In order to determine where we are on the Earth's surface and also
to be able to determine distances, directions and areas, we first need to know the size
and shape of the Earth. To make things easier for us, we take the Earth to approximate a
chosen reference surface that is easy to work with. This surface is called the reference
spheroid (a sphere which is flattened at the poles). In South Africa we currently
use the Clarke 1880 (modified) spheroid, but with the increasing use of
satellite position fixing methods (GPS), a more appropriate reference surface is being
used, namely the WGS 84 spheroid,
which is the reference surface that will be used in the future.
To assist surveyors and others in their work, a network of permanently marked and accurately surveyed trigonometrical beacons has been established throughout the country, from which one can measure the position and height of any other point. The network of trigonometrical beacons is represented mainly by white pillars with black vanes, which can be seen on the tops of mountains, buildings and water towers throughout the country.
Until recently, the positions of the beacons were determined by surveyors using theodolites and distance measuring apparatus. Nowadays, however, satellite positioning methods are used and, depending on the type of equipment and methods, the positions can be determined to within a few centimetres.
It is this network of beacons that permits us to position and orientate a map correctly. We can also easily join two maps together. The latitude and longitude, co-ordinate grid and true north shown on the map have been determined from this network of beacons.
The trigonometrical beacons are used not only to make maps but also for the survey of civil engineering projects, such as the building of roads and water pipelines, sites of scientific interest and for the cadastral surveys of land parcels.
