Home Articles A Common and modern African Geodetic Reference System

A Common and modern African Geodetic Reference System

H.O.Farah, C.M. Kamamia, W.K. Ottichillo
Regional Centre for Mapping of Resources for Development Ruaraka, Nairobi
[email protected]

Countries have traditionally maintained their own geodetic reference systems resulting in maps in neighboring countries not edge-matching properly at the borders. Apart from increasing the potential for misunderstanding and conflicts, this situation makes it difficult for countries to share information and to work on joint plans and projects.

Information on one country’s maps could not be easily referenced to that on another country’s maps. As we move towards more regional integration, and adopt regional approaches to peace and security, environmental management, trade and industry, we need maps that are continuous across national boundaries. This will be possible through the establishment of a common geodetic reference frame.

The African Geodetic Reference Frame (AFREF) was conceived as a unified geodetic reference frame for Africa to be the fundamental basis for the national and regional 3D reference networks. It is expected to be consistent and homogeneous with the International Terrestrial Reference Frame (ITRF) Standards. ITRF is the global reference frame system for the earth as adopted by the International Association of Geodesy (IAG).

When fully implemented, it will consist of a network of permanent Global Navigational Satellite system (GNSS) stations, continuous or otherwise, such that a user any where in Africa would have free access to GNSS data nd products, and would be at most 1000 km from such stations. Its full implementation will include a unified vertical datum and shall support efforts to establish a precise African Geoid.

All African countries have started embracing the use and applications of GNSS technologies particularly Global Positioning system (GPS) in the various geo information applications, services and products. GPS uses World Geodetic System 1984 (WGS84) WGS 84 system is a modern, global and uniform system best fitting the earth. The International Terrestrial Reference System (ITRS) is the global terrestrial reference system officially adopted by the International Association of Geodesy (IAG). The WGS84 reference system, which is widely used in the world and in Africa as stated earlier, is now identical to ITRS at centimeter level. GPS technology may therefore be used in the implementation of AFREF.

GNSS, such as GPS technology is very accessible, precise, economical and sustainable. Most GPS products do not require the user to have in depth knowledge of its technology as their production is application based and user friendly. With the increased use and application of GPS and the requirements to relate the GPS solutions with the already existing mapping products based on local and national coordinates reference systems, there is an urgent need to establish and determine the transformation data to and from such systems to GPS references systems. This shall be achieved on full realization of the AFREF project.

The realization of AFREF has vast potentials for geodesy, mapping, surveying, geo-information, natural hazards mitigation, earth sciences, etc. Its implementation will provide a major springboard for the transfer and enhancement of skills and knowledge in surveying, geodesy and especially Global Navigation Technologies (GNSS) with its applications along coastal regions.

The concept of a unified geodetic datum for Africa is not entirely new. An effort was made in the 1980s to establish a unified datum using satellite techniques via the African Doppler Survey (ADOS) project. The ADOS project was started in 1982 and was completed in 1986.

EUREF is the common geodetic reference frame for Europe. Permanent tracking stations form the backbone of the EUREF network, which are densified at the national level using local campaigns of finite duration.

The main components of the network are permanent GPS stations, Operational Centres, Local Data Centres, a Regional Data Centre, Local Analysis Centres, a Regional Analysis Centre and a Network Coordinator. The creation of EUREF took advantage of the existence of various components, adding the network coordinator function to coordinate activities that were already on-going in member countries.

EUREF is the European regional component of the Global Network of IGS. As such it delivers free-network solutions from EUREF local analysis centres to IGS for the maintenance of the International Earth Rotation Service (IERS). SIRGAS (Sistema de Referencia Geocentrico para Americas del Sur) is the equivalent of EUREF for the South American countries. It was initially established in 1993 during the International Conference on the Definition of a South American Geocentric Datum, sponsored by the IAG.

Fig. 1 Workflow and products of the photogrammetric/remote sensing process
OBJECTIVES
The following are some of the identified objectives of AFREF

  • Define a continental geodetic reference frame for Africa
  • Establish precise and uniform African geoid
  • Establish permanent GNSS base stations such that users have free access to GNSS data and product from such stations
  • Provide sustainable development for GNSS and ICT technology transfer within the continent
  • Establish an in-country expertise for implementation, operation, management, analysis and presentation GNSS data and products.

PROPOSED

Fig. 2 Organizational Structure

IMPLEMENTATION STRATEGY
The successful implementation of AFREF depends on the application of Global Navigation Satellite system (GNSS), and in particular the Global Positioning System (GPS). The UN Office for Outer Space Affairs (UN OOSA) is the coordinating body for the peaceful use of space, including GNSS.

AFREF has been presented and discussed at USA/UN OOSA sponsored workshops, including one held at Lusaka, Zambia in July 2002, and Vienna, Austria in December 2004, and its importance for the development of Africa was accepted. A number of other meetings have been held including Governing Councils (GOC) meetings for the Regional Centre for Mapping of Resources for Development (RCMRD), UN ECA CODI meeting, IAGG/IAG and IGS international meetings amongst other where AFREF issues have been discussed.

International Association of Geodesy (IAG) promotes the concept of unified regional geodetic reference frames all over the world. IAG and its service organizations, in particular the International GNNS Services (IGS) has established a network of continuous GPS observation stations across the globe. To date more than 200 such stations have been established all over the world since 1992. GPS data is available free from the IGS web site and its data processing centers. In Africa, approximately 18 (see Figure 1) such stations have been established by various IGS organizations.

Most of the stations are distributed in the Eastern and Southern Africa and part of Western African coastal region. Central and Northern part of Africa are not covered.

Densification of IGS networks with

its products in Africa is the first step toward the realization of AFREF. For practical effectiveness, the following implementation and coordination structures have been developed and proposed through various workshops and accepted by Economic Commission of Africa (UN ECA).

The proposal is based on continental and regional coordination with national implementation. The following are the proposed implementation regions based on United Nations economic blocks

  • NAFREF, North Africa Reference Frame for North Africa comprising of Algeria, Egypt, Libya, Mauritania, Morocco and Tunisia.
  • SAFREF, South Africa reference frame for SADAC countries including Botswana Lesotho, Malawi ,South Africa, Swaziland, Namibia, Zambia, Zimbabwe
  • EAFREF, East African Reference Frame for IGAD countries including Burundi, Djibouti, Ethiopia, Eritrea, Kenya, Somalia, Rwanda, Tanzania and Uganda
  • CAFREF, Central Africa Reference frame (for Central Africa)
  • WAFREF, West Africa Reference Frame (for West Africa) Following the discussions at various forums including the United Nation Economic Commission for Africa (UN ECA) Committee on Development Information (CODI) meetings, the implementation of AFREF will follow an approach consisting of following three major phases:
  • The establishment of a frame work of continuous permanent GPS base stations throughout the regions that will become part of the worldwide IGS network of stations. These stations must comply with the internationally accepted standards as set out by IGS. During the Governing Council meeting of the Regional Centre for Mapping of Resources for Development (RCMRD) held in December 2000 at Windhoek Namibia, it was declared that densification of IGS network be carried out to at least 1000km by the National Mapping Organizations(NMO) with assistance and collaboration of IGS. A number of sites were identified for establishment of such points.
  • The densification of the network of permanent base stations, largely on a country-by-country basis, to determine the relationship between the national geodetic system and the ITRF, and to refine the transformation parameters necessary to relate the national systems to ITRF. The densification may be carried out by individual countries by way of establishing GPS networks through either continuous or semi continuous permanent GPS stations.
  • The third and equally important phase of the project will be to address the development of a more refined geoid model for Africa and the definition of a common vertical datum for the continent. The unification of national land levelling networks will follow from this. This phase of the project can run parallel to the two phases described above.

Countries will be expected to actively participate in the planning, management and execution of field campaigns, and in the processing, computation and interpretation of the observations in all phases of the project. They will also be expected to maintain electrical and communication facilities at the continuous permanent stations, and arrange for the delivery of requisite data sets to the data centres.

ORGANISATION STRUCTURE
The organization structure of AFREF is illustrated in Figure 2. The Steering Committee is nominated by United Nations Economic Commission of Africa, Commission on Development Information (UN-ECA-CODI) and sub-commission on geo-information (CODI-GEO).

It is responsible for the continental coordination of the implementation of AFREF.

It draws its membership from the United Nations Regional Centres in

surveys and mapping technologies, IAG and AFREF implementation regions as listed below;

  • Regional Centre for Mapping of Resources for Development (RCMRD),
  • African Organisation of Cartography and Remote Sensing (AOCRS),
  • Regional Centre for Training in Aerospace Surveys (RECTAS),
  • International Association of Geodesy (IAG) sub commission on reference frames and AFREF (SC 1.3d)
  • Egypt, for NAFREF,
  • Tanzania, for EAFREF
  • Nigeria, for WAFREF
  • Namibia, for SAFREF

The steering Committee liaises with international partners and a scientific advisory group and links them to the regional bodies and countries.

Regional bodies dealing with surveys and mapping disciplines including geo-information are expected to coordinate the implementation at regional level. Regional centres are expected to provide the linkages between the Steering Committee, regional working groups

and National Surveys & Mapping Organization. National Mapping Organizations (NMOs), whose responsibility amongst others is the establishment and maintenance of geodetic networks are expected to lead and coordinate AFREF national implementation. NMOs should be supported by other GNSS stakeholders within the country. They are therefore expected to form working groups with participation from relevant public institutions and private sector including professional bodies such as surveyors, engineers and geophysicist. GNSS data users such as those in the transportation, aviation, construction, natural resources management and military should also be incorporated into the working groups. National Mapping Organizations are members of UN ECA CODI GEO, Steering Committee, regional coordination working groups and finally national coordination and implementation working groups.

The coordination bodies main function is to ensure commonality in funding and acquisition of hardware and software, implementation procedures, dissemination of data. The implementation is therefore national with regional and continental coordination.

REQUIRED RESOURCES
Personnel and institutional resources are required at both national and regional levels. This requires persons well versed in the field of geodesy and particularly in establishing, manning, and processing of GNSS data and products. AFREF participants will be expected to use up-to-date positioning equipment, mainly GPS of appropriate precision for the global network. These will include receivers and other ancillary components. The permanent computing stations will need dedicated computers and storage peripherals to hold the data. Software packages and hardware are required for the processing of GPS data.

In the second phase of AFREF, software and expertise will be required for the re-computation and adjustment of national coordinate products to the new reference system for surveying, mapping and scientific communities. Communications and network connectivity are essential components for the successful implementation of the reference network. AFREF being part of the global network, there will be constant need to upload and download data to and from designated data centres and IGS centres.

CURRENT STATUS
International Association of Geodesy (IAG) supports AFREF under the commission for developing countries and reference frames and sub commission on regional reference frames. A dedicated sub commission (SC1.3d) for AFREF was established.

International GPS services (IGS) with its Associates also supports AFREF as evidenced by the Continuously Operating Reference Stations (CORS) currently established in Africa.

IGS and partners support the AFREF project. As described earlier 18 (eighteen) such stations have been established to date and are continuously being established.

As described in Figure 1, a sparse network IGS stations are available within the continent though most were not established and manned through the support of the National Mapping Organizations (NMO). With the support of NMOs it’s expected that densification of IGS network will take place at faster rate. A one week technical workshop was held at the University of Cape Town, South Africa in July 2006. The purpose of the workshop was to bring together representatives from African National Mapping Organizations and Universities and International experts in geodesy and GNSS to discuss and debate the technical issues involved in the implementation of the AFREF project.

It drew participation from 18 African countries and 2 Regional Centres dealing with surveying and mapping. World-renowned scientist from 10 international agencies attended and shared their knowledge and experiences with the workshop participants. Representatives from 5 GPS vendors also attended.

The AFREF Steering Committee is inviting organizations to participate by providing the resources to implement AFREF. The participation is open to a broad range of organizations such as National Mapping Organizations, Universities and research organizations dealing with earth and environmental sciences including geo informatics, seismology, geophysics, meteorology, GNSS hardware and software vendors and donor community. Call for participation documents are available at AFREF web site:

The demonstration phase of AFREF has already started with many organizations responding positively to take part in AFREF. South Africa, Egypt, Morocco, Namibia, and Ivory Coast have already established CORS. Ten other countries namely, Nigeria, Cameroon, Tanzania, Malawi, Ghana, Mozambique, Ethiopia, Namibia, Benin, Kenya and Botswana have committed themselves to establish at least one CORS by the end of 2006. The collection and analysis of AFREF data is expected to start in 2007.