It was declared a National Monument under old NMC legislation on 26 February 1965.

When Johannesburg was proclaimed in 1886 on the triangular site Randjeslaagte, the area of the town was nine square km. Randjeslaagte was a piece of ‘uitvalgrond’ – land left over from the Farms surveyed around it, which was not considered suitable for Farming. The Beacon marks the apex of the triangle with its base running along Commissioner Street, from End Street in the East to Ntembi Pilisio Street in the West. This nine square km. remained the Municipal Area of Johannesburg until 1901. The original Surveyor’s Beacon was a white pole fixed in a cairn of rock and concrete. It was declared a National Monument in 1965 and the cairn smoothed with cement.  In the early 1990s the Sandton Historical Association, with expert guidance from Professional Land Surveyor Werner Kirchhoff hoped to erect a Beacon Monument to mark the boundary line between two of Sandton's original Farms. Although the initiative did not get past the planning stage, the vision for the project was documented in an excellent article written by Avril Reid and Kirchhoff. Below are a few extracts from the article (SHA's 1993 annual magazine) revealing some fascinating history about beacons and boundaries.
 
A Beacon, or Boundary Corner Mark, marking the division between two of Sandton’s old Farms, was recently relocated at the South-Western corner of Sandown at the junction of West Street, Linden Road and Wierda Road East. It is at a point where traffic improvements will make a new Island and it is proposed to mark this Historic point with a concrete pyramid replacing the original stone cairn. This interesting Landmark has been Relocated by Werner Kirchhoff, Professional Land Surveyor, who was instructed by Nedcor to resurvey the Boundaries of Erf 535 Sandown Extension 44 where they are extending their Technical Information Centre.
 
During the early years of the Zuid-Afrikaansche Republiek (Transvaal), Land Ownership was extremely important - abundant Land was a principal reason for the Great Trek. The Sandton Beacon identified above marks a boundary point between the Voortrekker Farms of ‘Zandfontein’ and ‘Cyferfontein’. The registration of Farms became mandatory in the 1850s and some Sandton Voortrekker Farms were surveyed for example in 1865 and 1866, and parts of Zandfontein before 1891. The final Zandfontein Boundaries were surveyed only in December 1891, a few years after the Death of the Voortrekker, Jan Christoffel Esterhuysen when the Property portions had to be professionally Sub-Divided before they could pass to his Heirs. The official Document was signed by then State President, Paul Kruger on December 9th, 1891.
 
Boundaries and Beacons:
Boundaries between Farms were initially informal and agreed between owners for later confirmation by the Landrost. And the Landrost did not always agree. One area which was not accepted by the landdrost because the farms Braamfontein and Doornfontein would have been too large, was declared ‘uitvalgrond’ or unallocated State Land, and it was this ‘uitvalgrond’ which became Johannesburg in 1886.
 
In the Transvaal each original adult male Settler was entitled to one Farm, although many (Paul Kruger for example) owned numerous Farms throughout the Republic. On 23 September 1853 the Volksraad determined that the size of each holding would be measured by riding at walking pace for half-an-hour in each direction from a central point. In 1861, by which date all the Sandton Voortrekker Farms had been allocated, the time was increased to three-quarters of an hour. By the first method a farm of approximately 3750 Morgen was obtained and, of course, it was roughly circular. Access to water was always a consideration in consolidating Farm Boundaries. A land tax was imposed at an early date. According to Werner Kirchhoff, the relocation surveys for old original beacon positions are generally reliable to within a few centimetres. Already by the end of the last Century, the National Triangulation had started for the survey connection between the Royal Observatory at Cape Town and the Greenwich Observatory in England. The triangulation establishing the National Survey System connects all the fixed points - those familiar white beacons with the black signal vanes which are visible on most hilltops - to a common co-ordinate system, using the  equator as an axis. In urban areas the trigonometrical beacons are black vein signals on top of many high structures. A further extension of this basic reference grid control was established in Sandton with the placing and survey of the Town reference marks set in the middle of many roads and intersections and marked with large white paint crosses to control the ongoing updating of the Sandton aerial maps. 
 
Property surveys
 
The Survey of Property Beacons has always been of a very high standard in South Africa and connection to Local and National grid systems ensures their reliable replacement. It has ensured the high title security of our properties free of disputes about Boundary Positions. The replacement of the Historic original line point between Zandfontein and Cyferfontein on the long subdivision line defining the original Western side of Sandown and West Street has now occurred. The extended end of this line to the corner of Zandfontein and Driefontein, at what is now called the Driefontein Farmhouse, has also been replaced. 
Zandfontein’s survey of 1891 was carried out by Land Surveyor Burton Tucker, who was required to erect Beacons ‘according to law’. The certificate on his diagram was attested that “De backend zyn aangewezen door A.B. Heyneke and J. Esterjuysen en zyn behoorlijk opgerig volgens wet”. He also certified that notice of his survey had been given to neighbouring owners. These facts were published in the Staatscourant in September 1891, according to the requirement of the time, allowing President Paul Kruger to sign the diagrams as ‘goedgekeurd’ - Approved. 
Land Surveyor Kirchhoff believes that beacon points of Historical importance and interest should be marked with a Monument of some kind. As a past chairman of the Parktown Association he was involved with the National Monument Beacon for the Northern corner of Randjeslaagte, defining Johannesburg’s present boundaries.
 
Geolocation
-26° 10' 51.6", 28° 2' 45.6"
Further Reading
https://www.sahistory.org.za/article/discovery-gold-1884
https://www.sahistory.org.za/places/johannesburg
https://www.sahistory.org.za/jquery_ajax.../discovery-gold-1884
https://sahistory.org.za/jquery.../johannesburg-timeline-1800-1991
https://www.sahistory.org.za/.../origin-johannesburgs-name-explained
https://www.sahistory.org.za/.../historical-conservation-historical-datesJHB. html
https://www.sahistory.org.za/.../first-mines-are-proclaimed-johannesburg