3-D READJUSTMENT OF PARTS OF THE NIGERIAN PRIMARY TRIANGULATION NETWORK WITH GNSS DATA

3-D READJUSTMENT OF PARTS OF THE NIGERIAN PRIMARY TRIANGULATION NETWORK WITH GNSS DATA

ABSTRACT

Current development has shown many countries improving and strengthening their national control networks using modern space geodetic system. This study carried out 3D readjustment of part of the Nigerian Primary Triangulation Network using GNSS data obtained from Office of the Surveyor General of the Federation (OSGoF). First, the geometric analysis of the existing Nigerian Primary Network were evaluated using Triangle Inequality theorem and on how well the triangles in the network are conditioned. The result revealed that the network fulfilled the condition of the theorem however, when subjected to how well-conditioned the triangles were within the network, it was discovered that 56% of the triangles met the requirement while 44% did not meet the geometric conditionality. Different processing strategies are capable of giving different coordinate solutions for same point. Using fifty-two (52) GNSS station observational campaigns carried out within the period of October, 2010 – April, 2011, the study performed comparative evaluation of three different GNSS post-processing strategies with respect to points of reference originally processed with BERNESSE software from OSGoF. These processing strategies include; reducing observational campaign observed from pairs of stations (baselines) and combining these baselines into a network (Approach 1), taking GPS observations observed simultaneously at all stations directly into a network adjustment where all the coordinates of the network are presents as unknowns (Approach 2) and lastly, processing the observations using Precise point Positioning techniques (Approach 3). Due to the dissimilar nature of positioning, Trimble Total Control software was used to process Approach 1 and 2 solution (Relative solution approach) while GNSS-lab tool (gLAB) was used to process Approach 3 (Stand-alone solution approach). The residual (differences) in the horizontal and vertical component were computed for all observations. Out of the three solutions, Approach 3 gave solutions that were closest to the points of reference, followed by Approach 1 and then Approach 2. Poor performance of Approach 2 was attributed to some restraining factors that considerably induced errors within its solutions. Improvement on the study will be on how to develop a standard approach for harmonizing GNSS solutions in the near future.

CHAPTER ONE

INTRODUCTION

1.1         Background to the Study

A reliable coordinate reference system is a basic requirement for the successful execution of all survey related projects. The definition and densification of coordinate reference systems that serve as a common reference framework of control points is hinged on points whose 3D positions are known to a high degree of accuracy permitting the many and varied surveying, mapping and charting programs to be referenced to that common systemfor various types of user (Acheampong, 2008).

Geodetic control networks are established by various methods. The classical (traditional) methods are traversing, triangulation and trilateration (Schofield and Breach, 2007). Provision of control points defining these reference frames by these conventional methods are expensive, tedious, limited to intervisibility between beacons and the area of survey, thus reducing the effectiveness of networks at night and in poor weather conditions.

Modern methods include the use of satellite techniques such as the Global Navigation Satellite System GNSS), and satellite altimeters. Satellite techniques can be used to establish and densify three-dimensional networks (Latitude, Longitude and ellipsoidal height) without the need to measure angles and distances between intermediate points more rapidly, with greater accuracy and less difficulty than terrestrial techniques (Poku-Gyamfi, 2009). Survey control could now be established almost anywhere and it was only necessary to have a clear view of the sky so the signal from the GPS satellites could be received clearly.

The Nigerian geodetic control network as shown in Figure 1.1 where green circles represents station points and the yellow lines represents the baselines connecting the stations together forming the control network at the time before the advent of modern space geodetic techniques appeared to meet most user needs were referenced to a non-geocentric datum (based on Clark 1880 ellipsoids) referred to as the Minna to furnish provisional coordinates of stations for mapping purpose as well as to assess the quality of the network. Heights used for the reduction of observations were also obtained.

However, the production of the Nigerian primary triangulation network has a number of inherent deficiencies resulting to serious distortion in the network. Some of these problems highlighted by Uzodinma (2005) include:

1. In-accuracy of the scale factor by compression of the Clarke 1880 ellipsoid, thereby causing defect in distances measured.
2. The origin of the Nigerian network is poorly defined

• There is absence of geoidal height model

1. Difficulties in the determination of the transformation parameters

According to Arinola (2006), most of the stations have neither been visited nor utilized in any manner since their establishment. During the last decades, many geodetic pillars materializing the reference frame have been destroyed, and only a small percentage of beacons are still usable. Also, in the existing framework, controls are very sparse in some areas of the country making it difficult to reference landed properties to the National grid. Another common problem with this network is the obliteration of the reference point monuments, which has rendered most of the points unusable as they are sometimes inaccessible. It is difficult and expensive under the conventional survey methods, to re-establish them after they have been tempered with or destroyed, usually done out of ignorance. Moreover with the distortion of about 10m discovered in part of the network, it cannot meet the requirements of new technologies such as GPS and capable of creating an irreconcilable problem during the implementation of a country wide GIS where data from different sources have to be integrated (Arinola, 2006).

The need strengthen and re-enforce the existing first order Nigeria Geodetic Network caused the geodetic community and the National Mapping Agency of Nigeria; the Office of the Surveyor General of the Federation (OSGoF) to examine the integrity and reliability of the existing geospatial/geodetic infrastructure to meet and satisfy modern and future needs by leveraging and utilizing advancements in GNSS having the adopted WGS 84 ellipsoid as its global datum which is consistent with the International Terrestrial Reference Frame (ITRF) (Dodo, Yakubu, Usifoh, and Bojude, 2011).

Measurements made from these solutions will provide direct compatibility with GNSS measurements and mapping or Geographical Information System (GIS) without the need for unnecessary transformation by spatial users and allow more efficient use of an organization spatial data resource by reducing need for duplication and unnecessary translation and reduce the risk of confusion as GNSS, GIS and navigation systems become more widely used and integrated into business and recreational activities in Nigeria (Isioye and Fajemirokun, 2011). These solutions can also serve as a base for the realization and implementation of the National Spatial Data Infrastructure (NSDI) and other geodetic network applications.

In summary, the conventional geodetic network is not compatible with the current satellite techniques, which does not ensure compatibility across various geographic, land and survey systems at the global level by a common coordinate reference system through which all types of geo-referenced information can be interrelated and exploited reliably (Isioye and Fajemirokun, 2011).

Therefore with the aid of GNSS technology, adjustment of part of the Nigerian primary triangulation network can be achievedfor the purpose of all related survey and other geodetic can serve as a backbone of new precise geodetic networks that makes positioning system easier and accurate enough to meet modern needs.

1.2         Statement of Research Problem

The Nigerian geodetic control network has served the mapping and cadastral needs of Nigeria well over the past sixty years. However, with the discovery of errors within the system which appears to increase away from Minna (triangulation point L40) the origin of Nigerian Survey (Arinola, 2006), inherent problems and limitations of the local system such as inefficiencies and difficulties to relate to modern system hence, the need to embrace a global reference system that is at harmony with all other countries of the World (Nwilo, Dodo, Edozie, and Adebomehin, 2013; Abudu and Adebomehin, 2016).

The development of Satellite technology, especially its application in geodesy through the use of GPS has opened a new vista in the observation and strengthening of Geodetic Control Networks worldwide (Arinola, 2006). With more satellites being deployed, modernization programs on-going to improve systems against interference, additional signals for better atmospheric modeling, and receiver costs becoming cheaper, satellite positioning techniques provide brighter future for datum definition and control networks densification. These facts coupled with accuracy, speed, adaptability and flexibility in operation make space-based technologies effective tools for the acquisition of geo-information (Nwilo, 2013).

Positioning with certain accuracy implies time transfer capability with comparable accuracy.The high demand for accurate, and reliable positioning using GNSS for many survey applications lead to the advent and wide usage of different positioning techniques. Global Navigation Satellite Systems provides various types of positioning state solutions.Positions can now be determined in different ways depending on what the position is determined with respect to defines the positioning mode. This competitive positioning approaches have varying significant benefits over each other in terms of operational cost and complexity, efficiencies and different level of accuracy for many spatial applications at various fields.Depending on specific use of data and type of work being performed, there will be different needs for the accuracy of the locational data for different surveying applications thus the need to ascertain the level of accuracy performance different types of GNSS positioning can delivered for proper survey application.

In summary, different GNSS positioning techniques gives rise to different solution quality in post processing mode and these positioning techniques are usually employed without adequate knowledge of the accuracy determination capability of the techniques. Hence, the need to evaluate their positioning performance in terms of accuracy and reliability for suitable application on the ever increasing GPS applications.

The study attempts to answer the following questions;

1. What is the geometric status of the existing primary network in Nigeria?
2. What are the points solutions based on baseline, network and precise point positioning approaches for the primary geodetic stations?

• What are the differences between the results realized by various solutions with reference coordinates?

1.3        Aim and Objectives

The aim of this research study is carry out 3-D readjustment of part of the Nigerian Primary Triangulation Network with GNSS data.

The objectives of the study are to:

1. Determine the geometric status of the existing primary triangulation network in Nigeria.
2. Generate GNSS point solutions based on baseline, network and precise point positioning approaches for the primary geodetic stations

• Comparative evaluation of the various solutions with reference solution coordinates

1.4     Justification of Study

The national geodetic network is a pivotal infrastructure of any country by providing the foundation for all geo-referencing activities. It is the base for coherent multipurpose Land Information System (cadastre) and its subsequent maintenance. Such system plays a vital role in the economic development of the country by delimiting and monitoring changes in property, environment, and biodiversity. The geodetic network services included are not limited to land management, urban development, physical planning, the construction industry, mineral exploration, investment and road construction. It is also vital to both air and water transport. The geodetic network is very important in the management of land in a decentralized system. It is vital in the smooth implementation of the National Land policy and can also help in generating direct revenue to the local governments (Jatua, et al., 2010).

GNSS, an advanced improved method of surveying capable of giving high accuracy and reliable result with a standard error in sub-centimeters has gradually been replacing traditional procedures for conducting precise surveys. Moreover, the use of classical methods is limited by such requirements as inter-visibility between the instrument stations and target stations, favorable weather and atmospheric conditions and accessibility of stations (nature of the terrain). The classical networks established by terrestrial methods are insufficient to contemporary requirements but with GNSS, geodetic measurements that are consistent in three dimensionsover larger distances can be determined with cm-accuracy is possible. It is also easy to use, portable, less labour intensive, and coordinates of the GPS are referenced to the World Geodetic System (WGS 84), a global ellipsoid having its origin as the mass centre of the earth, and height, compatible with international systems.

1.5         Scope of Study

For the purpose of this study, limited available data were obtained from Office of Surveyor General of the Federation (OSGoF) consisting of fifty-six (56) first-order coordinates of stations referenced to Clarke 1880 and fifty-two (52) GNSS observational raw station data along with same points processed earlier with BERNESSE software. The GNSS campaign observations were processed using freely available software at no cost. Three processing strategies were used for the study. Trimble Total Control software was used to process two out of the three processing techniques (Baselines and Network approach) while GNSS-LAB (gLAB) tool was used to process the third approach (PPP technique) for evaluation.

1.6         The Study Area

Nigeria is located in western Africa on the Gulf of Guinea and has a total area of 923,768 km2 (356,669 sq mi) making it the world’s 32nd-largest country (after Tanzania). It shares a 4,047km border on the west coast by the Republic of Benin (773 km), on the north by the Republic of Niger (1497 km), on the east by the Republic of Cameroon (1690 km), and on the south by the Atlantic Ocean with a coastline of at least 853 km. (https://en.wikipedia.org/wiki/Nigeria). Nigeria lies between latitudes 4° and 14°N, and longitudes 3°E and 15°E is divided into thirty-six states and a Federal Capital Territory (FCT) as shown in Figure 1.2. 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 ChappalWaddi in the Northern state of Taraba (https://en.wikipedia.org/wiki/Nigeria).

3-D READJUSTMENT OF PARTS OF THE NIGERIAN PRIMARY TRIANGULATION NETWORK WITH GNSS DATA

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