Cartographic processes (Chief Directorate: Surveys & Mapping, South Africa)

The fair drawing process is a manual method of preparing linework, symbols and typology (names) in accordance with the map specification determined by the Chief Directorate. High quality linework is achieved by a process called "scribing" where the image is engraved on a coated film ensuring that line width specifications are carefully adhered to.

Each colour used on a map is scribed on a separate film colour plate and symbols and names are combined photo-mechanically to produce colour separates for printing. The map is printed using the offset lithographic process.

To improve our service to the public and to meet the growing demand for maps and map data, a computer assisted mapping system has been established. The introduction of computer assisted mapping and geographic information systems have added new and exciting dimensions to cartographic techniques and usage of spatial information. A number of products are now available in digital form. The large scale 1:10000 series orthophoto maps are produced from digitally rectified aerial photographs. The raster image is either reproduced on film which can be used for printing, or supplied on CD-ROM.

The medium scale 1:50 000 map series has been digitally captured and topologically structured. This data is stored in a topographical GIS database where it is continually maintained and updated.

From the GIS, the data is imported into the digital cartographic system where the data is structured into separate colour plates for lithographic printing. The 1:50 000 scale maps are available as printed paper copies or raster images on CD-ROM. Digital elevation models are a collection of heights in a digital form which can be used to determine the height of a point or place, the terrain profile between the points, the visibility from a point position and to calculate slopes and interpret the terrain forms. DEM information is supplied in 400 / 200 metre DEM and 50 metre DEM. This information is available as a raster (matrix) of elevations, with a limit on the east-west extent.

Maps are printed using the offset lithographic process, which is most commonly used for the printing of large images involving both the hue and tone in which fine lines are also important. By using fine grained printing plates it is possible to reproduce coloured images with great accuracy.

Modern offset printing presses may be single colour machines or may print up to six colours in sequence. The image is offset from the printing plate onto the transfer cylinder which holds a rubber blanket. The paper is carried on an impression cylinder which when in contact with the transfer cylinder offsets the image onto the paper. Most modern presses may print up to 10,000 impressions per hour making this process the most cost effective method of printing maps.

Legal Disclaimer | Revised: 15/04/02 15:27:44
Copyright © 1997 Chief Directorate: Surveys & Mapping
Private Bag X10, Mowbray 7705. Tel. +27-(0)21-658 4300
(Dept of Land Affairs, Republic of South Africa)