil Nadu 2174 738 2912 708

states total 18248 4258 22506 8675states and CGWB total 26698 6434 33132 15444

Perusal of the table shows that for CGWB about 6800 and for the states about 8700 wells stillneed to be connected to MSL. Hence, with the present stage of level survey work only limitedspatial quantitative analysis and interpretation of water level data is possible. It allows onlyfor the preparation of water level fluctuation maps and depth to water level but not for waterlevel contour maps.


4 Required accuracyThe accuracy of the geographical co-ordinates and elevation data is crucial for theinterpretation of the contour maps and other spatial products. Excessive errors in thegeographical co-ordinates affect the location of points on the map. The shift of pointlocations can substantially alter the shape of the contours thus changing the perceivedgradient of flow. Further, it can shift the positions of recharge and discharge areas and alterthe shape of the drainage basin.

Likewise, when the elevation values are erroneous the groundwater perceived gradient isaffected and reversal of groundwater flow in coastal areas can go unnoticed or interpretationsmay result in unnecessary alarms.

Thus, it is seen that elevation and geographical co-ordinate values can significantly affect thenature of interpretation.

4.1 Co-ordinate accuracyIn the HP Project area, the existing network of groundwater observation wells monitorsmostly the shallow phreatic aquifer. A limited number of wells observe deeper aquifers.

For a large number of wells, geographical co-ordinates still have to be established. Manyagencies have been using 1:250,000 scale topo-sheets or village maps instead of 1:50,000scale topo-sheets. Moreover, for many sites the well locations have not systematically beentransferred on to topo-sheet and therefore, the co-ordinate values lack accuracy. In manystates for villages with a number of wells, all wells have been assigned identical co-ordinates.In such a situation, the contouring software can handle the data pertaining to a single wellonly, thereby neglecting any variation within the village.

The co-ordinate accuracy requirements should be related to the use of the well data.Generally, a horizontal accuracy better than 25 m is sufficient for groundwater applications.

4.2 Level accuracyA distinction should be made between absolute and relative RL accuracy. For manyhydrogeological studies, e.g. flow analysis in aquifers, the water level gradient should beaccurately known. This dictates a high relative accuracy. Therefore, for RL measurements theaccuracy requirement is specified in relative terms, i.e. an error in mm per km.

In the non-coastal zone, an average RL accuracy of 50 mm/km is acceptable for routine andgeneral-purpose hydrological studies. Examples of such studies are the preparation of waterlevel contour maps and geological cross sections for aquifer correlation.

The areas with high accuracy requirements are mainly concentrated along the coast. In suchareas, the accuracy requirement for the elevation data is in the order of 10 mm/km. The RLaccuracy requirements, differentiated per type of terrain, are summarised in the Table 2below. The same Table also gives the size of the related area in the Project Area and anindication of the number of wells it concerns.


Table 2: RL accuracy requirement per type of terrain

terrain type accuracy area in km2 numbersflat coastal area ≤10 mm/km 348000 3300upland, hilly ≤50 mm/km 1305000 30000

The accuracy of the water level measurement proper, relative to ToC, should also beconsidered.


5 Reference surface for levellingIn hydrology, levels are commonly expressed relative to a reference surface, whichessentially is level and encompasses the entire globe.

The surface of free and static water is regarded as being level, i.e. under gravity and earthrotation forces the free surface of entirely static water has equal potential. A special equalpotential surface is the geoid, which by definition is closely related to Mean Sea Level(MSL). Due to gravity anomalies, the geoid is not a regular but an undulating surface.Elevations relative to MSL are expressed as distance above the geoid.

It is quite impractical to establish a network of MSL reference points for the full ProjectArea. Instead, use can be made of an existing and well-established reference network.Virtually the only high accuracy reference network existing in the Project Area consists ofGTS (Great Triangular Survey) benchmarks. Survey of India meticulously administrates andmaintains the GTS benchmarks.

As explained in Section 4.2, for groundwater levelling the absolute accuracy of the waterlevel data is not as important as the relative accuracy. Consequently, the main requirement isthat the GTS benchmarks have a good relative accuracy. Their accuracy and spacing arepresented in Table 3.

Table 3: Basic reference point accuracy

reference point type relative accuracy spacingGTS 1st order, VVP 1 mm @ 1 km 100 tot 500 kmGTS 2nd order, VP 3 mm @ 1 km 50 to 80 km

Tertiary, double run 12 mm @ 1 km 20 to 30 kmTertiary, single run 24 mm @ 1 km 20 to 30 km

Legend: VVP Very Very PreciseVP Very Precise

The accuracy degrades with the square root of the distance in kilometre. E.g. at a distance of4 km the GTS 2nd order accuracy is 6 mm.

The relative accuracy of the reference points is only of importance if more than one referencepoint is used within a single aquifer area. However, it is recommended to connect thelevelling network to more than one GTS benchmark point for error control purposes.

Further, it should be noted that the larger the separation between the reference points thesmaller the error per km is.


6 Conventional surveying techniquesConventional levelling is executed by automatic mechanical (non-electronic) levelinstruments. State-of-the-art surveying instruments comprise a lot of electronics; often asingle board personal computer is built-in to enhance the performance of the instrument andto cater for data recording.

6.1 Levelling methodsConventional levelling is essentially an incremental process. Each increment, the heights oftwo levelling staffs are observed and annotated in a logbook. The size of the incrementdepends on the magnification and the quality of the levelling instrument and the terraincondition (slope, obstructions). Each increment may cover a distance up to a maximum of200 m. It is estimated that on relatively flat terrain, the levelling instrument may have to beshifted and set up about 5 to 10 times. To obtain the specified accuracy, the survey teamshould be skilled and execute the work meticulously.

The levelling quality is monitored and increased by applying double run levelling, i.e. eachsub-trajectory is levelled from a reference point to a new point at some distance and back tothe reference point. The difference in level between the two runs should fall withinpredefined accuracy requirements.

Recently, electronic levelling instruments, denominated digital levels, became commerciallyavailable. Such instruments require very little adjustments by the surveyor. The digital levelautomatically takes the staff reading and records it, together with administrative andidentification data. It requires a special levelling staff which has a face with a bar codepattern precisely printed over it. To obtain a level reading, the digital level observes andanalyses the image of the bar code. The other face of the staff may have a conventional scaleto allow manual reading.

Another alternative for traditional levelling is the 'total station'; i.e. an electronic theodolitewith integrated distance meter and digital data recording. The instrument measures bearing,vertical angle and range to a retro-reflector (prism) at a distance. The elevation of the prism iscalculated from the vertical angle and the range. The co-ordinates are calculated from bearingand horizontal range.

6.2 Reference surfaceDue to the very principle of the levelling instrument, the instrument reference plane settlesitself parallel to the local geoid surface. Height obtained by levelling (H) is orthometricheight. Levelling heights are expressed as elevation above to MSL, i.e. height above thegeoid. For at least one reference point in the area, the elevation above MSL should be known.

6.3 AccuracyTable 4 summarises for each type of instrument the levelling accuracy in double runengineering mode. The figures assume engineering grade levelling. For automatic level andtotal station, higher accuracy is possible than indicated in the Table, but cost increases rapidlywith better accuracy. To obtain the accuracy for a certain levelling distance (L km) thepresented accuracy figure should be multiplied by √L. Over distance of 9 km with anautomatic level instrument, the estimated error would be 5√9 =15 mm, that is less than 2mm/km.


Table 4: Indicative instrument accuracy and range

instrument accuracy rangeautomatic level 5 mm @ 1 km 100 mdigital level 1 mm @ 1 km 100 mtotal station 15 mm @ 1 km 2500 m

The listed accuracy figures apply to good quality instruments used in double-run surveying.The accuracy of the total station is comparable with that of single-run engineering levelling,however, the measuring range is much higher. At short range, say less then 750 m, a singleprism suffices. Larger ranges require multiple prisms. Second order effects caused byundulation of the geoid shape, adversely affect the accuracy of the total station over largerdistances. A precise estimate cannot be made, as no geoid shape data are available to theConsultant yet. However, it is assumed that these effects are insignificant.

6.4 Practical aspectsThe methodology and use of the automatic level is rather straightforward and wellunderstood. Coverage is highest in flat terrain but is adversely affected in sloping terrain.

The time to set-up and take measurement with a digital level is very short; consequently, thedaily coverage primarily depends on transport efficiency. The possibility of making a mistakeis much reduced by the electronic reading and data recording.

A major advantage of the total station is its capability to cover more than 1 km perobservation. It can also measure along slopes. Much like with conventional levelling, line-of-sight between the station and the retro-reflector is required. In urban areas and many otherterrain types, such as woodland, and along winding roads this may limit the coverage. Thepossibility of making a mistake is much reduced by the electronic reading and data recording.

While taking measurements using automatic level or digital level instruments, the surveyorand labourers are not far apart and can easily communicate with each other. In case of thetotal station, much larger distances are common practice, so for effective communicationbetween surveyor and labourers walkie-talkies are required. The transport should also be wellorganised to benefit of the speed and efficiency of the total station. The total station deliversaccurate co-ordinates as a side product.

6.5 Staff, duration and costFor each of the instrument types a team of 4 to 6 persons can effectively execute thelevelling. The teams may comprise a surveyor with a labourer to carry the instrument and anumbrella, and a labourer for each staff/prism. For transport, a vehicle and a driver arerequired.

Table 5 summarises the estimated levelling coverage in wells per month, per team and totalteam years for the Project Area. The calculations are based on average figures, between andwithin states other, more dedicated, figures may be applied. The coverage assumes a workingday of 6 hours.


Table 5: Estimation of remaining work for optical instruments

CGWB SGWD15.5 km/well 6800 wells 11.5 km/well 8700 wells

instrument type coverage wells/monthper team

team years wells/monthper team

team years

automatic level 6 km/day 7.7 146.6 10.4 139.2digital level 12

km/day15.5 73.3 20.8 69.7

total station 24km/day

30.9 36.6 41.6 34.8

With digital levels, the daily coverage can be about twice as much as with automatic levelsprovided that proper transport is available. From the figures above the total throughput timecan be calculated for a given number of teams. For example, the SGWDs would have todeploy 42 teams to finish the levelling within one year.

The daily coverage of a total station can be quite large, in particular in flat terrain. In slopingterrain, the coverage depends on the line of sight conditions. In flat terrain, the functioning ofthe total station is not hampered by the slope but the coverage largely depends on thetransport times.

Table 6: Estimation of operational and instrument cost for optical instruments

CGWB SGWD6800 wells 8700 wells

instrumenttype

investmentin Rs lakhs

operationalcost inRs/well

instrumentcost in Rs/wellper instrument

operational cost inRs/well


automaticlevel

0.5 4786 7.4 3555 5.7

digital level 1.8 2393 26.5 1777 20.7total station 6.0 1196 88.2 889 69.0

The cost components of the levelling comprise:1. instrument investment costs2. operational costs including:

• cost of staff• cost of transport

It is assumed that the investment costs for the instruments are entirely written-off, though ifproperly handled, at the end of the project the instruments may still be operational and couldbe used for other work then. The operational costs are based on a monthly staff cost of Rs37,000 per team including vehicle, driver, surveyor, labourers, daily allowance, board andlodging. The average travel speed, which largely affects the daily coverage, is assumed to be35 km/hour. The productivity calculations are based on 20 working days of 6 hours permonth and 6 months per year.

The operational costs depend on the cost of staff, labour, and transport. The larger the speedof levelling, i.e. the daily coverage, the lower the cost per well. The instrument cost per wellis calculated for a single instrument only, e.g. for the 8700 wells of the combined SGWDs


investment cost for one instrument is shared by all wells. As can be concluded from Table 5,it would take 147 team years to level all remaining SGWD wells with a single automatic levelor 35 years with a total station.

To speed up progress, more teams should be deployed. To calculate the instrument cost perwell, the related instrument cost figure should be multiplied by the number of instruments. If18 total stations were deployed by the SGWDs, then the instrument cost per well would be 18x 69.0=1242 Rs per well. In this example the combined operational and instrument costwould amount to 2131 Rs per well.

6.6 TrainingIt is assumed that little or no training is required for the use of the conventional automaticlevel instruments.

Although not complicated in its application, a few days of training may be required for theintroduction to digital level instruments. In particular the concept of the instrument, the use ofdigital technology and data transfer to a PC should be addressed.

The same applies for the use of the total station. It is expected that the surveyors are familiarwith the methodology. The surveyors may benefit from training in the operation of the totalstation and data transfer to a PC. The accuracy aspects should also be covered, in particularthe effect of meteorological conditions on level reading accuracy.


7 Global Positioning System based surveying techniquesThe Global Positioning System (GPS), delivers x,y,z co-ordinates and time, worldwide. TheGPS comprises satellites in orbit (the space segment), a control system (the control segment)and the user's equipment (the user segment), that is the GPS receiver. At any instant of time,the co-ordinates and velocity of each of the satellites are accurately known relative to an earthbound co-ordinate system. Each satellite transmits its co-ordinates, precise time and otherdata to the receiver.

From the received data, the GPS receiver can calculate its distance (pseudo range) to eachsatellite. Pseudo ranges to at least four satellites are required to calculate the receiver's co-ordinates and elevation. Position data are expressed in the WGS84 system. Co-ordinates inthe WGS84 system can be accurately transferred to e.g. UTM.

The positioning accuracy is adversely affected in many ways, e.g. by the functioning of thecontrol system, the satellites and during radio wave propagation to the receiver. In thiscontext, it should be noted that the GPS system supports two accuracy classes, one formilitary and another for civilian use. The civilian accuracy is purposefully degraded bymanipulation of transmitted data. The propagation speed of electromagnetic waves from thesatellites to the receivers is not constant but varies with amongst others the amount of freeelectrons in the ionosphere and the content of water vapour in the troposphere/atmosphere.For civilian use, the accuracy of a single measurement is about 100 m horizontally, and 150to 200 m vertically.

A differential GPS technique that largely improves accuracy deploys two (or more) receiversconcurrently. One receiver, the reference or base receiver, is operated at a benchmark withknown co-ordinates. The other receiver, the mobile or roving receiver, is used to measure atthe unknown points. By combining the data obtained from both (or more) receivers, many ofthe errors that are common to both receivers, e.g. fluctuations of wave propagation speed, canbe largely reduced. Further, the effects of the civilian accuracy degradation can be virtuallycompletely removed. Both receivers should have at least four satellites in common. Thiscombination of GPS receivers can be implemented in real time, e.g. in Differential GPS(DGPS), or static. In real time mode, a data communication system is required to deliver thedata from reference receiver at the mobile receiver. In static mode, the GPS receivers areequipped with data loggers to record the received data for later analysis. In particular in staticdifferential mode, very high accuracy can be achieved.

7.1 Levelling techniquesGPS can offer several accuracy grades, depending on the receiver technology. Below, threefrequently applied receiver technologies are summarised. In all three technologies, two ormore receivers are deployed in a kind of static differential mode. The reference receiver isinstalled at a point with known co-ordinates and height above MSL. The mobile receiver isdeployed at the point of interest. Both receivers are operated in a data-logging mode,concurrently receiving and recording data. For best accuracy and reliability, as manysatellites as possible have to be observed. Both receivers should have at least four satellites incommon. The receiving and data recording process may be continued for a few minutes up toseveral hours, depending on the required accuracy and receiver characteristics.


Subsequently, the recorded data are retrieved from the receivers and processed at aconvenient time and place. The processing yields co-ordinates and heights of the measuredpoints and quality indicators. Applying a proper geoid model, the heights can be convertedinto RL values for the observation wells.

The basic receiver technologies are summarised on the following.1. Travel time or code phase receiver.

This type of receiver merely measures the travel time of the coded satellite datamessages, i.e. the time between transmission by the satellite and the reception by thereceiver. In differential mode (DGPS) the accuracy is about 1 m horizontally and 1.5 to 2m vertically.

2. L1 receiver.The L1 receiver not only measures travel time but also the phase of the L1 radio carrierwave. This significantly enhances the accuracy both horizontally and vertically. Theheight accuracy is in the order of 20 mm plus several ppm over distance, (1 ppm is equalto 1 mm/km). As a rule of thumb, the maximum baseline length (distance between basestation and mobile station) should not be more than some 15 km. Beyond that distanceaccuracy will decrease rapidly, amongst others due to unresolved phase ambiguity; atlarge separations the system cannot reliably distinguish between successive L1 wavelengths. Longer baselines could be split into practical sections and intermediatemeasurements could be taken. Measurements at intermediate positions cost extra time forset-up and receiving. Moreover, accuracy is adversely affected.

3. L1/L2 receiver.This receiver type measures phases of the L1 and the L2 carrier waves. This combinationgives the receiver a much larger operational range without the non-resolvable phaseambiguity restriction. At short baselines, the observation time can be a few minutes orless. At larger baselines, beyond 15 km, observation time increases to say 15 minutes to1 hour. Accuracy improves to sub centimetre levels plus about 1 ppm (1 mm/km) overdistance. The L1/L2 receivers are most expensive.

7.2 Reference surfaceGPS height (h) is not relative to MSL but relative to an ellipsoid, the aforementionedWGS84, and it does not reflect any gravity effect. For conversion from ellipsoid height (GPS)to geoid height (MSL) the local separation (N) between the geoid and the ellipsoid is to beknown. Commonly, the following equation is used to convert from ellipsoid height to geoidheight:

H = h – N.

h = ellipsoid heightH = geoid heightN = separation

The value of N varies with the location on earth. Presently, major efforts are being made,world wide, to model the geoid as accurately as possible as a function of place. Advancedmodels yield a relative accuracy in the order of 1 to 4 mm/km. In absolute terms, very goodmodels have estimated errors of ±0.3 m. Both error components, absolute and relative, arebeing further improved by better modelling, better data, etc. Such models e.g. those of USA,Europe and Australia are based on millions of point gravity measurements.


A worldwide geoid model relative to WGS84 (EGM96) is available from NIMA, USA. Themodel grid has a spacing of 0.25 degree (N and E). Its accuracy is insufficient for levellingpurposes, however, the EGM96 model could be used to assess the separation between geoidand ellipsoid (WGS84).

It appears that for peninsular India no accurate and comprehensive models are available, yet.However, SoI maintains an extensive network of primary and secondary GTS (GreatTriangular Survey) geodetic points which could be used as reference points for GPS assistedlevelling, see Chapter 5.

7.3 Overall accuracyThe accuracy requirements of RL values, which depend upon the application of the data,were explained in Chapter 4. Depending upon the implemented technology, GPS survey canyield level accuracy better than 10 mm/km. High accuracy receivers (GPS L1/L2) and precisemethodologies can deliver an accuracy of 10 mm plus 1 mm/km (1 ppm). Moderate accuracyreceivers (GPS L1) may have an accuracy of 20 to 50 mm plus 2 to 5 mm/km (2 to 5 ppm). Inthe following example, the relative accuracy for the 50 mm GPS L1 receiver is calculated.Example: if the reference receiver and the mobile receiver are 10 km apart, then the accuracywould be 50 mm + 10 x 5 mm = 100 mm, i.e. 10 mm/km. Most GPS L1 receiver types woulddeliver better accuracy and the GPS L1/L2 receivers would deliver a much better accuracy.

Standard DGPS receivers may deliver an accuracy in the range of 1 m, very low costreceivers 3 to 5 m (see Table 7). Simple hand held GPS receivers deliver, for a singlemeasurement and without combination with a reference receiver, an accuracy of about 100 mhorizontally and 150 m to 200 m vertically. It should be noted that the stated accuracy isrelative to the ellipsoid WGS84.

Table 7: Indicative accuracy and operational range

instrument accuracy indication(relative to ellipsoid)

range

DGPS 1 m ≥1000 kmGPS L1 20 mm + 5 mm/km 15 kmGPS L1/L2 8 mm + 1 mm/km 100 km

The horizontal positioning accuracy of the GPS receivers is better than the elevation accuracyand for use under HP, it does not need enhancement. The conversion from ellipsoid to UTMis rather straightforward and formulae, constants and software are readily available frommany sources. It should be noted that accuracy pertains to the measurement relative to thebase station.

7.4 Practical aspectsAfter a geoid model has been established, levelling by GPS can be fast, but it is currently notin vogue in the country.

For conversion of GPS heights to MSL, the separation between ellipsoid and geoid datum isto be established by taking GPS observations at reference points. If the ellipsoid and geoidsurfaces are parallel to each other, at least within a small margin, then one GPS observation ata precisely known GTS benchmark point would suffice to establish the height difference.


However, in India the surfaces are tilted, hence, the height correction has a spatial variation.A simple local approximation of the geoid would be a (flat) plane with known elevation andtilt (East and North).

To establish such a plane, GPS observations from three precisely known (co-ordinates andelevation relative to MSL) reference points are needed. The reference points, e.g. GTSbenchmark points, should be chosen at the corners of a triangle of approximately equal sides.For each of the reference points, the separation between geoid and ellipsoid is calculated.These three separation values define a planar geoid model.

Having established such a separation/correction model, the GPS observations, collected in thearea enclosed by the GTS benchmark points, can be converted to geoid levels by linearinterpolation. It should be noted that the GPS observations have to be collected at the point ofinterest, e.g. at the observation wells while simultaneously the base station is operated at aGTS benchmark point.

In coastal areas, this methodology could be feasible for groundwater application if the localgeoid gradients along N and E directions are relatively constant over the area covered by theGTS benchmarks. As an indication, the geoid gradient should not vary by more than 5mm/km. If GPS data and MSL levels from more than three GTS benchmarks would beavailable, then a more complex model could be developed to achieve more accurate levelconversion.

Generally, GPS elevations (h), relative to the ellipsoid, have an accuracy that is 1.5 to 2 timesworse than the horizontal accuracy (E, N). The main reason is that for elevation data, onlysatellites above the observation point can be received. The other satellites are below thehorizon and consequently the radio signals are obstructed by the earth.

The satellite geometry, that is the number of visible satellites and their relative position in thesky, must be adequate to ensure accurate results. The receivers and/or the post processingsoftware can detect the quality of the geometry and may neglect data of poor geometry. Thegeometry can also be affected by local conditions, e.g. the receiver is surrounded by highbuildings, covered by tree canopy etc. For best results, each receiver should have anunobstructed view to the sky. GPS measurements are hardly affected by weather conditions,however, during thunderstorms data loss may occur.

Given the high accuracy of the GPS readings, the final accuracy of level data, relative toMSL, largely depends on the accuracy of the model that is used to convert from ellipsoid datainto geoid data.

The accuracy of the GTS benchmark points directly affects the final results.

7.5 Staff, duration and costA team of about 2 persons can effectively apply the GPS equipment in the field. During thedata collection by the GPS receiver, the ToC level of the well and possibly other points/benchmarks may be connected to the GPS receiver. For that and depending on the site conditions, asimple levelling instrument might be useful. As large distances have to be covered daily, avehicle and a driver are required.

The instrument costs are directly related to the accuracy of the equipment.


Next Table 8 summarises estimated levelling production in wells per month. The calculationsare based on average figures, between and within states other more precise figures may beapplied.

Table 8: Estimation of remaining work for GPS instruments

CGWB SGWD15.5 km/well 6800 wells 11.5 km/well 8700 wells

Instrument type range wells/monthper team

team years wells/monthper team

team years

GPS L1 15 km 83 13.6 90 16.1GPS L1/L2 100 km 101 11.3 111 13.0

The GPS L1 receiver has an effective range of about 15 km. To cover larger ranges anincremental approach should be adopted, much like with levelling. The increments can be 10to 15 km. This range is measured as a straight line from the reference receiver to the mobileunaffected by intermediate obstructions. Obviously, the intermediate measurements takemore time.

The GPS L1/L2 receiver has a much larger operational range and hence very fewintermediate measurements are required. It is estimated that, given the prevailing terrain andtransport conditions, the daily coverage of a GPS L1/L2 receiver set is about 33% higher thanthat of the GPS L1 receiver set.

Table 9: Estimation of operational and instrument cost for GPS instruments

CGWB SGWD6800 wells 8700 wells

instrumenttype

investmentin Rs lakhs





GPS L1 15 445 221 410 172GPS L1/L2 24 368 353 333 276

It is assumed that the investment costs for the instruments are entirely written-off, though ifproperly handled, at the end of the project the instruments may still be operational and couldbe used for other work then. The operational costs are based on a monthly staff cost of Rs37,000 per team including vehicle, driver, surveyor, labourers, daily allowance, board andlodging. The average travel speed, which largely affects the daily coverage, is assumed to be35 km/hour. The productivity calculations are based on 20 working days of 6 hours permonth and 6 months per year.

For the GPS L1 receiver it is assumed that the maximum time on site is 1 hour and for theGPS L1/L2 receiver the onsite time is set at 45 minutes. The base receiver is set-up at thebeginning of the day and recovered at the end of the day. Only a guardsman should stay withthe base receiver.


It should be noted that the presented investment costs are at the conservative side. Thetechnology is rapidly being enhanced with new features, increased accuracy, etc. As a resultGPS receivers of present day state-of-the art are quickly replaced by more advanced receiverswhich reduces the prices of old technology receivers.

The operational costs depend on the cost of staff, labour, and transport. The larger the speedof levelling, i.e. the daily coverage, the lower the cost per well. The instrument cost per wellis calculated for a single instrument only, e.g. for the 8700 wells of the combined SGWDsinvestment cost for one instrument is shared by all wells. As can be concluded from Table 8,it would take 13 team years to level all remaining SGWD wells with a GPS L1/L2 receiversystem. To speed up progress, more teams should be deployed.

To calculate the instrument cost per well, the related instrument cost figure should bemultiplied by the number of instruments. If 7 GPS L1/L2 receivers were deployed by theSGWDs, then the instrument cost per well would be 7 x 276= 1932 Rs per well. In thisexample the combined operational and instrument cost would amount to 2265 Rs per well.

7.6 TrainingRecent developments in the GPS industry resulted in accurate, reliable and easy to useinstruments. Taking measurements virtually has come down to setting up of the receivers andswitching them on in data logging mode. Because of this ease of use, only limited GPSknowledge is needed for field operation. Hence, training requirements for field operation arerather limited.

Planning, quality control and data processing require a rather detailed understanding of theGPS system, geodesy and in particular the shift from ellipsoidal heights to geoidal height.These activities should be executed by geodetic staff.

The geodetic staff requires in-depth training, covering of GPS principles, planning, dataprocessing and datum shift e.g. to UTM and geoid. Such training, covering theory andpractice, may take about 1 month.


8 Comparison of surveying techniques

8.1 Available optionsHigh accuracy surveysThe requirements for the high accuracy surveys can be met by using the automatic level ordigital level equipment without special measures. Also a total station featuring a verticalangle accuracy of 2" or better could meet the requirements if it is carefully used.

With GPS, the feasibility depends on the local spatial distribution of the separation betweenthe ellipsoid and the geoid. In case the separation has a constant slope, then a simple linearinterpolation (plane surface) would deliver an accurate local geoid model. Unfortunately, ageoid model or detailed separation data are not available. Consequently, a proper errorassessment is not possible. A rough estimate is that at any point in the Project Area theseparation gradient is less than 40 mm/km.

Hence, starting form a reference point with known level, e.g. a GTS benchmark, the addederror due to the uncertainty in the separation gradient is expected to be less than 40 mm/km.

A geoid separation model could be developed by surveying a (large) number of GTSbenchmarks in the Project Area. Based on these data a model could be developed. Assumingthat such a model is available with sufficient accuracy, the GPS L1 receiver could deliver therequired data provided that the surveyed points are at least several kilometres apart andwithin a distance of not more than 15 km from the reference point. In many cases,intermediate measurements are needed because the average distance between GTSbenchmarks in the Project Area is about 21 km. In quite a few cases, the distance betweenGTS benchmarks is in the range of 30 to 50 km. The GPS L1/L2 receiver could deliver therequired accuracy at short and long distance.

Moderate accuracy surveysThe accuracy specification for the moderate accuracy surveys is 50 mm/km. Technically, allthe three discussed surveying instruments (auto level, digital level and total station) can meetthe accuracy requirements. Assuming that geoid separation slope is less than 40 mm/km itcan be concluded that both geodetic GPS receiver types (L1 and L1/L2 receivers) can deliverthe required accuracy without difficulty. Still, a simple separation model would enhance thedata quality against little effort.

8.2 Comparison of techniquesConventional surveying is a well-understood technique, which can meet the accuracyrequirements. A disadvantage is the relatively small coverage.

Application of digital levels would increase the obtained accuracy at a significantly highercoverage. A digital level though, is more vulnerable, costlier and requires training in its useand understanding of some automation aspects. Investment costs are higher than for theconventional levels and staffs. Personnel requirements are similar to conventional levelling.


A total station offers less accuracy than properly executed conventional levelling. Dailycoverage can be much higher though. The instrument is more vulnerable, costlier andrequires training in the basic concepts, instrument use and meteorological effects onaccuracy. Further, understanding of some automation aspects has to be obtained by thesurveyor. Investment costs are considerably higher than for the conventional automatic levelsand levelling staffs. Personnel requirements are similar to the conventional automatic levels.

Levelling by GPS is a totally different technology. It does not deliver elevation relative toMSL/geoid but height relative to the WGS84 ellipsoid instead. The ellipsoidal heights have tobe converted to geoid heights. For that, a geoid model is required. Presently there is nonational Indian geoid model available to the CGWB and SGWDs of the Hydrology Project.Consequently, such a model should be established for each area where GPS levelling isapplied. That model can be very simple for moderate and low accuracy application.

Daily coverage can be much higher though. The receivers are rather rugged but costly.Training is required to familiarise operators and surveyors/engineers with the basic concepts,deployment and data processing. Investment costs are high. The engineer, who is responsiblefor data processing, planning and quality assurance should have a high level of education andtraining. The field teams could consist of a surveyor and an assistant. Further, a vehicle isrequired.

A summary of the various instruments, accuracy and costs is presented in Table 10. Thefigures are based on 6800 pending wells for CGWB and 8700 for SGWDs respectively. Itshould be noted that in the table the stated GPS accuracy is instrument accuracy relative toWGS84. RL accuracy, relative to MSL, depends on the applied geoid model, i.e. themodelled separation between ellipsoid and geoid.

Table 10: Tabular summary of total costs

instrument accuracy units

operationalcosts in

Rs lakhs

Instrumentcosts in

Rs lakhs

total costsin Rslakhs years

Central Groundwater Boardautomatic level 5 mm @ 1 km 74 325.47 37.00 362.14 1.98digital level 1 mm @ 1 km 37 162.74 66.60 229.34 1.98total station 15 mm @ 1

km19 81.37 114.00 195.27 1.93

GPS L1 20 mm +5mm/km

9 30.27 135.00 165.27 1.51

GPS L1/L2 8 mm +1mm/km

9 25.03 216.00 241.03 1.25

State Groundwater Departmentsautomatic level 5 mm @ 1 km 70 309.32 35.00 344.32 1.99digital level 1 mm @ 1 km 35 154.66 63.00 217.66 1.99total station 15 mm @ 1

km18 77.33 108.00 185.33 1.94

GPS L1 20 mm +5mm/km

7 35.67 105.00 140.67 2.30

GPS L1/L2 8 mm +1mm/km

7 28.96 168.00 196.96 1.86


The displayed costs are in Rs lakhs. The number of units (instruments) is optimised forcompletion of the levelling in about two years time. For the CGWB, the minimum number ofinstruments was limited to 9, i.e. the number regions. For the SGWDs, a similar limit wasapplied. Each state should have at least one unit of the chosen instrument. However, it isassumed that Gujarat can finish the levelling without investment in new total station or GPSinstruments. Consequently, the total duration in some of the GPS cases fell short of twoyears. As mentioned under Sub-Section 7.5, the applied GPS receiver costs are on theconservative side.

Further refinements of the cost and time calculations, based on the number pending wells, aresummarised in Table 11 and Table 12. The number of teams/instruments is optimised forcosts and workload. The estimates are based on the data of the following Tables:

Table 1: Status of RL survey, as per MIS 30 September 1998Table 5: Estimation of remaining work for optical instrumentsTable 6: Estimation of operational and instrument cost for optical instrumentsTable 8: Estimation of remaining work for GPS instrumentsTable 9: Estimation of operational and instrument cost for GPS instruments

Automatic levelling instruments are not included in the tables because of the large number ofteams/instruments and the excessive costs involved.

Table 11: RL survey costs and duration for digital level and total station

digital level total stationstate RL

pendingteams costs in

Rs lakhsyears teams costs in

Rs lakhsyears

Andhra Pradesh 1281 5 32.31 2.05 3 29.05 1.71Gujarat 121 1 3.05 0.97 1 2.74 0.48Karnataka 2010 8 50.70 2.01 4 45.58 2.01Kerala 649 3 16.37 1.73 1 14.72 2.60Madhya Pradesh 665 3 16.77 1.77 2 15.08 1.33Maharashtra 2388 9 60.23 2.12 5 54.16 1.91Orissa 853 4 21.51 1.71 2 19.35 1.71Tamil Nadu 708 3 17.86 1.89 2 16.06 1.42

states total 8675 36 218.80 1.93 20 196.74 1.74

Table 12: RL survey costs and duration for GPS L1 and GPS L1/L2 receivers

GPS L1 GPS L1/L2state RL

pendingteams costs in

Rs lakhsyears teams costs in

Rs lakhsyears

Andhra Pradesh 1281 1 25.13 2.37 1 36.06 1.92Gujarat 121 0 2.37 0.00 0 0.00 0.00Karnataka 2010 2 39.43 1.86 2 56.59 1.51Kerala 649 1 12.73 1.20 1 18.27 0.97Madhya Pradesh 665 1 13.04 1.23 1 18.72 1.00Maharashtra 2388 2 46.84 2.20 2 67.23 1.79Orissa 853 1 16.73 1.57 1 24.01 1.28Tamil Nadu 708 1 13.89 1.31 1 19.93 1.06

states total 8675 9 170.16 1.78 9 244.23 1.45


9 Conclusions1. All of the assessed instruments can deliver the required accuracy, both for coastal and

upland/high relief terrain.2. Whatever instruments are used, benchmark reference data are required. Therefore, the

agencies should have access to the benchmark co-ordinates and elevations above MSL ofthe GTS benchmark points of SoI.

3. To execute the levelling work with automatic and digital levelling instruments anexcessive number of instruments and teams are required.

4. If labour, transport and investment costs are taken into account, then the GPS L1equipment appears to be most cost and time efficient. One GPS receiver per agency,would suffice to level the remaining wells). in the Project Area in about two years (inNovember 1998 about 60% CGWB wells and 40% of SGWD wells were not yetconnected to MSL).

5. Further, use of GPS, and to a lesser extent the total station, would free several vehicles forother work.

6. GPS delivers x,y,z, data, i.e. not only heights above a datum but also accurate co-ordinates.

7. GPS brings a rather different way of surveying. In India, levelling by GPS is not yetapplied on a large scale. It may take some practical use to become fully familiar with themethodology. On a worldwide scale, more and more levelling projects make use of GPStechnology. Eventually, GPS levelling will become the only methodology for engineeringlevelling where accuracy demands are not extremely high.

8. A simple geoid model would be sufficient to deliver level data of sufficient accuracy forupland and sloping areas. The geoid model could be based on the second order GTSbenchmarks.

9. For the surveyors/engineers, in particular those who are responsible for planning and GPSdata processing, considerable training is needed. Because the GPS receivers manufacturednowadays are rather easy to use, the field teams need only limited training.

10. GPS receivers are rather costly, hence, not only the training effort is large also theinvestment costs are substantial.

11. Introduction of digital levels or total stations would not require a significant adaptation ofthe surveying teams and/or methodology. After a relatively short training, the surveyorand his/her assistant would be capable to execute the levelling in an accurate and effectiveway.

12. The total station also delivers co-ordinates as a side product.


10 Recommendations1 Continue levelling by traditional means and first concentrate on coastal areas where

accuracy requirements are highest.2 The number of wells to be levelled is too large to be done by conventional levelling. An

alternative method should be adopted.3 It is recommended to introduce GPS based levelling, in particular for upland areas. One

set of GPS L1 receivers per agency/region would be sufficient to cover all the remainingwells within two years.• A proper training of the personnel selected for handling GPS equipment is

mandatory, so that thoroughly trained procedures and methodologies can beimplemented in conducting surveys and correct data are obtained.

• For conversion of GPS heights to MSL, the separation between ellipsoid to geoiddatum is to be estimated. This requires GPS observations from at least threeprecisely defined (co-ordinates and elevation relative to MSL) reference points. Thethree reference points should be approximately at equal distances from each other,i.e. in a equal sided triangle. If more reference data points are available, thenmodelling of these on a geoid surface would improve the conversion accuracy andreliability. Possibly, the whole state/basin, or even the full Project Area, could becovered in a single geoid model.

• For purposes of quality control, one or more verification points, with preciselyknown co-ordinates and elevation above MSL, should be included in the observationprogram. Further, for GPS verification measurements could be made along at leasttwo levelled reference lines.

• The major issue with GPS levelling is the conversion from ellipsoid heights toMSL/geoid heights. Most likely, the applied geoid model can be considerablyimproved when more geodetic data come available over the years. Then, the resultsof the GPS levelling campaign could be reprocessed while using the improved geoidmodel. This would result in more accurate piezometer levels. Hence, the GPS levelswill be beneficial in the future. Therefore, it is recommended to carefully archive allGPS and other relevant data, properly described, annotated and reported.

• As only one GPS receiver set per agency is recommended, a contingency plan isrequired for the event that the Agency's GPS receiver set fails.

• While planning the first survey, assistance may be sought from experts, e.g. theequipment vendor, SoI, or others.

• A pilot project with GPS L1/L2 receivers could be started, possibly in co-operationwith the training institute of SoI in Hyderabad.

4 As an alternative to GPS, total stations might be considered for levelling and positionfixing. The total station approach is slightly costlier than GPS L1 technology andrequires many instruments, which makes co-ordination and logistics more complicated.To facilitate communication between the surveyor and the prism men of the total station,walkie-talkies are needed.

5 Co-ordinate levelling activities between state and central agencies, to optimise thelevelling process.

6 In some cases, digital levels may be introduced to accelerate the levelling coverage andto improve the accuracy and reliability of the data.


Manufacturers and representativesTable 13 lists GPS equipment manufacturers, which are represented in India. This table maynot be complete. Some of the representatives also have survey equipment in their productline, which is regarded as an asset.

Table 13: Known GPS equipment manufacturers represented in India

Manufacturer Local representation Surveyinstruments

Ashtech Scientific Instrument Co. Ltd. noLeica Elcome Technologies yesTrimble Mekaster yesSokkia Toshni-Tek International yes

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