An Assessment Of The Existing Continuously Operating Reference Stations (CORS) In Nigeria An Exploration Using Geographical Information System (GIS

American Journal of Geographic Information System 2014, 3(4): 147-157
DOI: 10.5923/j.ajgis.20140304.01
An Assessment of the Existing Continuously Operating
Reference Stations (CORS) in Nigeria: An Exploration
Using Geographical Information System (GIS)
Ali I. Naibbi1,*
, Samir S. Ibrahim2
1
Department Geography University of Portsmouth, Portsmouth, UK
2
Office of the Surveyor General of the Federation of Nigeria, Garki II Abuja Nigeria
Abstract The modernisation of the Global Navigation Satellite Systems (GNSS) has brought about significant changes
in the methods of positioning and navigation. More recently, countries are adopting the continuous observation and
measurements using a network of GNSS receivers called Continuously Operating Reference Stations (CORS) as the standard.
For example, Nigeria has established 11 permanent GNSS Network (NIGNET) or CORS since 2008, which has provided the
incentive for the adoption of the geocentric datum in all its geodetic activities. Unfortunately, the spatial coverage of the
Nigerian CORS stations on a distance of more than 300km (between the stations) seems inadequate based on the global
station densification standard of the International Terrestrial Reference Frame (ITRF). This study explores the CORS
network and determines the suitability of improving the density of the existing CORS stations with a view to extending their
spatial coverage in Nigeria. Data obtained from various sources are processed and analysed using buffer and Digital
Elevation Model (DEM) techniques. The result shows that the existing CORS stations in Nigeria only covers about 25% of
the country. Therefore, the study proposes new CORS stations that improve the distribution and the spatial coverage of the
existing stations (to about 96%) in line with the ITRF’s standard.
Keywords CORS, AFREF, Nigerian Geocentric Datum, GNSS, NIGNET
1. Introduction
The establishment of survey control points involves the
transfer of the control points closer to the site of the survey,
which provides a geospatial data infrastructure in the area,
and also assist the user with an easy access to reliable
controls. Survey control points helps in the sustainable
management of spatial data [1] and also plays a vital role in
Navigation, Geodetic Surveying, Mapping, Photogrammet
ry, Civil Engineering and Geographic Information Systems
(GIS). This present study investigates the existing surveying
control points used in Nigeria and offers recommendations
to improve their spatial coverage.
The national mapping of Nigeria relates to the old
systems of surveying (basic surveying procedures), which
depended on using chains, the odolites and traversing
techniques to establish the primary triangulation network of
the country in the 1960s [2]. These traditional survey
control systems are referenced to a non-geocentric datum
(based on Clark 1880 ellipsoids) referred to as the Minna
* Corresponding author:
sarkinaibbi@yahoo.com (Ali I. Naibbi)
Published online at http://journal.sapub.org/ajgis
Copyright © 2014 Scientific & Academic Publishing. All Rights Reserved
datum (L40) [2]. The L40 datum, which is regional in
nature, has the following adopted geodetic co-ordinates;
Latitude φ = 09° 38 09 N, Longitude λ = 06° 30 59 E,
Height H = 279.6m above the geoid [3]. The old survey
technique faces some deficiencies such as the absence of
Geoidal height model [4]. The Geoidal height model is a
mathematical model of the earth that uses gravity
measurements to correspond with the mean ocean surface
level on the earth [5]. Another deficiency that the old
survey technique faces is that of transformation parameters,
which involves the shift from one datum to another (local
datum to global datum and vice versa). These are difficult
to determine due to the old methods used in establishing the
datum [5]. This suggests that there is a need for an upgrade
(of the old survey techniques) to improve the accuracy of
surveys and mapping activities in Nigeria for suitable and
sustainable development.
The adoption of geocentric coordinate frames as a
national geodetic datum has now become the common trend
[2] in both developed (for example, United States of
America (USA), United Kingdom (UK), Austria)and
developing countries (for example, South Africa and
Kenya). For example, the space based technology made it
possible using satellite to establish geocentric datum [2].
The high level of developmental activities in Nigeria in the

148 Ali I. Naibbi et al.: An Assessment of the Existing Continuously Operating Reference Stations
(CORS) in Nigeria: An Exploration Using Geographical Information System (GIS)
areas of construction, power transmission, mechanised
farming and so on, suggests the need to expand and update
the Nigerian Primary Geodetic control Network (NPGN),
and employ the geocentric datum that are commonly used
in other countries. This will provide an avenue to meet up to
the world standard, as well as to satisfy user’s high
expectations (engineers, town planners, surveyors,
geographers etc.). Even though, Nigeria has already
established 11 permanent GNSS Network (NIGNET) of
Continuously Operating Reference Station (CORS), this
study examines the spatial coverage of these existing CORS
network (as part of the Nigerian Geocentric Datum (NGD)),
with a view to suggesting ways of improving their standard
in line with the International Terrestrial Reference Frame
(ITRF).
1.1. Geographical Location of Nigeria
Nigeria lies between latitudes 4° and 14° N and longitudes
2° and 15° E, and is located in the western part of Africa [6].
The country has a total area of about 923,768km2
making it
the 32nd
largest country in the world [7]. It shares about
4,047km borders with four countries (Benin Republic to the
west, Niger Republic to the north, Chad to the north-east and
Cameroon to the east- see Figure 1). It has a coastline of at
least 853 km that stretches along the Atlantic Ocean in the
southern border.
Nigeria has a varied landscape; to the southwest of the
Niger is a rugged highland and to the southeast of the Benue
is the Mambilla Plateau, which forms the highest Plateau in
the country. The highest elevation point in Nigeria
(2,419metres above sea level) is located in Chappal Waddi
in the northern state of Taraba. The country has two major
rivers, namely; the River Niger and River Benue that
converged in a “Y” shaped valley in Lockoja, and emptied its
waters into the Atlantic Ocean in the Niger Delta area of the
southern part of the country [6].
Figure 1. Map of Nigeria (Source: Adapted from Porter et al. [8], p.372)

American Journal of Geographic Information System 2014, 3(4): 147-157 149
2. Background
2.1. Continuously Operating Reference Station (CORS)
Nicholas ([8]) defines CORS as Continuously Operating
Reference Station, which can take the place of a traditional
base station used in differential GNSS positing. CORS can
give an instant high positional accuracy of ± 20 mm. CORS
Network began in 1994 when NGS officially installed
permanent GPS receiver in the National Institute of
standards and Technology, Gaithersburg Campus, Maryland,
and few months later in Colorado, which were later
incorporated with three other continuously operating GPS
stations that are originally part of CIGNET to become the
first five stations in CORS Network ([9 -10]). By 2000, more
than 200 CORS stations are installed and incorporated into
the NGS’ CORS Network. By the first quarter of 2014, the
CORS network increased to more than 1,900 sites, which is
why it is now regarded as the primary source for geodetic
survey community to access National Spatial Reference
System (NSRS) data.
CORS networks are now established in different countries
and by different organisations due to its high geodetic data
accuracy (± 20 mm). The development of CORS networks
are categorised into three tiers as noted by Rizos ([11], p.10).
These are as follows;
(1) Tier 1 stations are the stations with International
Global Service (IGS) grade 1 standards
(2) Tier 2 stations as National Geodetic Network, and
(3) Tier 3 stations are networks established by private
institutions, state and local agencies.
Schwieger et al. ([12], p.7) noted that the Tier 1 stations
are high accuracy stations (± 20 mm) with class 1 geodetic
receivers and well stabilised antenna monument capable of
tracking any Regional Navigation Satellite System (RNSS)
and GNSS signals. These stations are used to define and
establish global reference frame and are also used in
scientific research and geodetic sciences. Similarly, the Tier
2 are also high accuracy stations, but are maintained at state
and national government levels. They are also equipped with
good quality geodetic receivers and their antennas are
stationed on a stabilised monument. They can also track
signal broadcast from GNSS and RNSS. These stations are
mostly established for the purpose of national geospatial
reference frame (geocentric datum). Conversely, the Tier 3
are high stations but with minimum requirements compared
to tier 1 and 2 stations. The Tier 3 stations are mostly
established by local government, state, private organisations
and agencies for positioning purposes. They are also referred
to as “fit for purpose” stations. They are equipped with
receivers that can track interoperable GNSS and RNSS
signals.
CORS stations are now established as networks that define
regions at local, continental and global level due to its
stability, continuity and effectiveness [9]. However,
associations of scientists, geodesists and other earth
observing bodies that form the first global reference frame
(consisting of GNSS sites) established specifications for
establishing CORS stations across the globe through the
ITRF [12]. The ITRF reference systems are made to be a
standard for all geodetic and earth science fields (see for
example, [13-15]). The main task of the ITRF as noted by
Schwieger et al. ([12], p.2) is to” maintain the stability and
to provide open access to the geometric and gravimetric
reference frames as well as time series of data and products,
by ensuring the generation of uninterrupted state-of-the-art
global observation”. It should be noted that all the stations
are situated in open accessible areas (especially public places)
to provide security and less obstruction to the sky by tall
buildings and trees, which may cause multipath errors and
reduces accuracy.
2.2. Nigerian GNSS Reference Network (NIGNET)
Reflecting on the geodetic datum (Minna datum (L40) in
Nigeria [2], which is not compatible with satellite derived
positioning system, suggests that the new geocentric datum
(CORS networks) should be adopted. This need prompted
the Office of the Surveyor General of Nigeria (OSGOF) to
initiate the NIGNET project in 2008 that derive the
establishment of the 11 CORS stations in various locations
in the country (see for example, [2] & [17]). These stations
are coordinated by nine international Global Navigational
Satellite System (GNSS) and IGS that are part of the
International Terrestrial Reference Frame (ITRF2008) [2].
However, since the establishment of the 11 CORS stations
in the country covering only 25% of the country, no attempt
has been made to upgrade and extend their coverage.
Conversely, developed countries with more CORS stations
focuses on both improving the integrity of their CORS
stations as well as improving the distribution and density of
the spatial coverage of these stations. Such countries include
Australia with about 250 CORS stations at regular intervals
of between 50km to100km [11], United States of America
with over 1,350 stations (as of 2008) at an average spacing of
70km between stations [9-11] & [18], and United Kingdom
with about 110 stations at an average spacing of 60km [19].
Naturally, the availability and spatial coverage of the stations
increases the number of IGS stations globally to strengthen
the ITRF. Overall, geospatial applications are better with
higher spatial coverage (density of the stations), because
higher density provides good reception coverage for
positioning results, prediction and monitoring of natural
disasters.
In 2007, prior to the establishment of CORS by OSGOF,
the GPS team of OSGOF obtained the WGS 84 values of
L40 (Nigerian Datum) by repeated GPS measurements using
differential positioning techniques on the station (L40) [1]. A
mean of means was obtained from the measurements and the
value was used for adjusting a nationwide surveying
campaign that commenced from Minna in Niger state,
Nigeria [1]. By the end of 2007, the Regional Centre for
Training in Aerospace Surveys (RECTAS) established the
first permanent reference station (CORS) in Ile Ife, Ibadan,

Nigeria, which consist of a receiver, an antenna, operating
software, site server software for quality control and data
analysis (see Figure 2). This was done in line with the
African Reference Frame’s (AFREF) project initiative [20].
Figure 2. First CORS station in Nigeria (Source: [20])
The 11 CORS networks in the country are linked with
ITRF2008 by acquiring GPS data from nine International
GNSS Service stations (IGS) as shown in Table 1 [2]. The
new reference frame that this present study is proposing will
also be linked with the existing reference frame of Nigeria to
enhance understanding and preserve legacy for geospatial
data, and will create a platform for use in modern space
techniques of positioning [21].
Table 1. IGS Stations used
Station ID Station location Country
Ellipsoidal
Height (m)
HARB Pretoria
Republic of
south Africa
1555.0000
NKLG Libreville Gabon 31.4800
RABT Rabat Morocco 90.1000
RBAY Richards bay South Africa 31.7927
SUTH Sutherland South Africa 1799.7659
CAGZ Capoterra Italy 238.0000
MAS1 Maspalomas Spain 197.3000
NOT1 Noto Italy 126.2000
SFER Sanfernando Spain 85.8000
Source: Nwilo et al. ([2], p.5)
NIGNET, which is a tier 1 station, was set up with
up-to-date geodetic equipments. The data from these stations
are processed at the central station located in Abuja where it
is corrected, computed and provided to the users [21]. The
optimisation of efficiency is a priority to the system and their
location, which are mostly positioned on concrete rooftops
for security reasons and signal obstruction [18]. Figure 3 is
one of the 11 CORS and Figure 4 shows the locations of the
11 existing stations across Nigeria.
The objectives of creating the 11 CORS stations by the
OSGOF is to cover the country with a relatively homogenous
distribution of CORS stations in order to increase the spatial
coverage of the networks and to provide an up to date
networks that are compatible with GNSS, GIS and ITRF
based datum [21].
Figure 3. Trimble R8- CORS at OSGOF roof top Abuja (Source: [22])
Overall, the existing 11 CORS stations in Nigeria (with an
average spacing distance of over 300km apart) seems
inadequate given the size of the country (about 923,768km2
)
as compared with the less than 100km spacing (based on the
NGS’s requirements)adopted in the USA and United
Kingdom. Therefore, increasing the number of the CORS
network in Nigeria as suggested in this study will improve
the signal multipath issues, site issues, and signal
environment issues experienced in the country. Also,
sufficient and well-defined networks of geocentric datum
(CORS) will provide among others; direct compatibility with
GNSS measurements, mapping and GIS. It will also promote
and optimise the use and application of spatial data through
single user friendly data environment, thereby increasing
efficiency in spatial data resource by reducing duplication
and unnecessary translation ([4], p. 979).
3. Methods
3.1. Data Sources
The data used for this study are obtained from different
secondary sources. A vector map of the Nigeria’s
administrative boundary, Nigerian Primary Geodetic
Network (NPGN) GPS control points as well as the existing
CORS stations datasets are provided by the OSGOF. Other
data sources include the U.S. Geological Surveys (USGS)
Earth Explorer, Geo Community and Diva-GIS (see table 2
for more details).
The data obtained from Earth Explorer are two satellite
derived digital elevation models (DEM) (Global
Multi-resolution Terrain Elevation Data 2010)
(GMTED2010) of northern and the southern Africa, which
has an improved detail elevation data, spatial resolution and
topographic data. The satellite image obtained is a 7.5
arc-seconds resolution and has a Root Mean Square Error
(RMSE) of between 26 and 30 metres. The Geo Community
data are vector files of road networks, rail network and water
bodies of Nigeria. The Diva-GIS data includes airports and
elevation shape files of Nigeria.

Figure 4. The Nigerian Permanent GNSS Reference Network (NIGNET) (Existing CORS stations) (Source: Adapted from Nwilo et al. [2])
Table 2. Summary of data and processing
Data Source Data Type Data format Data processing
Office of the
Surveyor General
of Nigeria
(OSGOF)
1) Base map of Nigeria’s
administrative
boundaries.
2) A vector map of
Nigeria’s boundary and
Nigerian Primary
Geodetic Network
(NPGN) GPS control
points.
3) Existing CORS stations
in Nigeria.
1) Portable Document
Format (PDF).
2) The vector map is
provided in a shapefile
format, and the NPGN
GPS points are provided
in a Microsoft office
Excel and Word.
3) The existing CORS
stations points are
provided in PDF format.
1) The administrative base map was
provided in PDF format, which is
georeferenced and digitized in
order toextract some important
features.
2) Both the administrative map and
the Nigeria boundary polygon
were set to WGS84 geographic
coordinate projection system.
3) The GPS coordinates the existing
CORS stations are then imported
into ArcMap as a text (*.txt) file,
and converted toEsri database files
(*.dbf),
U.S Geological
Surveys (USGS)
-Earth Explorer
Two satellite images of
derived Digital Elevation
Models (DEM)
Global Multi-resolution
Terrain Elevation Data
2010 (GMTED2010). The
images are 7.5 arc-seconds
resolution with a Root
Mean Square Error
(RMSE) of between 26 and
30 metres.
The DEM (GMTED 2010) are two
separate images, which are mosaicked
and clipped with the boundary map of
Nigeria to trim the image to the area of
interest. The DEM images are later
converted to points heights.
Geo Community
data
Nigeria’s Road Networks,
Rail Network and Water
Bodies
Vector shapefiles

3.2. Data Preparation and Analysis
3.2.1. Data Processing
The processing of the collected data to map the existing
CORS stations and the proposed sites for the establishment
of new ones is summarised in table 2. From the table, the
various data formats are standardised. This enables the
integration of the various data sets into the research process.
However, an interoperability issue was observed in the
datasets, which are presented in both vector and raster
formats, and most of them are in different referencing
formats as they are acquired from different sources. As such,
the files were converted to one projection system (World
Geodetic System 1984 (WGS84)). The study used ESRI Arc
GIS 10.1 software version to perform the analysis. This is
because the software allows the exploration of the various
data formats and has vast functionalities that suit the analysis
conducted here.
3.2.2. Data Analysis
The analysis commences by creating buffers of 100km
radius from the existing 11 CORS stations in the country in
order to observe their spatial coverage. This is because the
study adopted the United States of America’s CORS map
stations as a standard, bearing in mind the NGS’s guidelines
for establishing new CORS stations which states that
“proposed CORS will be no closer than 70km from an
existing CORS” ([18], p. 5). This cannot be overlooked, as it
is a requirement for establishing new CORS stations. The
buffers established from the existing CORS stations are then
converted to line features, and then to polygon features using
ArcGIS 10.1 software, which allows the calculation of their
spatial coverage in relation to the Nigerian area polygon.
Satellite elevation imageor Digital Elevation Model
(DEM) is used in proposing the location of the new stations
because CORS, unlike its predecessor (Theodolites) in the
field of surveying [13], are GPS instrument and do not use
line of sight for baseline establishment (triangulation). Also,
higher grounds have less obstruction of rarely more than 10
degrees above thehorizon [18].
The DEM, which is a raster image, has been converted to
points that allows for the determining of the height points
and further classifications. The points are then classified
according to the elevation difference. Care was observed
during the classification in order to obtain a smooth and
average spacing between the points. When selecting the
proposed stations, it occurred that points lower than
100metres close to the riverside appeared in the valley (water
bodies). Therefore, an effective measure was taken to move
them further away from the water bodies by increasing the
measurement of the height while avoiding random selection,
as station stability is a priority. Also, the majority of the
points are selected according to their proximity to public
facilities, such as road networks. This is because
accessibility is one of the priority factors in any
developmental project and one of the core objectives in the
guidelines for establishing bench marks and CORS stations
([18] & [22]).
However, the stationing of some of the points on low land
areas is to regulate the physical effect ofsoil characteristics.
For example, areas with coarse-soil are best siltation sites
because fine-grained or clay soil are susceptible to variation
in moisture content and susceptible to erosion [22]. These
areas (coarse-soil) are also easier to be accessed for
maintenance than the stations on highlands or hillcrests.
4. Results and Discussions
4.1. The Spatial Coverage of the Existing and Proposed
CORS Stations in Nigeria
As highlighted earlier, CORS are categorised in tiers and
the Nigerian CORS are operated under Tier 1 stations.
Therefore, this study used the NGS’s benchmark guidelines
for creating CORS stations, which was promoted by the
United States NGS ([18], P.6).
Figure 5 shows the coverage of the existing 11 CORS
stations in Nigeria at 100km buffer. From the figure, it can be
observed that the existing 11 CORS stations based on the
100km buffer covers only a quarter of the country’s area
(306,598km² covered and 605,131km² uncovered). This is
relatively poor at this radius and it is clear that there is a
requirement for an increase in the number of the CORS
stations to enhance the spatial coverage and distribution of
the stations in order to meet up with the global standard.
Hence, the need for proposing more condensed networks in
the country.
One possible way of achieving this is to propose at least
one CORS station in each of the 36 states in the country and
the capital city Abuja.The idea of using each of the states to
propose the allocation of the stations was derived from the
USGS’ High Accuracy Reference Network (HARN) (also
referred to as precise geodetic controls), which used similar
method in the late 1980s (see for example, [9]). Figure 6
shows the proposed stations in each of the country’s state
using the Nigerian Primary Geodetic Network (NGPN) GPS
points, which gives broader area coverage across the
country.
While there is an improvement of coverage (77% of the
total area of the country) from the existing CORS stations in
the country (see figure 6), the proposed state coverage is yet
to cover many parts of the country. Particularly, uncovered
areas are observed in the west-central, north-east and
south-eastern parts of the country. Therefore, despite
providing a remarkable result by tripling the spatial coverage
of Nigeria (see figure 6), the 23% uncovered by the proposed
state cannot be ignored, as it is almost the spatial coverage of
the existing 11 CORS stations in the country. Given that this
study intends to propose CORS stations that can cover the
entire country, there is also a room for improvement to
accommodate some of the uncovered spaces. Figure 7
shows a different criterion used to establish the CORS that

partially covered the entire country by incorporating the
elevation point data (DEM) of the country in the analysis.
The elevation data was used to classify the height
differences in order to locate suitable areas for the
establishment of the new CORS stations. Road networks and
water bodies are also used alongside the elevation because of
their importance in defining suitable locations. For example,
even if the height points are selected, the areas are unknown
without reconnaissance survey. The results of this selection
(see figure 8) show a spatial coverage of 96% (876070km²)
which is three times the spatial coverage of the existing
CORS in the country. The remaining uncovered areas (4%)
are fragments close to the borders and the River Niger.
Although, the areas can also be mapped out, site factors were
considered, particularly proximity of stations to avoid excess
signal multipath errors, and also to balance the distribution of
the stations at the minimum distance of 70km and a
maximum distance of 100km between each station.
Generally, establishing a station close to the road after
considering Right of Way (ROW) serves as a protection to
the station as land disputes and mowers may result in areas
not being public [22]. Moreover, stability of the station is of
utmost importance, as it has to be made on a stable ground or
building structure in order to avoid any future crustal
deformation. Water bodies can have a great impact on soil as
the flowing water can wash it away. Also, expansion and
contraction of the soil may occur, which may change the land
formation by centimetres or metres [22], thereby affecting
the accuracy of the CORS station. Such areas must be
avoided since CORS stations are millimetre accuracy points
(± 20 mm) [18]. Therefore, the water bodies and road layers
used here provide measures for avoiding the error of placing
the station points in a river, and the placement of the stations
close to the road for proximity and accessibility during future
maintenance.
Figure 9 shows the final output of both the 49 proposed
and the existing CORS stations in Nigeria. It provides an
improvement in the density and distribution of the stations
compared to the existing 11 CORS stations in the country.
The map shows that the coverage is denser in the southern
part of the country due to the nature of the administrative
boundary distribution, which has small area compared to the
northern part (see for example, [6]).
Figure 5. The spatial coverage of the existing CORS stations in Nigeria at 100km Buffer

Figure 6. The state proposed CORS stations using the NPGN GPS control points at 100km Buffer
Figure 7. The proposed stations using the elevation data in areas not covered by the marginalisation of the existing and the proposed state CORS
stations

Figure 8. The spatial coverage of the proposed and the existing CORS stations in Nigeria at 100km Buffers
Figure 9. The proposed map of Existing and Proposed CORS stations in Nigeria

5. Conclusions
Nigeria has already adopted the new geocentric datum,
which commenced in 2007/2008 by installing CORS
networks across the nation. This study used the international
standard criteria for CORS establishment, as well as the
objectives of the ITRF, AFREF and OSGOF to propose a
condensed and more robust spatial network of CORS
stations in Nigeria with baselines shorter than the existing
300km apart. However, the data sets used for the study are
obtained from secondary sources and are only verified using
other means such as the Google earth. Even though, the new
proposed CORS stations maps are unable to provide any
further detailed information about the landscape at each of
the proposed stations due to the large area coverage and
insufficient data, the study has highlighted the steps and
potentiality of improving the existing CORS coverage map
of the country. Therefore, the study serves as a template for
future applications and fields that requires spatial coverage
for their research or commercial use such as geospatial
institutions and communication institutions.
It should be noted that the study only focussed on spatial
coverage, which limits other aspects of establishing CORS
stations such as equipment specifications, data and
communication procedures to be considered in this study.
Also, there are many practical aspects of site configuration
(for example, site monumentation, obstructions etc.) ([10],
p.4& [18], p.7) that are considered during the establishment
of CORS stations that this study overlooked because of lack
of data.
So far, it is evident from the results that the spatial
coverage of the existing 11 CORS stations in Nigeria is
inadequate, and therefore the proposal for the establishment
of new ones is paramount. The new proposed 49 and the 11
existing CORS stations (totalling 60 CORS stations)
established here covered about 96% of the total area of
Nigeria as compared to the 11 existing stations that covers
only 25% of the country. Also, the proposed stations have
baseline distance of less than 200km apart, which provides a
denser and well-improved distribution of networks.
The proposed CORS networks can be linked to other
existing CORS network within and outside the country,
which can help reduce baseline dependent errors, improve
positioning services and continuous referencing of GNSS
measurements to the reference frame and datum, which is the
sole objective of ITRF (see for example, [12] & [23]). It will
also provide a common reference frame across the country
and other continents for optimal collection, storage,
exchanging and visualisation of information [12]. Similarly,
the availability of well distributed CORS stations in Nigeria
will make it possible to understand, expand and answer
relative GNSS and GPS questions since it is the common
standard technology in Navigation. This study opined that
updating the current CORS stations in the country would
accelerate the evolvement of the Nigerian terrestrial
reference system into a complete NIGNET of 60 stations,
which would potentially strengthen the Nigerian Primary
Geodetic Network (NPGN).Overall, achieving a wider
spatial coverage of the CORS networks (at near regular
intervals), particularly now that the government is
advocating for new reforms on infrastructural development
across the nation is paramount.
ACKNOWLEDGEMENTS
The authors wish to thank the OSGOF, the U.S.
Geological Surveys (USGS), Geo Community and Diva-GIS
for the data used in this study. We also like to thank the
reviewers for their useful comments and suggestions.
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