Wadiso 6 User Guide > Model input and display of results in the Albion environment > Performing special operations on the system > WGS System Conversion

Introduction

Before 1 January 1999 the official co-ordinate system for South Africa was based on the Cape Datum and the modified Clarke 1880 ellipsoid. As from 1 January 1999 the Hartebeesthoek94 Datum is used and the World Geodetic System 1984 ellipsoid (WGS84). In addition to the transformation, flaws and distortions, which have become evident through the use of the Global Positioning System (GPS), have been rectified. At this stage all heights will remain referenced to mean sea level, and are therefore not affected by the transformation.

Projection

In South Africa the Gauss Conform Projection is used to project ellipsoidal latitude and longitude (degrees) co-ordinates onto a plane surface. The projected co-ordinates are commonly known as the “Lo. Co-ordinate system”. Only the area within one degree of longitude on either side of a central meridian (e.g. 19 degrees east) is projected. The width of each segment or “belt” is therefore two degrees of longitude. Only these projected coordinates can be presented in Wadiso.

Magnitude of transformation

The Hartebeesthoek94 Datum (WGS84) X (Southing) coordinate is typically between 289 and 295 metres greater than the Cape Datum (Clarke 1880) counterpart at a point of interest in South Africa. The Hartebeesthoek94 Datum (WGS84) Y (Westing) coordinate is typically between 20 and 67 metres greater than the Cape Datum (Clarke 1880) counterpart at a point of interest in South Africa. These values (and the direction) should be noted and used to check any transformation.

WGS84 Conversion Module

In order to transform co-ordinates accurately from the old to the new system (or vice versa) a dedicated WGS84 Conversion module is included in Wadiso. This module performs a very accurate conversion on CAD, hydraulic model or data points, as it makes use of thousands of control points (typical the coordinates of trigonometric beacons). It therefore requires data files with these control points, known as block files. One block file covers an area of 1 degree square, i.e. approximately an area of 100 km x 100 km. For example the block file 3318 for the Western Cape covers the area from 18 to 19 degrees East and 33 to 34 degrees South. In order cover the whole of the Cape Town Metropole for example, one would require the block files 3318 and 3418. The user must therefore acquire the block files for the area to be transformed. These files can be ordered from GLS Consulting, and are typically delivered electronically via email.

Accuracy

As the WGS84 conversion module makes use of block files, a very high degree of accuracy can be guaranteed. The means square error of transformation between datums is 15 mm. This is based on a calculation for all 22 000 town survey marks and 29 000 trigonometric beacons in South Africa. The worse single point error in South Africa is about 0.2 m. However, for the vast majority of points the error is less than 10 mm. Other conversion systems, which do not make use of block data files, cannot provide better accuracy.

Documentation

In order to avoid confusion, it very important to note in what system a drawing or model has been stored. This can be achieved by adding a corresponding identifier (e.g. “WGS-19” or LO-19) in the title block of a drawing, the job name of a model or in the file name of a drawing or model.

WGS84 Background

WGS84 Background