Cartography

Piece of a history lesson to surveyors

PUBLISHED : Sunday, 20 April, 1997, 12:00am
UPDATED : Sunday, 20 April, 1997, 12:00am

When the Rifleman's bolt was first hammered into the bedrock on the shoreline of Hong Kong Island in 1866 by swarthy weather-beaten sailors, Sir Richard MacDonnell had just arrived to govern a young colony of 125,000 people.


Six years before, the Qing dynasty had ceded three square miles of the Kowloon peninsula to Britain after Lord Elgin sacked the Summer Palace outside Beijing. In London, Queen Victoria was in her prime.


Today, the copper bolt sits on a desk in the Murray Building office of Chan Hak. Fourteen centimetres long, it is an important historical relic for surveyors.


By tapping it into the bedrock 131 years ago, the sailors of Her Majesty's Surveying Vessel Rifleman began mapping the land and waters of Britain's new territory in 1866. Now Mr Chan, who is head of the Survey and Mapping Office, wants to put the Rifleman's bolt back as close as possible to its original position, near the present-day Lippo Centre in Admiralty.


'It's a part of the history of surveying in Hong Kong, 1866 was the first time that a vessel came to Hong Kong to do a sounding survey,' Mr Chan said.


The Rifleman's bolt establishes a reference surface for all heights and depths in Hong Kong, which surveyors measure against the lowest tide. The lowest water level recorded by the crew of the Rifleman was 1.23 metres below the average sea level, and this became what surveyors call the Principal Datum.


A mark at the Principal Datum itself, though, would be virtually useless for surveying purposes as it would nearly always be covered by water, so surveyors commonly establish a mark on land.


The crew of the Rifleman knocked the bolt into a handy rock, and then calculated that it was 17 feet 10 inches (5.43 metres) above the Principal Datum.


By calculating heights against the Rifleman's bolt, and then adding 17 feet 10 inches, surveyors could work out heights against the Principal Datum.


Even today, the Rifleman's bolt is the first lesson for surveying students in the territory, according to Ronald Chan Chi-duen, the Survey and Mapping Office's head of training. 'It is the fundamental for all surveying in Hong Kong,' he said.


The surveyors on the Rifleman took soundings of the harbour to produce the first navigation charts showing accurate measurements of its depth. According to Chan Hak, this would be a difficult task because the water level changes constantly with the tide.


'During the survey they would have had to send someone to take tidal readings at the bolt every half an hour to record the variation of the water surface,' he said.


By noting the time each sounding in the harbour was taken, and comparing it to the tidal reading taken at the same time, the surveyors could calculate the water depth accurately against the Principal Datum.


'At the same time,' Mr Chan said, 'development was also taking place on shore, and of course that mark would also be referred to for all the land surveys.


'Even today,' added colleague Simon Kwok Chi-wo, a senior land surveyor, 'the Principal Datum, 17 feet and 10 inches below the bolt, is an important surface for all types of development.


'All construction and infrastructure works have to be related to the Principal Datum. For example, your building should not be higher than a certain height, that refers to the Principal Datum.


'And there are 2,000 benchmarks, height reference marks, along all the main roads for people to refer to when they are doing construction surveys.


'They all tie up to the same reference and this is the Principal Datum. Ordinary people have a relationship with this, but they do not realise it.' The Rifleman's bolt was embedded in rock for only a year, according to Mr Chan, before it was moved to the wall of the Royal Navy's storehouse number 12 on Tamar Street near the former naval dockyard in Admiralty, with a stone plaque commemorating the Rifleman's visit.


When the storehouse was demolished in 1962, the bolt and plaque were attached to the wall of a building on HMS Tamar naval base, then in Admiralty, as a memorial to the history of surveying in Hong Kong. On the wall, Mr Chan said, the bolt was not at its original height of 17 feet 10 inches above the Principal Datum.


Last month the British military removed the bolt and offered it to the Survey and Mapping Office, according to military spokesman Roger Goodwin.


'There are a number of historical items around the military establishments in Hong Kong,' Mr Goodwin said. 'Those that can be moved are being donated to organisations or departments which are considered to have the greatest interest in them. A number of mementoes from the Hong Kong Military Service Corps, for instance, have gone to their Regimental Association.


'The Rifleman's bolt, which has a considerable significance for the survey people in Hong Kong, has been given to them for safe-keeping. For them it will mean something and they will look after it.' The Museum of History, according to Mr Chan, also wanted the bolt. 'They happen to be on a hill,' he said. 'The important thing is to restore the original level of the bolt and also its location. Otherwise we might as well just keep it in our office.


'Now we want to restore it to somewhere near its original location and more importantly, put it back to the original level.' Finding somewhere to put the Rifleman's bolt could prove difficult, however, even if there are no objections because of bad effects on fung shui, resurrecting a possibly colonial symbol, or a small monument proving a hazard to pedestrians.


Due to successive construction and road building projects, the ground around the Lippo Centre is already well above the required 17-foot-10-inch level.


'It would be below the level of the road,' Mr Chan said. 'We may have to put it in Admiralty MTR station, just inside the entrance.'