Boulders Beacon
The cache, a small camo-taped tablet pot, is hidden towards the southern end of the Boulders area in the middle of a group of these distinctive huge granite boulders between Froggy Pond and Fisherman’s Beach on the west coast of False Bay.
It was placed in recognition of an earlier cache nearby, Defence10 & Zambesiboy’s GC3B8AT Cobra Cache, which was hidden on 21/1/12 and archived on 11/3/20 after some 104 finds – the FTF for which was made by yours truly!
To Reach the Cache Location: park in the lay-by @ at S 34 12.346 E 18 27.461 and after securing your cachemobile, carefully cross the road and make your way down to the beach, then across, above, down and around to the cache location.
Geocachers are of course familiar with GPS and coordinates as the basis of their sport. So, it was of interest to note that the huge granite boulder below which the cache is hidden is also home to a (trig) beacon.
This is part of the South African integrated survey system (ISS) called the National Control Survey Network (NCSW) - a network of 78,829 (2019 figure) control points throughout the country, each with its own identifying number. See here for a clearly explained and well-illustrated presentation on the 2019 status of the Network – all figures below are derived from this.
It is gradually being replaced by 67 active GPS base stations which form Trignet – a continuously operating GNSS (Global Navigation Satellite Systems) network which provides post-processing and real-time GPS correction data.
The NCSW comprises:
a) 29,437 trig beacons [such as the one near GZ] on mountains, hilltops and prominent man-made structures (eg. high buildings and water towers). Each consists of a white concrete pillar 1.2m high, 40cm diameter with a 40cm x 40cm square black steel vane set 1.2m above it. The positional accuracy is 10mm and height accuracy is within 100mm.
On the most used local map they are identified by a number and elevation, eg. the cache beacon is shown as 206 9.
b) 28,964 town survey marks (TSM) consisting of a 13mm brass pin/stud/peg set in concrete in a circular 228mm diameter cast iron box, 228mm diameter, normally in the tarred road surface of 122 urban areas.
The 152mm cast-iron lid has a square pattern on it, with a raised triangle in the middle indicating a survey marker, and can be opened without any special tools. The lid only rests on a collar, but often sand/soil/gravel prevents it from being opened easily. In rainy season they are often filled with water or Mud.
They are each identified by a number stamped onto a copper/aluminium/ stainless steel plate (25mm x 75mm) on a 250mm chain attached inside. Positional accuracy is also 10mm, but in height they are accurately levelled to within just 1mm. Their location may be highlighted by thick white painted lines - often as an arrow-head - or a white painted surround.
See here for an interactive map showing the location of all the 11,866 TSM and Benchmarks in Cape Town. From this same page can be downloaded a related spreadsheet with recognisable coordinates enabling plotting of the control points on Google Earth. So now you have no excuse for not knowing where your local control points are . . . 😊
By the way, the relevant law ((Land Survey Act) only states that you are not allowed to destroy them without written permission, so you are free to open them and have a look inside . . . maybe hide a little something?!
Note: the detailed info for a) and b) above was stumbled upon here - in a Geocaching Forum!
c) 20,431benchmarks which are precisely levelled points located originally along railway lines but nowadays (also) along the main road transport routes. They consist of a brass peg/stud set into the centre of a concrete base. Each has a height which can be used by surveyors for local survey projects (no images yet found).
Of course, nowadays, GPS has radically changed the way geodesists and surveyors do their work – and the philosophy surrounding national control survey networks not only in South Africa but in many other countries around the world.
The network of trigonometrical (trig) beacons on top of mountains and tall structures and buildings is a passive network since a beacon simply shows the position of the coordinates assigned to it and cannot update or monitor its position. In contrast, current practice is to establish active control survey networks which utilise GPS as the primary positioning tool.
The New South African Datum is referred to as the Hartebeesthoek 94 Datum (see here for info on this and other datums, and coordinate systems) and uses the World Geodetic System 1984 (WGS84) as reference ellipsoid – the mathematical representation of the imperfect (ie. non-spherical) shape of the earth.
The datum was implemented nationally on 1 January 1999. The benefit is that surveys and geo-spatial data all have a common datum, which helps in data exchange, contributes to uniform standards of accuracy and reduces the costs of survey and spatial data to the public.
The Hartebeesthoek 94 Datum enables South Africa to retain its status of having one of the most up to date and advanced integrated survey reference systems in the world, which gives users the freedom to integrate their geo-spatial data in the world arena. It also provides South Africa with the opportunity to assist its southern African neighbours with technical and technological expertise in upgrading their geodetic systems.
Note: South African Trig Beacons is a category of the geocaching-related scavenger hunt Waymarking which will ‘provide you with the tools to share and discover unique and interesting locations on the planet’. The number of waymarks recorded so far recently passed the 1,000,000 mark. Geocachers can also log into Waymarking with the same User Name & Password.
Another interesting beacon of a different type in the area, marked on the most popular local map and located a short distance southwest of the cache location on a boulder outcrop off Oatland Point is a Marine Beacon* which when aligned with a similar beacon on the lower slopes of Simonsberg points the way to Whittle Rock (an extensive rocky offshore reef – and an obvious danger to marine craft) some 9.3km SW of the former.
*a fixed navigation mark recognisable by its shape colour, pattern, top mark, or light character, or a combination of these. The one at Oatlands Point was originally a wooden tripod, since replaced by a simple post. The nearby dive site is called the A-Frame after the shape of the earlier beacon.