From reality to map :
Map Projections used in South Africa

The map maker has the task of bringing the real world to the map user. This is no easy task as the space available on the map is limited and the real world must be represented by symbols (points, lines and area fills). The process of making the map involves collecting data and making measurements (usually from aerial photographs) of objects in the real world. This information is then translated into understandable symbols and names and other relevant information are added which the map user can interpret to get knowledge about the real world

The Earth is round while the map is flat and so the map maker has to project the round surface on the Earth onto the flat surface on the map. This process is known as a map projection. There are different map projections, each with different properties of preserving true shape, area or distance.
Top: Aerial photo
Bottom: The corresponding map

For a map to be useful it must be a good representation of the real world. This means that as things change, such as new roads or dams or houses, the map must be changed to show these changes. It is important, therefore, that maps show the date at which the information is valid. It is also these changes that make it necessary for maps to be updated at intervals. The updated map is shown as a new edition of the map with a new date.

 

Map projections

The geodetic and mapping products available from the Chief Directorate : Surveys and Mapping will be obtained with reference to a particular ellipsoid and a particular projection.

South Africa utilizes the Modified Clarke 1880 spheroid as reference, at present. Its parameters are:

The 1st January 1999 has been identified as the date of implementation for the new datum based on the WGS84 spheroid. Its parameters are:

All geodetic products and mapping products down to a scale of 1:250 000 use the Gauss Conform projection.

Other smaller scale mapping products utilize one of the following projections:

 

Gauss Conform projection

Points are allocated unique geographical co-ordinates. These are latitude, longitude and height. The height could be either orthometric (above the geoid (defined as mean sea level)) or above the surface of the reference spheroid (ellipsoidal height). In order to map these co-ordinates, they need to be projected onto the plane. In South Africa we use the Gauss-conform projection. The projection effectively projects the latitude and longitude coordinates onto a surface referenced by a particular longitude and the equator. The South African system is defined in belts of two degrees in longitude. Every odd meridian is the central meridian of reference (simply referred to as that "Lo" or Longitude of origin). The Lo.s are 17, 19, 21, 23, 25, 27, 29 and 31. Each two degree belt is therefore flattened with reference to that Lo. (ie. Lo.19 has a flat surface representing the area between 18 degrees east and 20 degrees east)

The projected co-ordinates are referred to two axes being Y, east/west, and X, north/south. The Y co-ordinate is positive west of the Lo., zero on that Lo. and negative east of that Lo. The X co-ordinate is positive south of the equator and zero on the equator. It is therefore obvious that projected co-ordinates will represent a degree of distortion relative to the distance that co-ordinate is away from its Lo. as they are plane representations of a curved surface. There is no distortion on the Lo. For this reason, the South African system is restricted to two degree belts.

Note: The South African system closely resembles the UTM system. The UTM system in the Southern Hemisphere has a false northing at the equator being 10,000,000 metres and decreases southwards, and a false easting of 500,000 metres on the Lo. increasing eastwards. This system consists of six degree belts centred on a Lo. and these belts are referenced with zone numbers. Zone 33 covers Longitude 12 to Longitude 18 and is centred on Lo.15.

The following simple conversion is applicable only where the Lo. of the South African system and the central meridian of the UTM system coincide ie. 15șE 21șE 27șE and 33șE

Y(Lo.)=(500 000-E(UTM))/0.9996
X(Lo.)=(10 000 000-N(UTM))/0.9996

The UTM system incorporates a scale distortion of 0.9996 at the Lo. to reduce distortion at the edges of the belt.

 

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Copyright © 1997 Chief Directorate: Surveys & Mapping
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(Dept of Land Affairs, Republic of South Africa)