SA Primary trig control

Here are some photos I have taken while visiting a few of the over 29000 trig control beacons installed by department of surveys and mapping .

The trigonometric beacons (trig beacons) in South Africa originated from the Western Cape, Cape Town to be exact and they were based on star observations and precisely measured baselines . Maclear between 1833 and 1870 and Sir David Gill between 1879 and 1907 whose initial geodetic objectives were to verify the size and shape of the earth in the Southern Hemisphere and later to provide geodetic control for topographic maps and navigation charts. From these beginnings the network was extended to eventually cover the entire country and comprises about 29000 highly visible trigonometric beacons on mountains, high buildings and water towers.

We still use them in our everyday work still do freestationing and to orientate onto when not using gps. Most of the metal vanes are gone and they are no longer maintained as they have been replaced with a nationwide RTN. Their paint is faded and quite hard to find unless they are well silhouetted or you know where to look.

There is great confidence to be had when free stationing off a known point, shooting a horizontal angle to a trig and then adding another trig, tell the robot to turn to it and unless you cocked it up, you always hit it within 5 seconds, average it out and survey away.

Here are a few I have been to.

Hogs back, the eastern cape 2010

Lions Head, Cape Town 2003

Making way for new developments

Brittania rock, Paarl 2013

Helderberg 2013
This one has an extended vane for better visibility and sighting.

The effort to install and reference them all must of been great.