Visualisation technical Datum transformations using the NTv2 grid by Julie Lovesay, Chief Directorate: National Geo-spatial Information (CD:NGI)

Prior to 1999, all geospatial data in was referenced to the Cape datum (Modified Clarke 1880 ellipsoid). Since the introduction of the Hartebeesthoek 94 datum (utilising the WGS84 ellipsoid) in 1999, GIS practitioners have faced challenges combining historical data in the Cape datum and more modern data in the Hartebeesthoek 94 datum. I have investigated an approach that can easily be used to transform data from the Cape datum to Hatebeesthoek 94. This is called the NTv2 grid which is software independent. However, it was tested within the ARC environment as this is one of the most widely used platforms.

he transformation of point files from one datum to another is Trelatively simple, and can be done with a Helmert transformation, provided the area is smaller than 40 km and at least two points are known in both systems. In smaller areas of only a few kilometres, a straight shift is acceptable if surrounding trigonometrical (trig) beacons or town survey marks are consistent. However the transformation of GIS vector data such as shapefiles does not involve only point conversion, but rather points, polygons and lines. This makes the transformation more complicated as the polygons and lines may contain many vertices and can possibly be geographically extensive.

Currently, the vast majority of GIS projects utilise WGS84/

Hartebeesthoek94 as the default Fig. 1: Left - Water pipes in Mossel Bay in Cape Datum overlaid on an image rectified in the datum. GIS practitioners are regularly Hartebeesthoek94 datum. Right – The water pipes after NTv2 transformation, vector data now faced with the problem of having correctly georeferenced. historic vector data in the another datum; mostly shapefiles in the "old" Cape Datum, while the rest of the data The National Transformation version 2 of the countries national control points, is in Hartebeesthoek94. When data (NTv2) is a datum transformation a regular spaced grid of shifts at an from two datums is combined and an method relatively unheard of within interval of 0,01 degrees (±1 km) was appropriate transformation strategy the South African surveying and GIS interpolated. This translated to roughly is not applied, a very visible shift will communities. It was originally created 1,55-million points, which are used to occur when the data is overlaid and by the Geodetic Survey Division of apply shifts to the shapefiles. displayed. Geomatics to transform files Because the density of South Africa’s from NAD1927 to NAD1983. This This is easy to spot, as there is national control points is irregular, the method has now been extended to generally a difference of around 300 m accuracy of the transformation will accommodate , , in x (northing) and 50 m in y (easting) vary. In highly populated urban areas, and South Africa. components. Applying this shift to it is possible to find over 5000 trig the shapefiles, or even the default The NTv2 method uses grid files to beacons and town survey marks in less transformation in ArcGIS will result in transform shapefiles from Cape Datum than a degree square; while rural areas very poor accuracies, as the horizontal to Hartebeesthoek94. The file contains such as the Northern Cape can have shift varies across the country by up binary information relating to the less than 200 trig beacons and no town to 80 m. ArcGIS’s default 3D shift will difference in the two (unprojected) survey marks in an equal area. As a produce results in the order of 15 m. coordinate systems. Using all 50 000 regular grid has been interpolated from

PositionIT – April/May 2010 49 VISUALISATION technical

(parent) grid, providing more accurate The accuracy of the NTv2 grid was transformations. NTv2 then checks tested by applying a transformation to all the grids, to find the one with the 91 trigonometrical beacons distributed highest density. Knowing which grid was throughout the country. By comparing used for the transformation can give the the transformed coordinates with the user an idea of the accuracy achieved. known Hartebeesthoek94 coordinates, South Africa does not currently use the conclusions could be drawn. A mean method of overlaying a sub-grid. variation of 1 cm with a standard deviation of 1 cm was achieved. This It is important to remember that the accuracy is suitable for both GIS and grid file applies to each vertex and no surveying applications and has the global rotation or scale factor is applied. potential to save time by reducing the The shifts in this grid have specifically need for re-digitising. been calculated to transform Cape Datum to Hartebeesthoek94. Thus for Users using this file for surveying systems such as the old Gold Fields applications should be aware that Fig. 2: The grid system works by overlaying datum, the NTv2 grid is unsuitable, the shifts applied in the NTv2 file smaller and more accurate grids on top of and could not be used even if it was use the published Cape Datum and the parent grid. developed. Hartebeesthoek94 coordinates on 1 January 1999. Older publications of This conversion can be done in ArcGIS coordinates may contain variances from on any central meridian/longitude all control points, it is difficult to infer the latest issue, making it unsuitable of origin (Lo), by defining a specific information regarding the accuracy of for cadastral applications. The file is transformation and the location of the these shifts. freely available to the public, and can NTv2 grid (a .gsb file). A new shapefile be acquired from ESRI South Africa. In the future, in areas where there with the suffix _project.shp is created in are more control points, (such the same folder. The same conversion Contact Julie Lovesay, CD:NGI, as urban areas) smaller grids or can be applied to unprojected Tel 021 658-4479, sub-grids may overlay the main (geographical) shapefiles. [email protected]

50 PositionIT – March 2010