A Common and modern African Geodetic Reference System
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.