Who planted the beacon on Beacon Island?

13 November 2011

Speaker:   Andrew Duminy                          

The local legend: According to Pat Storrar, the beacon was erected on the Island in 1772 (the Van Plettenberg website says 1771), inscribed with the latitude and longitude of Plettenberg Bay. The caption on the photograph of the beacon in her book Portrait of Plettenberg Bay states that it was ‘replaced twice’, but she does not provide details of when this took place (according to the Website, it was replaced by Harbour-Master John F Sewell by a stone beacon in 1881).

This is more or less repeated by Clare Storrar in his 1999 edited version of the Sewell diaries: “The navigational beacon was used by mariners to set their chronometers to Greenwich Mean Time for longitudinal and astronomical calculations.

The original beacon was a baulk of stinkwood erected in 1772. It has been replaced/moved several times since then. It was last moved when the second Beacon Island Hotel was built in 1971. Inscribed on this stone beacon is: (Latitude) 34° 3' 38˝S, (Longitude) 23° 22' 55˝E. Magnetic declination 7' 29˝ W.1864.” (Its actual position is 34° 3' 45"S, 23° 22' 32"E.)The story of the erection of the beacon has become part of Plettenberg Bay folklore. We are unfortunately not able to ask the Storrars about the source of their information. They were both meticulous researchers. I intend to show, however, that there was no beacon in 1772 and that it must have been placed there much later.

How were latitude and longitude established?

Latitude can be established with a reasonable degree of accuracy by means of erecting a vertical pole on a flat surface. If the length of the shadow is measured at midday – when the shadow is at its shortest – the angle formed by the end of the shadow from the top of the pole can be calculated by trigonometry. This angle is the degree of latitude, once allowance has been made for the northerly and southerly movement of the sun between the solstices.  At sea, this measurement was made with a simple instrument known as a cross-staff and, by the 1750s, the invention of the sextant had made these measurements much more accurate. (A sextant makes it possible to view objects simultaneously through paired mirrors, one of which is fixed to a moving plate. When the two objects are lined up, the angle formed can be read off on a scale on the bottom of the instrument by means of an indicator on the moving plate.)

Establishing longitude is much more difficult. Degrees of longitude are measured from a zero meridian. The Dutch usually used Amsterdam or Tenerife. The French used Paris or the Ile de Fer in the Azores. The British used Greenwich and the Greenwich meridian was internationally adopted in 1884 – many years later. Degrees of longitude east or west of this position are calculated by a difference in time. Because the world is a sphere, there are 360 degrees of longitude in a daily rotation; and, as there are 24 hours in a day, each hour translates into 15˚ of longitude.

During the 18th Century, there were three ways of establishing longitude. The first was developed by the famous French astronomer Cassini, who established the French Observatory in Paris in 1669. He discovered, using a powerful telescope, that the moons of the planet Venus have a regular orbit; and was able to time these movements, using two pendulum clocks, the one beating every second and the other every half-second. This made it possible to establish the longitude of other positions on the earth’s surface by means of making these observations from there and comparing them with those recorded in Paris. All that was necessary was the transportation of a powerful telescope and an accurate clock to the other positions.  The French Academy of Sciences sent expeditions to a number of places on the earth’s surface to do this, so making it possible to draw more accurate maps of the world and to discover the size and shape of the earth (was it round or was it pear-shaped?).

In 1685 such an expedition called at Cape Town when the Jesuit priest Guy Tachard was the first to attempt to calculate its longitude. This was followed by another French project in 1752, when the astronomer Nicolas Louis Lacaille was sent to the Cape. Using a more powerful telescope and armed with Cassini’s tables, he situated his laboratory in Strand Street and, after making numerous observations, concluded that its position was 16˚8" east of Paris (or 18°20'22" east of Greenwich; its actual position is 18°27'E). Lacaille was also instructed, once he had established this longitude, to measure a section of the Cape Town meridian in order to assist in the calculation of the size and shape of the earth. He did so by first measuring out a baseline somewhere near Darling and then surveying a chain of triangles between there and Cape Town. His measurement created something of a stir in scientific circles because they did not tally with measurements that had been made in the northern hemisphere – with significant implications for the size and shape of the earth.

The second means of establishing longitude was by means of what were called ‘lunar distances’. This method came about as the result of the refinement of the sextant and after numerous observations and calculations had been made by astronomers in Europe regarding the movement of the moon relative to the stars and planets. In order to establish longitude, three observations had to be made and repeated as often as possible. The first was the height of the moon above the horizon. The second was the altitude of the chosen star and the third was the distance between the star and the moon. These readings then had to be ‘corrected’ for refraction and parallax and were then compared with records that had been compiled at observatories in Europe, published in what soon became known as ‘nautical almanacs’.

The third method was by means of a chronometer, a spring-driven clock which keeps time accurately over long distances and is not affected by changes in heat, gravity, barometric pressure, or movement (most importantly the movement of a ship at sea).  A trial version of the H3 took 19 years to develop and was first tried in 1761. It thereafter took many years to perfect, so that it was not until the early 19th century that it came into general usage. The chronometer made the calculation of longitude much simpler because it showed the time at the zero meridian. The establishment of longitude therefore became greatly simplified, because all that was necessary was that the time of day at the other position had to be determined.

Who could have placed the beacon in 1772 (or 1771)?

In 1772 Plettenberg Bay was an extremely remote corner of the world. As Pat Storrar herself goes on the explain, six years later (in 1778) the Dutch ship Katwyk aan Rhyn visited the Bay and the captain recorded that the bay itself was not marked on his charts, let alone the beacon. Strange as it may seem, the Dutch avoided this coastline on the journey to and from the East, keeping well to the south to avoid the treacherous rocks as well as the Agulhas Bank. The most accurate and detailed survey of the coastline had been carried out by the French mariner d’Apres de Mannevillette in 1752. This was a running survey and he established longitudes by means of measuring ‘lunar distances’ with a sextant, a technique he had helped to perfect.  He did not land at Plettenberg Bay, so could not have erected the beacon on the island.

There are only four persons who set foot in Plettenberg Bay at this time who might have been interested in establishing its co-ordinates and who may have possessed the expertise to do so. They were:

Carl Peter Thunberg. After studying Medicine and Natural Philosophy, he arrived in Cape Town in April 1772 to collect specimens for Carl Linneaus, the Swedish botanist.  During the next few years, he travelled extensively and visited Plettenberg Bay late in 1772, having come on horseback (some of the way on oxback) over the difficult terrain east of what is now George. With two unnamed companions, he was the first to travel from Plett, through what is now Prince Alfred’s Pass to the Langkloof, where he rejoined his baggage van. In March 1775 he proceeded to Java and Japan.  As he had passed through Plett only briefly and was on horseback, he would not have been able to transport a bulky and delicate telescope. He also makes no mention of any astronomical observations in his recollections.

Robert Jacob Gordon, who traveled around the Cape Peninsula with Thunberg and Masson in May 1773. According to Gordon’s biographer (Patrick Cullinan, pages 21-2), he then undertook a long journey and it is speculated that he may have been one of the two unnamed companions who accompanied Thunberg on his brief visit to Plettenberg Bay. He was certainly interested in mapping, but, if he did in fact visit Plett with Thunberg during 1773, would not have been here long enough to make accurate observations. It must also be said that he paid a second visit to Plett 14 years later, travelling down what is now the Prince Alfred’s Pass from the Langkloof on horseback. On this occasion, he spent four days exploring and produced the large panoramic painting of the Keurbooms River mouth from the top of the hill. His biographer Patrick Cullinan says Gordon took ‘various bearings of the area’ while staying at Wittedrift (page 57) but there is no mention of his having established longitude. Before his suicide in 1795, he was busy drawing a huge and detailed map of the Cape, now in the Rijksmuseum in Amsterdam. The southern parts of the Cape are blank. This seems to indicate he did not have this information and consequently could not have been responsible for the beacon on the Island. 

Van Plettenberg.  On his expedition, which visited Plettenberg Bay in November 1778, the Governor was accompanied by the surveyor Leiste, who subsequently drew a map showing the route that had been followed. While here, van Plettenberg erected the VOC beacon proclaiming Dutch ownership. Leiste’s map contains very little accurate information and places Plettenberg Bay (and Beacon Island) at latitude 4.5˚ east of Cape Town (22˚32' east of Greenwich, an inaccuracy of nearly a minute). These positions were probably obtained from Mannevillette’s 1752 chart or from a later chart produced by the Dutch East India Company.

Le Vaillant spent about 6 months in this area in 1782, collecting birds and shooting anything that moved, including nearly 30 red hartebeest and an elephant. Although he compiled a map after his return to France, that contains many inaccuracies, indicating that he was not interested in recording latitudes accurately, let along longitudes.

None of these individuals had the skill or the necessary equipment to calculate longitude.

One other possibility is Francois Renier Duminy, a French mariner who had learnt the latest techniques of navigation during his service in the French East India Company (possibly from Mannevillette himself). He carried out a new survey of the southern Cape coast between 1781 and 1787. This had become necessary because Mannevillette, for reasons best known to himself, had produced a revised and completely inaccurate version of his 1752 chart in 1775. During this survey, Duminy did a detailed survey of Plettenberg Bay in 1786. It shows the Robberg, Beacon island, the Lookout rocks, and the coast stretching as far as the Storms River mouth. Using a sextant, he calculated the latitude of Plettenberg Bay as 30° 50'S and the longitude as 5°15' east of Cape Town (23°42'east of Greenwich).

As Duminy was the first to make these calculations of latitude and longitude, he was the only one of the early visitors who would have been able to inscribe these details on a beacon. There are two reasons, however, why I do not think he did so. The first is that he did not show a beacon on his detailed chart of the bay. The second is that his co-ordinates do not coincide with those that were inscribed on the beacon. The latitude on the beacon is given as 34°3'38"S and longitude is given as 23°22'55", the most significant discrepancy between these and Duminy’s being a difference about 3 degrees of latitude.

My conclusion therefore is that there was nobody during the years 1752 and about 1790 who could have erected the beacon on the Island.

Beacon not shown on early maps

In further support of my argument, it can be shown that a beacon does not appear on any early map of Plettenberg Bay drawn after it was supposed to have been erected. If it was there and, if it was of such importance that navigators used it to ‘set their chronometers to Greenwich Mean Time and to make longitudinal and astronomical calculations’, it surely would have been shown.

I have already mentioned that it does not appear on Leiste’s map, on Mannevillette’s chart or on Duminy’s charts. The first reliable map of the area was drawn in about 1789 after a survey of the region by Johan Friderici and Josephus Jones. This was not a geodetic survey, based upon the establishment of the latitude and longitude, but was carried out using a ‘plane table’. At that time, map-drawing began with the shape of the coastline, obtained from a running survey such as those of Mannevillette in 1752 and Duminy between 1781 and 1787. Inland details were filled in after different landmarks had been observed from various points of observation. Positions were calculated by means of estimating the distance and the direction that had been traveled. Longitude, were therefore entirely a matter of guesswork.  These early surveyors would therefore have been very interested indeed in a beacon which showed the longitude of Plettenberg Bay and they would surely have indicated this on their map if it was there. Friderici’s map, however, shows Beacon Island as a bare outcrop with nothing on it but stones and coastal scrub.

The next map to be drawn of the coast was by Lieutenant McPherson Rice in 1795. He had completed a new running survey that year after the British occupation.  Like Duminy, he produced a detailed chart of Plettenberg Bay. He would also have had the opportunity to set foot on Beacon Island to erect a beacon there. I think it unlikely however, for the same reason as I have rejected the possibility that Duminy did so. This is that McPherson Rice did not show a beacon on his chart. His estimates of latitude and longitude also do not tally with those inscribed on the beacon: he estimated the latitude to be 34° 6’ 36” S (a variance of 2 minutes from that marked on the beacon) and longitude 23° 48’ E (a difference of 26 minutes) His magnetic variance was 27’ 12” (a difference of nearly 20 minutes).

The most accurate survey of the Cape coast was that of Captain William Owen in 1822, as part of the Great African survey undertaken by the British Admiralty. The main purpose was to survey the virtually unknown coastline of Africa from the Keiskamma River to Cape Guardarfui (the headland of the Horn of Africa in Somalia) but Owen surveyed the southern Cape as well. This was one of the most ambitious survey projects yet undertaken and Owen used no less than nine chronometers to provide him with accurate Greenwich Mean Time. He did not, however, land anywhere along the coast and so would not have been able to place the beacon on the island or to read the inscription on a beacon, if it was already there. His chart shows the longitude of Plettenberg Bay at about 23˚E.

The absence of any record of a beacon on any of these early maps and charts supports my contention that the beacon did not exist and was not ‘used by ships at sea to set their chronometers for longitudinal or astronomical calculations’.


As there is today a beacon on the island and as it could not have been placed there before 1772, we must assume  that the beacon was placed there at a later date. The first reference I can find to a beacon is in Sewell's diaries. (He was Harbour-Master of Plettenberg Bay between 1874 and 1897.) On May 12 1893 he refers to a notice calling for tenders to enclose ‘the beacons on the island and van Plettenberg’s. (I don’t think he was referring to more than one beacon on the island, but was using the plural ungrammatically). On 10 January 1894, he then refers to the fact that a wall had been built around it (Storrar, pages 185, 193). The beacon was therefore definitely there in 1893 and it must have been there for some time if Sewell was worried about its deteriorating condition.

The first surveys. Before we go on to discuss further the question as to who placed the beacon there, I have to digress and tell you about another chapter in the history of mapping in South Africa. This is land-surveying. The drawing of accurate maps depends upon the establishment of the geodetic co-ordinates of as many positions as possible by means of a chain of triangles. It begins with the measurement of a baseline, measured from a position the co-ordinates of which have been established by means of astronomical observations (this is known as the ‘datum’). Once this has been done, the position of other landmarks can be established by means of triangulation (trigonometry).  The instrument used to measure these angles was called a theodolite. Those used in the first great surveys undertaken in the nineteenth century were huge and heavy (that used in the great survey of  India led by Sir George Everest, for example, weighed over 500 kilograms). They had to be transported from one position to another, up and down hills, across rivers and to the mountain summits. Survey work was therefore extremely laborious and time-consuming. The Great India survey, led by Sir George Everest, took 64 years to complete.

The first survey undertaken in South Africa was that of Lacaille. The next survey was that of Charles Michell, who became Cape Surveyor-General in 1828. He started his survey in 1834 in the Tsitsikamma region. When I read this, I for a moment imagined that our problem had been solved, but his biographer (Gordon Richings) informs us that Michell commenced his survey in the Langkloof and visited Plettenberg Bay only briefly on 16 November on his way from Great Brak to Avontuur (page 76). Michell abandoned his survey shortly after he had begun this work because he had to proceed to the Eastern frontier, where war broke out at the end of the year. He visited Plett again in 1836, when his records show that he ‘took bearings’ of Driefontein, Tsitsikamma, the mouth of Witte Els River, Plettenberg Bay Point (the Robberg) and Formosa Peak (page 100). There is no mention of Beacon Island.

Because of the discrepancies between Lacaille’s measurements and those obtained in surveys in other parts of the world, the Cape Astronomer Sir Thomas Maclear was instructed in 1839 to repeat Lacaille’s survey of the arc of the Cape Town meridian. As no trace of Lacaille’s positions could be found, he measured a new baseline and, between 1841 and 1846, surveyed 31 triangles, extending as far south as Cape Agulhas. This, for the first time, established a number of positions to which other surveys could be linked. (Maclear marked his positions with stone cairns 14 feet high so that they would not disappear as Lacaille’s had done.)

Captain William Bailey. That brings me to the last possibility and to the conclusion of my talk. Between 1859 and 1862 a major survey of the southern Cape was carried out by Captain William Bailey on behalf of the British Admiralty. The reason for this was that in 1852 the tragic loss of the Birkenhead had occurred at Danger Point, near Hermanus. The reason for this loss was the complete unreliability of the existing charts. The aim of the survey was therefore to establish the precise geodetic positions of recognisable landmarks along the coastline.

Between 1859 and 1862 Bailey surveyed a chain of triangles stretching from Cape Agulhas to Port Elizabeth, erecting beacons (some of stone and some just a wooden stake in the ground, like that on the Beacon Island). Most of his data, together with his two large theodolites, were lost when the ship that was taking them back to Cape Town was wrecked near Cape Agulhas. Fortunately, Bailey had kept his notes and many of his readings and, from them was able to produce a report which showed his triangles, extending eastwards as far as a position near Grahamstown in the Eastern Cape. Fifty years later, the beacons he had erected could not be found when Henry Fourcade did his surveys of the mountains. That is another story.

My conclusion is that the beacon on the island was placed there by Bailey during this survey. If this is so, it was one of the few remnants of this ill-fated episode in the history of South African mapping. His equipment and his records were lost in the wreck of the Waldenstein, but the beacon survived where it had been placed, thanks largely it seems to Captain Sewell’s interest. It is a great pity that the original, like so many other historical relics of Plettenberg Bay, has been lost. I hope, however, that I have succeeded in uncovering the truth surrounding its history.

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