One continent, one representation: article on AFREF progress

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The GeoConnexion magazine publishes an article entitled: "One continent, one representation". IGN FI, IGN France and Uneca look at progress on AFREF, a common geodetic reference frame that can be used by all the countries in Africa. 

Geodesy has long been a specialist field. It was first of all used to understand the Earth’s shape and dimensions, then to position ourselves in the best way possible on the Earth’s surface. But while rich countries started to create networks that enabled them to position points homogeneously across their entire territory down to the nearest decimetre, African countries remained with just a few astronomical points the accuracy of which could only be counted in decametres for many years. 

As André Nonguierma, SIG expert for the United Nations Economic Commission for Africa, says: “The heterogeneity of technical approaches used by the various countries was a real hurdle in terms ofthe need for analysis and regional planning. It was therefore essential to rapidly obtain a good model for global representation of the Earth’s surface which would allow us to clearly describe the geometric shape of objects as well as reference altitudes for the planet’s surface.”

The AFREF (African Geodetic Reference Framework) was created in 2002 as part of the GGRF (Global and National Geodetic Reference Frames), a worldwide project that aims to create between now and 2030 a common geodetic reference frame that can be used by all the countries in Africa. In its present form, the AFREF project consists of a network of permanent global navigation satellite systems (GNSS), spread over the whole continent so that any potential user is always within 1,000 km of the nearest reference station.

But the countries’ need for a transnational reference frame has become even more evident in recent years with the creation of regional project funders who are looking to open up access to certain countries and improve the mobility of goods and people between different African countries; also factors are the adoption of the Programme for Infrastructure Development in Africa (PIDA) and a call from the African Union of African States to delimit their borders between now and 2017.

IGN FI’s geodetic expert Cyril Romieu looks back over the various technical advancements that have made the AFREF project possible. “It is of course the introduction of the first satellite positioning systems in the 1970s followed by the GPS system at the end of the 80s which will enable the creation of reference systems (mainly geographical institutes and mapping agencies) technically possible and more financially accessible. Modern GNSS technology now allows access to any national reference via several means: materialised networks using terminals, permanent GNSS stations distributing data via internet, PPP (Precise Point Positioning) type positioning. “The technological progress made in topography varies widely from one African country to another, more than on any other continent. It is therefore crucial to ensure that a national reference system is accessible not only by the most advanced users with GNSS receivers in real-time GSM mode, for example, but also for those still using traditional angular measurement techniques and an optical method.” [...]

Read the complete article

References - Geodesy and metrology