Digital Products

With developments in computer technology, and in particular geographical information systems, a number of products are available in digital form.

Digital topographical information has been captured from the 1:500 000 map series and the 1:50 000 map series. This information has been topologically structured and sorted as features (objects). This information has been prepared primarily for use in geographical information systems (GIS) but can also be used in computer-aided drawing (CAD) systems (Note: use in CAD systems can lead to some loss of information).

The applications to which the digital topographical information can be put are wide, similar to uses of the printed map, with the difference that users can use the power of computer application software that they are using to process and analyse spatial relationships between objects and other phenomena and thereby generate new information.

The information is available in various formats CD-ROM. The information is in a continuous form and is not sheet based.

Digital elevation models are a collection of elevations (heights) in a digital form, giving a representation of the surface of the Earth. A digital elevation model can be used to determine the height of a point or place, the terrain profile between two points, the visibility from a point, and to calculate slopes and interpret the terrain forms. The DEMs are determined from photogrammetric (aerial photograph) measurements and computed in a regular grid of elevation points.

Two types of DEMs are available :

• 400 / 200 metre DEM - has a grid spacing of 400m x 400m in most areas or a grid spacing of 200m x 200m in the metropolitan and more mountainous areas. The absolute height accuracy is ±10 m.
• 50 metre DEM - has a grid spacing of 50m x 50m and is available in the metropolitan areas and growth areas. The height accuracy is ±2.5 m.

The DEM information is supplied as a raster (matrix) of elevations, with a limit on the east west extent.

Digital orthophoto images are obtained by digitally rectifying aerial photographs. The digital orthophoto is a photographic image of the terrain - but more importantly, it is true to scale and therefore accurate distances and areas can be measured. These images are popular as 'backdrops', similar to the digital map, but have the advantage over the digital map in that it is a real image of the terrain and not one interpreted by the map maker.

Both the digital map and digital orthophoto image are available on CD-ROM in TIFF format.