New national projection and cartesian coordinate system proposed

November 1st, 2018, Published in Articles: EE Publishers, Articles: PositionIT, Featured: EE Publishers, Featured: PositionIT

South Africa is planning to change its national coordinate and projection system, which has profound consequences for surveying, mapping, engineering and high accuracy positioning applications, and for measurement more generally. The proposed changes are open for public comment until 30 November 2018.

South Africa currently uses the Gauss Conform Projection (a special case of the Transversal Mercator Projection that is limited to two-degree zones). While the projected coordinates are given in the Gauss Conform Projection, which is used in all cadastral surveying and other applications, it has shortcoming and presents operational challenges to users.

Being a conformal projection, the Gauss Conform Projection preserves shape locally. The projection distortions increase exponentially with distance from the central meridian, and for this reason the Gauss Conform Projection is restricted to two-degree zones – one degree on either side of the central meridian. For many applications the limitation of two-degree zones is restrictive, preventing applications working over coverages larger than two degrees in longitude (east-west direction). To cover the extent of continental South Africa requires nine such zones.

The Chief Directorate: National Geo-spatial Information (CD:NGI) is mandated by section 3A(d) of the Land Survey Act (Act 8 of 1997) to establish and maintain a national control survey system. This includes the definition of the South African spatial referencing system. In order to address the limitation of the zones in the Gauss Conform Projection for applications with large coverages, the CD:NGI has researched alternative projection systems. In particular, the Universal Transverse Mercator (UTM) and the Lambert Conformal Conic Projection with Two Standard Parallels have been assessed.

The UTM is used extensively internationally and provides for six-degree zones. Four zones cover South Africa, but the projection distortions are considerably greater than the Gauss Conform Projection. These projection distortions will be acceptable for some applications, but are too large for applications requiring greater accuracy, such as cadastral surveys.

South Africa is greater in extent in the east-west direction than the north-south direction, and as such a conical projection is more suited for South Africa. For this reason, the Lambert Conformal Conic Projection with Two Standard Parallels provides an advantage. Two options were considered, either having three zones (covering four degrees in extent) or four zones (covering three degrees in extent). The projection distortions are indicated in Table. 1. The four-zone option is then comparable with the projection distortions in the Gauss Conform Projection, and should then be acceptable for cadastral surveys.

Table 1: Comparison of the projections.

Table 1: Comparison of the projections.

The current projected coordinate system is a left-handed coordinate system, with the first axis (+y) pointing west and the second axis (+x) pointing south. Directions are measured clockwise from south. This coordinate system is not compatible with most application software, creating problems for users. It is proposed then to change to the greater accepted right-handed coordinate system of having the first axis (x) pointing east and the second axis (y) pointing north, with directions measured clockwise from north.

Fig. 1: Current vs proposed coordinate system.

Fig. 1: Current vs proposed coordinate system.

In the proposed coordinate system:

  • If the projection is the Gauss Conform Projection, then the origin is the intersection of the central meridian of the zone with the Equator, with +x to east and +y to north, however to keep the value positive and smaller over the continental South Africa, a false co-ordinate of 4,000,000.00 will be applied at the Equator; or
  • If the projection is the Lambert Conform Conic Projection with Two Standard Parallels, then the origin is the intersection of the central parallel for the zone and longitude 25°E, with +x to east and +y to north.

It must be noted that the coordinate system can be changed without changing the current projection used.

Interested parties can submit comments and preference by completing the form (https://wp.me/a5dDng-16TO) by no later than 20 November 2018. The completed proposal form can either be posted to: Chief Directorate: National Geo-spatial Information, (Attn: Ms T Makapela), Private Bag X10, 7705, Mowbray; or emailed to thando.makapela@drdlr.gov.za.

Contact Thando Makapela, CD:NGI, thando.makapela@drdlr.gov.za