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Government of India & Government of The Netherlands
DHV CONSULTANTS &
DELFT HYDRAULICS with
HALCROW, TAHAL, CES,
ORG & JPS
VOLUME 4
GEO-HYDROLOGY
FIELD MANUAL - PART V
REDUCED LEVEL SURVEYS

Field Manual – Geo-hydrology (GW) Volume 4 – Part V
Geo-hydrology March 2003 Page i
Table of Contents
GENERAL 1
1 STEPS IN REDUCED LEVEL SURVEY 2
1.1 INTRODUCTION 2
1.2 REQUIRED ACCURACY 3
1.3 LEVEL ACCURACY 3
1.4 REFERENCE SURFACE FOR LEVELLING 3
2 CONVENTIONAL SURVEYING TECHNIQUES 4
2.1 LEVELLING METHODS 4
2.1.1 THE DUMPY LEVEL 4
2.1.2 TOTAL STATIONS 5
2.2 REFERENCE SURFACE 5
2.3 ACCURACY 6
2.4 PRACTICAL ASPECTS 6
2.5 STAFF, DURATION AND COST 6
2.6 TRAINING 7
3 GLOBAL POSITIONING SYSTEM (GPS) 7
3.1 PRACTICAL ASPECTS 10
4 AVAILABLE OPTIONS IN SURVEYING TECHNIQUES 11
4.1 HIGH ACCURACY SURVEYS 11
4.2 MODERATE ACCURACY SURVEYS 12
5 COMPARISON OF TECHNIQUES 12
6 MANUFACTURERS AND REPRESENTATIVES 13

Geo-hydrology March 2003 Page 1
GENERAL
The Field Manual on Geo-Hydrology comprises the procedures to be carried out to ensure proper
execution of design of the groundwater water level monitoring network, operation and maintenance of
observation well and piezometers. The operational procedures are tuned to the task descriptions
prepared for each Hydrological Information System (HIS) function. The task description for each HIS-
function is presented in Volume 1 of the Field Manual.
It is essential, that the procedures, described in the Manual, are closely followed to create uniformity
in the field operations, which is the first step to arrive at comparable hydrological data of high quality.
It is stressed that water level network must not be seen in isolation; in the HIS integration of networks
and of activities is a must.
• Volume 4 of the Field Manual deals with the steps to be taken for network design and
optimisation as well as for its operation and maintenance. It covers the following aspects.
• Part I deals with the steps to be taken for network design and optimisation. Furthermore, site
selection procedures are included, tuned to the suitability of a site for specific measurement
procedures.
• Part II details with piezometer construction procedure with details of the different elements and
the significance of different elements in the piezometer construction
• Part III comprises the preparatory activities and procedures for carrying out aquifer tests. The
procedures to be adopted for analysis of pumping test data is briefly discussed
• Part IV comprises the testing and installation of DWLR’s. Procedures to be followed for
procurements and installation are outlined in Volume 4 of the reference manual.
• Part V deals with the need for carrying out Reduced Level Surveys and the procedures in
carrying out the survey are outlined.
• Part VI deals with the standardised procedures to be adopted for manual collection of water level
data from open wells and piezometers.
• Part VII deals with the standardised procedures to be adopted for retrieval of data from DWLR
and integration with the software.
• Part VIII, deals with procedures to be adopted for regular inspection and maintenance of
piezometers and DWLR’s.
The procedures as listed out in this manual are in concurrence with the ISO standards as far as
available for the various techniques and applicable to the conditions in Peninsular India.

1 STEPS IN REDUCED LEVEL SURVEY
1.1 INTRODUCTION
In this chapter an overview of steps in reduced level survey is presented. An important field practice
that will ensure the quality of ground-water-level data includes the establishment of permanent datum
and measuring their altitudes. The locations and the altitudes of the observation wells and
piezometers should be accurately surveyed to establish horizontal and vertical datum’s. Inaccurate
datum’s can be a major source of error for water-level measurements used as control points for
contoured water-level or potentiometric-surface maps and in the calibration and sensitivity analysis of
numerical ground-water models. Recent advances in the portability and operation of surveying
equipment, and the popularity of Global Positioning System (GPS) technology, have simplified the
process of obtaining a fast, accurate survey of well location coordinates and datum’s.
Levelling is the operation of determining differences of elevation by measuring vertical distances
directly on a graduated rod with the use of a levelling instrument such as a dumpy level, theodolite,
Total Station or GPS. Levelling measures the height difference between any two points along a
horizontal line of sight. Summing the relative height differences along a levelling line yields the
elevation of those BM’s (Bench Mark) with respect to the height of the first BM.
Elevation values of piezometers and observation wells are some of the parameters that have to be
accurately measured for enabling correct interpretation of the variables measured in the field.
For each groundwater monitoring structure, the accurate elevation of ground level the top of casing (in
m above MSL) are to be measured, see Figure 1.1. The measured values need to be verified with the
elevation contours represented in topo-sheets.
Figure 1.1:
Protective cover for piezometer
showing measuring point and ground
level reference point
It is advisable to standardise the position of the BM for all the monitoring wells. This should be a
reference point preferably in the NE part of the piezometers/observation wells. The BM should be at
the observation well/piezometer necessarily at two points i.e.: Ground Surface and the Measuring
Point. These are the reference points that have to be surveyed to establish their position above sea
level. The ground level reference point should be a permanent marking (preferably a concrete
foundation anchored to the ground with its surface matching the ground level) such as near the base
Ground level
Measuring Point
Height of Measuring Point
(RL above MSL)

of the platform and the measuring point should be on the top of the casing (provide a V notch for
identification) in the case of the piezometers and top of the parapet wall in the case of dug wells. For
easy identification, the BM should be painted with white paint or white washed and the elevation value
written on it with red paint. The altitude of the Measuring point should also be painted on the
protective cover, preferably in the inner side and on the display board of the observation
well/piezometer.
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 any area such as
groundwater flow paths and gradients, recharge and discharge areas and groundwater basin
boundaries, etc. Hence, it should be given utmost care.
1.2 REQUIRED ACCURACY
The accuracy of the elevation data is crucial for the interpretation of the contour maps and other
spatial products. Any error can substantially alter the shape of the contours thus changing the
perceived gradient of flow, reversal of groundwater flow in coastal areas can go unnoticed or
interpretations may result in unnecessary alarms.
The co-ordinate accuracy requirements should be related to the use of the well data. Generally, a
horizontal accuracy better than 25 m is required.
1.3 LEVEL ACCURACY
A distinction should be made between absolute and relative reference level (RL) accuracy. For many
hydro-geological studies, e.g. flow, analysis in aquifers, the water level gradient should be accurately
known. This dictates a high relative accuracy. Therefore, for RL measurements the accuracy
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 and general-
purpose hydrological studies. Examples of such studies are the preparation of water level contour
maps and geological cross-sections for aquifer correlation.
The areas with high accuracy requirements are mainly concentrated in flat terrain such as along the
coast. In such areas, the accuracy requirement for the elevation data is in the order of 10 mm/km. The
RL accuracy requirements, differentiated per type of terrain, are summarised in Table 1.1.
Terrain type Accuracy
Flat coastal area ≤10 mm/km
Upland, hilly ≤50 mm/km
Table 1.1: RL accuracy requirement per type of terrain
The accuracy of the water level measurement proper, relative to top of casing (ToC), should also be
considered.
1.4 REFERENCE SURFACE FOR LEVELLING
In hydrology, levels are commonly expressed relative to a reference surface, which essentially is level
and encompasses the entire globe.
The surface of free and static water is regarded as being level, i.e. under gravity and earth rotation
forces the free surface of entirely static water has equal potential. A special equal potential 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 use of the groundwater
agencies. Instead, use can be made of an existing and well-established reference network. Virtually
the only high accuracy reference network existing in the country is GTS (Great Triangular Survey)
benchmarks. Survey of India meticulously administrates and maintains the GTS benchmarks. The
GTS accuracy and spacing are presented in Table 1.2
Reference point type Relative accuracy Spacing
GTS 1
st
order, VVP 1 mm @ 1 km 100 to 500 km
GTS 2
nd
order, VP 3 mm @ 1 km 50 to 80 km
Tertiary, double run 12 mm @ 1 km 20 to 30 km
Tertiary, single run 24 mm @ 1 km 20 to 30 km
Legend: VVP Very Very Precise
VP Very Precise
Table1.2: Basic reference point accuracy
The accuracy degrades with the square root of the distance in kilometer. E.g. at a distance of 4 km
the GTS 2nd
order accuracy is 6 mm.
The relative accuracy of the reference points is only of importance if more than one reference point is
used within a single aquifer area. However, it is recommended to connect the levelling 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, the smaller the
error per km is.
2 CONVENTIONAL SURVEYING TECHNIQUES
Conventional levelling is executed by automatic mechanical (non-electronic) levelling instruments.
State-of-the-art surveying instruments comprise a lot of electronics. Often a single board personal
computer is built-in to enhance the performance of the instrument and to cater for data recording.
2.1 LEVELLING METHODS
Levelling is the operation of determining differences of elevation by measuring vertical distances
directly on a graduated rod with the use of a levelling instrument such as a dumpy level, or theodolite.
This method is called direct leveling or differential leveling. Indirect leveling can be done using the
principle that differences in elevation are proportional to the differences in atmospheric pressure. The
difference in elevation between two points can also be determined trigonometrically, using vertical
angles and horizontal or inclined distances. The benchmark Reference Point is also very important in
levelling. The bench marks are also permanent objects of known elevation located by Irrigation, Public
Works Department, Railways and Highway Authorities with details of their locations. These are
usually concrete posts set close to the ground, where there is least likelihood of disturbance.
2.1.1 THE DUMPY LEVEL
The Dumpy Level is probably the most common surveying instrument. The Dumpy is set up on a
tripod and needs to be level. This is done by the use of adjustable legs and fine control screws on the
base of the instrument (which is essentially a spirit level with a telescope attached). Once the
instrument has been levelled it provides a 360° horizontal plane of view. This can then be used in

conjunction with a measuring staff (a very large ruler) to measure the height of any point on the site.
To set this up, a "backsight" is taken on a known height related to the sea level (this point is normally
taken from a known GTS benchmark (see Figure 2.1). A "foresight" is then taken on the particular
feature or object being recorded. The difference between the backsight and the foresight is then
added to the known height and value for the height of the object above (or below) sea level can be
attained. Each increment may cover a distance up to a maximum of 200 m. It is estimated that on
relatively flat terrain, the levelling instrument may have to be shifted and set up about 5 to 10 times.
To obtain the specified accuracy, the survey team should be skilled and execute the work
meticulously.
Figure 2.1:
Principle of levelling with Dumpy Level
The levelling quality is monitored and increased by applying double run levelling, i.e. each sub-
trajectory is levelled from a reference point to a new point at some distance and back to the reference
point. The difference in level between the two runs should fall within predefined accuracy
requirements.
Recently, electronic levelling instruments, denominated digital levels, have become commercially
available. Such instruments require very little adjustments by the surveyor. The digital level
automatically takes the staff reading and records it, together with administrative and identification
data. It requires a special levelling staff, which has a face with a bar code pattern precisely, printed
over it. To obtain a level reading, the digital level observes and analyses the image of the bar code.
The other face of the staff may have a conventional scale to allow manual reading.
2.1.2 TOTAL STATIONS
The 'Total Station' is an electronic theodolite with 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 is calculated from the vertical angle and the range. The co-ordinates are
calculated from bearing and horizontal range. The Total Station stores the information digitally so that
all the information can then be processed through a computer. A theodolite once set up and levelled,
not only can measure in a horizontal plane in a similar manner to a Dumpy Level, but it can also
measure in a vertical plane. Once measurements can be taken in three dimensions, several
properties can be measured e.g.: height, angle of slope and distance.
2.2 REFERENCE SURFACE
Due to the very principle of the levelling instrument, the instrument reference plane settles itself
parallel to the local geoid surface. The height obtained by levelling (H) is the orthometric height.
Levelling heights are expressed as elevation above to MSL, i.e. height above the geoid. For at least
one reference point in the area, the elevation above MSL should be known.

2.3 ACCURACY
Table 2.2 summarises for each type of instrument the levelling accuracy in double run engineering
mode. The figures assume engineering grade levelling. For Automatic Level and Total Station, higher
accuracy is possible than indicated in Table 2.1, but cost increases rapidly with better accuracy. To
obtain the accuracy for a certain levelling distance (L km) the presented accuracy figure should be
multiplied by √L. Over distance of 9 km with an Automatic Level Instrument, the estimated error would
be 5√9 =15 mm, that is less than 2 mm/km.
Instrument Accuracy Range
Automatic Level 5 mm @ 1 km 100 m
Digital Level 1 mm @ 1 km 100 m
Total Station 15 mm @ 1 km 2500 m
Table 2.1: Indicative instrument accuracy and range
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 single prism suffices. Larger
ranges require multiple prisms. Second order effects caused by undulation of the geoid shape,
adversely affect the accuracy of the Total Station over larger distances. A precise estimate cannot be
made, as no geoid shape data are available. However, it is assumed that these effects are
insignificant.
2.4 PRACTICAL ASPECTS
The methodology and use of the Automatic Level is rather straightforward and well understood.
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, the daily
coverage primarily depends on transport efficiency. The possibility of making a mistake is 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 per observation. 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 other terrain types, such as woodland,
and along winding roads this may limit the coverage. The possibility 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 surveyor and
labourers are not far apart and can easily communicate with each other. In case of the Total Station,
much larger distances are common practice, so for effective communication between surveyor and
labourers walkie-talkies/mobile phones are required. The transport should also be well organised to
benefit of the speed and efficiency of the Total Station. The Total Station delivers accurate co-
ordinates as a side product.
2.5 STAFF, DURATION AND COST
For each of the instrument types a team of 4 to 6 persons can effectively execute the levelling. The
teams may comprise a surveyor with a labourer to carry the instrument and an umbrella, and a
labourer for each staff/prism. For transport, a vehicle and a driver are required.

The time taken for Automatic Level coverage could be around 6 km/day, Digital Level 12 km/day,
while using Total Station it can be as much as 24 km/day.
With Digital Levels, the daily coverage can be about twice as much as with Automatic Levels provided
that proper transport is available. From the figures above the total throughput time can be calculated
for a given number of teams.
The daily coverage of a Total Station can be quite large, in particular in flat terrain. In sloping terrain,
the coverage depends on the line of sight conditions. In flat terrain, the functioning of the Total Station
is not hampered by the slope but the coverage largely depends on the transport times.
The cost components of the levelling comprise:
1. instrument investment costs
2. operational costs including:
• cost of staff
• cost of transport
The operational costs depend on the cost of staff, labour, and transport. The larger the speed of
levelling, i.e. the daily coverage, the lower the cost per well.
To speed up progress, more teams should be deployed.
2.6 TRAINING
It is assumed that little or no training is required for the use of the conventional Automatic Level
instruments.
Although not complicated in its application, a few days of training may be required for the introduction
to Digital Level Instruments. In particular the concept of the instrument, the use of digital 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 familiar with
the methodology. The surveyors may benefit from training in the operation of the Total Station and
data transfer to a PC. The accuracy aspects should also be covered, in particular the effect of
meteorological conditions on level reading accuracy.
3 GLOBAL POSITIONING SYSTEM (GPS)
3.1 GENERAL
The Global Positioning System (GPS), delivers x,y,z co-ordinates and time, worldwide. The GPS
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 earth bound co-ordinate
system. Each satellite transmits its co-ordinates, precise time and other data to the receiver.
From the received data, the GPS receiver can calculate its distance (pseudo range) to each satellite.
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 in the 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 the control
system, the satellites and during radio wave propagation to the receiver. In this context, it should be
noted that the GPS system supports two accuracy classes, one for military and another for civilian
use. The civilian accuracy is purposefully degraded by manipulation of transmitted data. The
propagation speed of electromagnetic waves from the satellites to the receivers is not constant but
varies with amongst others the amount of free electrons 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 150 to 200 m vertically.
A differential GPS technique that largely improves accuracy deploys two (or more) receivers
concurrently. One receiver, the reference or base receiver, is operated at a benchmark with known
co-ordinates. The other receiver, the mobile or roving receiver, is used to measure at the unknown
points. By combining the data obtained from both (or more) receivers, many of the errors that are
common to both receivers, e.g. fluctuations of wave propagation speed, can be largely reduced.
Further, the effects of the civilian accuracy degradation can be virtually completely removed. Both
receivers should have at least four satellites in common. This combination 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 the data from reference receiver at the mobile receiver.
In static mode, the GPS receivers are equipped with data loggers to record the received data for later
analysis. In particular in static differential mode, very high accuracy can be achieved.
GPS can offer several accuracy grades, depending on the receiver technology. Below, three
frequently applied receiver technologies are summarised. In all three technologies, two or more
receivers are deployed in a kind of static differential mode. The reference receiver is installed at a
point with known co-ordinates and height above MSL. The mobile receiver is deployed 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 many satellites as possible have to be observed. Both
receivers should have at least four satellites in common. The receiving and data recording process
may be continued for a few minutes up to several hours, depending on the required accuracy and
receiver characteristics.
Subsequently, the recorded data are retrieved from the receivers and processed at a convenient time
and place. The processing yields co-ordinates and heights of the measured points and quality
indicators. Applying a proper geoid model, the heights can be converted into RL values for the
observation wells.
The basic receiver technologies are summarised in the following.
• Travel time or code phase receiver.
This type of receiver merely measures the travel time of the coded satellite data messages, i.e.
the time between transmission by the satellite and the reception by the receiver. In differential
mode (DGPS) the accuracy is about 1 m horizontally and 1.5 to 2 m vertically.
• L1 receiver.
The L1 receiver not only measures travel time but also the phase of the L1 radio carrier wave.
This significantly enhances the accuracy both horizontally and vertically. The height accuracy is
in the order of 20 mm plus several ppm over distance, (1 ppm is equal to 1 mm/km). As a rule of
thumb, the maximum baseline length (distance between base station and mobile station) should
not be more than some 15 km. Beyond that distance accuracy will decrease rapidly, amongst
others due to unresolved phase ambiguity; at large separations the system cannot reliably
distinguish between successive L1 wave lengths. Longer baselines could be split into practical
sections and intermediate measurements could be taken. Measurements at intermediate
positions cost extra time for set-up and receiving. Moreover, accuracy is adversely affected.
• L1/L2 receiver.
This receiver type measures phases of the L1 and the L2 carrier waves. This combination gives
the receiver a much larger operational range without the non-resolvable phase ambiguity
restriction. At short baselines, the observation time can be a few minutes or less. At larger

baselines, beyond 15 km, observation time increases to say 15 minutes to 1 hour. Accuracy
improves to sub centimetre levels plus about 1 ppm (1 mm/km) over distance. The L1/L2
receivers are most expensive.
GPS height (h) is not relative to MSL but relative to an ellipsoid, the aforementioned WGS84, 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 be known. Commonly, the following
equation is used to convert from ellipsoid height to geoid height:
H = h – N (3.1)
where: h = ellipsoid height
H = geoid height
N = 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. Advanced models yield a relative
accuracy in the order of 1 to 4 mm/km. In absolute terms, very good models have estimated errors of
±0.3 m. Both error components, absolute and relative, are being 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. The model grid
has a spacing of 0.25 degree (N and E). Its accuracy is insufficient for levelling purposes. However,
the EGM96 model could be used to assess the separation between geoid and 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 (Great Triangular
Survey) geodetic points, which could be used as reference points for GPS assisted levelling.
The accuracy requirements of RL values, which depend upon the application of the data, were
explained in Chapter 1. Depending upon the implemented technology, GPS survey can yield level
accuracy better than 10 mm/km. High accuracy receivers (GPS L1/L2) and precise methodologies
can deliver an accuracy of 10 mm plus 1 mm/km (1 ppm). Moderate accuracy receivers (GPS L1)
may have an accuracy of 20 to 50 mm plus 2 to 5 mm/km (2 to 5 ppm). In the 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 accuracy would
be 50 mm + 10 x 5 mm = 100 mm, i.e. 10 mm/km.
Most GPS L1 receiver types would deliver 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 cost receivers 3 to 5
m (see Table 3.1). Simple hand held GPS receivers deliver, for a single measurement and without
combination with a reference receiver, an accuracy of about 100 m horizontally and 150 m to 200 m
vertically. It should be noted that the stated accuracy is relative to the ellipsoid WGS84.

Instrument Accuracy indication
(relative to ellipsoid)
Range
DGPS 1 m ≥1000 km
GPS L1 20 mm + 5 mm/km 15 km
GPS L1/L2 8 mm + 1 mm/km 100 km
Table 3.1: Indicative accuracy and operational range
The horizontal positioning accuracy of the GPS receivers is better than the elevation accuracy and for
use under HP, it does not need enhancement. The conversion from ellipsoid to UTM is rather
straightforward and formulae, constants and software are readily available from many sources. It
should be noted that the accuracy pertains to the measurement relative to the base station.
3.2 Practical aspects
After a geoid model has been established, levelling by GPS can be fast, but it is currently not in vogue
in the country.
For conversion of GPS heights to MSL, the separation between ellipsoid and geoid datum is to be
established by taking GPS observations at reference points. If the ellipsoid and geoid surfaces are
parallel to each other, at least within a small margin, then one GPS observation at a 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 and tilt (East and North).
To establish such a plane, GPS observations from three precisely known (co-ordinates and elevation
relative to MSL) reference points are needed. The reference points, e.g. GTS benchmark 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 the area
enclosed by the GTS benchmark points, can be converted to geoid levels by linear interpolation. It
should be noted, that the GPS observations have to be collected at the point of interest, e.g. at the
observation wells, while simultaneously the base station is operated at a GTS benchmark point.
In coastal areas, this methodology could be feasible for groundwater application if the local geoid
gradients along N and E directions are relatively constant over the area covered by the GTS
benchmarks. As an indication, the geoid gradient should not vary by more than 5 mm/km. If GPS data
and MSL levels from more than three GTS benchmarks would be available, then a more complex
model could be developed to achieve more accurate level conversion.
Generally, GPS elevations (h), relative to the ellipsoid, have an accuracy that is 1.5 to 2 times worse
than the horizontal accuracy (E, N). The main reason is that for elevation data, only satellites above
the observation point can be received. The other satellites are below the horizon 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 the sky,
must be adequate to ensure accurate results. The receivers and/or the post processing software can
detect the quality of the geometry and may neglect data of poor geometry. The geometry can also be
affected by local conditions, e.g. the receiver is surrounded by high buildings, covered by tree canopy,
etc. For best results, each receiver should have an unobstructed 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 to MSL, largely
depends on the accuracy of the model that is used to convert from ellipsoid data into geoid data.
The accuracy of the GTS benchmark points directly affects the final results.
A team of about 2 persons can effectively apply the GPS equipment in the field. During the data
collection by the GPS receiver, the ToC level of the well and possibly other points/bench marks may
be connected to the GPS receiver. For that and depending on the site conditions, a simple levelling
instrument might be useful. As large distances have to be covered daily, a vehicle and a driver are
required.
The instrument costs are directly related to the accuracy of the equipment.
The GPS L1 receiver has an effective range of about 15 km. To cover larger ranges an incremental
approach should be adopted, much like with levelling. The increments can be 10 to 15 km. This range
is measured as a straight line from the reference receiver to the mobile unaffected by intermediate
obstructions. Obviously, the intermediate measurements take more time.
The GPS L1/L2 receiver has a much larger operational range and hence very few intermediate
measurements are required. It is estimated, that, given the prevailing terrain and transport conditions,
the daily coverage of a GPS L1/L2 receiver set is about 33% higher than that of the GPS L1 receiver
set.
For the GPS L1 receiver it is assumed, that the maximum time on site is 1 hour and for the GPS L1/L2
receiver the onsite time is set at 45 minutes. The base receiver is set-up at the beginning of the day
and recovered at the end of the day. Only a guardsman should stay with the base receiver.
The GPS technology is rapidly being enhanced with new features, increased accuracy, etc. As a
result GPS receivers of present day state-of-the art are quickly replaced by more advanced receivers
which reduces the prices of old technology receivers.
Recent developments in the GPS industry resulted in accurate, reliable and easy to use instruments.
Taking measurements virtually has come down to setting up of the receivers and switching them on in
data logging mode. Because of this ease of use, only limited GPS knowledge is needed for field
operation. Hence, training requirements for field operation are rather limited.
Planning, quality control and data processing require a rather detailed understanding of the GPS
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 GPS principles, planning, data processing and
datum shift e.g. to UTM and geoid. Such training, covering theory and practice, may take about 1
month.
4 AVAILABLE OPTIONS IN SURVEYING TECHNIQUES
4.1 HIGH ACCURACY SURVEYS
The requirements for the high accuracy surveys can be met by using the automatic level or digital
level equipment without special measures. Also a Total Station featuring a vertical angle 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 between the
ellipsoid and the geoid. In case the separation has a constant slope, then a simple linear interpolation
(plane surface) would deliver an accurate local geoid model. Unfortunately, a geoid model or detailed
separation data is not available. Consequently, a proper error assessment is not possible. A rough
estimate is that at any point in the Penninsular India the separation gradient is less than 40 mm/km.
Hence, starting form a reference point with known level, e.g. a GTS benchmark, the added error 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 GTS benchmarks in
the Project Area. Based on these data a model could be developed. Assuming that such a model is
available with sufficient accuracy, the GPS L1 receiver could deliver the required data, provided that
the surveyed points are at least several kilometres apart and within a distance of not more than 15 km
from the reference point. In many cases, intermediate measurements are needed because the
average distance between GTS benchmarks in Penninsular India is about 21 km. In quite a few
cases, the distance between GTS benchmarks is in the range of 30 to 50 km. The GPS L1/L2 receiver
could deliver the required accuracy at short and long distance.
4.2 MODERATE ACCURACY SURVEYS
The accuracy specification for the moderate accuracy surveys is 50 mm/km. Technically, all the three
discussed surveying instruments (Auto Level, Digital Level and Total Station) can meet the accuracy
requirements. Assuming that the geoid separation slope is less than 40 mm/km it can be concluded
that both geodetic GPS receiver types (L1 and L1/L2 receivers) can deliver the required accuracy
without difficulty. Still, a simple separation model would enhance the data quality against little effort.
5 COMPARISON OF TECHNIQUES
Conventional surveying is a well-understood technique, which can meet the accuracy requirements. A
disadvantage is the relatively small coverage.
Application of Digital Levels would increase the obtained accuracy at a significantly higher coverage.
A Digital Level though, is more vulnerable, costlier and requires training in its use and understanding
of some automation aspects. Investment costs are higher than for the conventional levels and staffs.
Personnel requirements are similar to conventional levelling.
A Total Station offers less accuracy than properly executed conventional levelling. Daily coverage can
be much higher though. The instrument is more vulnerable, costlier and requires training in the basic
concepts, instrument use and meteorological effects on accuracy. Further, understanding of some
automation aspects has to be obtained by the surveyor. Investment costs are considerably higher
than for the conventional Automatic Levels and 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 to MSL/geoid
but height relative to the WGS84 ellipsoid instead. The ellipsoidal heights have to be converted to
geoid heights. For that, a geoid model is required. Presently there is no national Indian geoid model
available to the CGWB and SGWDs in Penninsular India. Consequently, such a model should be
established for each area where GPS levelling is applied. 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 responsible for data processing,
planning and quality assurance should have a high level of education and training. The field teams
could consist of a surveyor and an assistant. Further, a vehicle is required.
6 MANUFACTURERS AND REPRESENTATIVES
Table 6.1 lists GPS equipment manufacturers, which are represented in India. This table may not be
complete. Some of the representatives also have survey equipment in their product line, which is
regarded as an asset.
Manufacturer Local representation Survey
instruments
Ashtech Scientific Instrument Co. Ltd. no
Leica Elcome Technologies yes
Trimble Mekaster yes
Sokkia Toshni-Tek International yes
Table 6.1: Known GPS equipment manufacturers represented in India

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