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Spatial Reference - Map Projections in South Africa

The Co-ordinate Systems in South Africa are referenced to a particular Ellipsoid and uses a specific Projection. The Clarke 1880 spheroid was used as a reference Ellipsoid and many products and information that uses this Datum are still in circulation. After 1 January 1999 the a new Datum was implemented based on the WGS84 Ellipsoid.

On this Ellipsoid, points are defined by unique geographic co-ordinates. These are Latitude, Longitude and Height. South Africa uses the Gauss Conform Projection to project these points from the ellipsoidal surface to the mapping plane to obtain Cartesian Co-ordinates.

The Cartesian Co-ordinate Systems normally has its origin on the equator at a specific Longitude such that the X-values increase Southwards and the Y-values increase to the West. To avoid to much distortion, the whole of South Africa could not be reference by a single Cartesian System and several "Belts" of Co-ordinate Systems were defined along each odd longitude in South Africa. These are referred to as "Lo" Systems which is short for Longitude of Origin and are also called the Central Meridian of the Co-ordinate System. (In South Africa the Central Meridians start from Lo.17, then Lo.19 and so on...)

 

Why this is Relevant to the Services on this Website:

The Control Point search services requires input of the Central Meridian for a Co-ordinate Search. This is because Y Co-ordinates repeat for each System around a different Central Meridian.

As you may have noticed, the search services also requires a Rough Latitude to be specified. This is not for the same reason as explained above, but because the database containing the Co-ordinates to be searched is very Large and the Rough Latitude is merely used to narrow down and speed up the Search. X Co-ordinates start from zero at the equator and increases South as far a South Africa is defined by the System. It never "breaks up" and repeats like the Y Co-ordinates.

 

Further Information on the Topic:

      World Geodetic System 1984 (WGS84) and the International Terrestrial Reference Frame (ITRF)

 

 


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