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To the 1st century BC
1st - 15th century
16th - 20th century
Problems of projection
Mercator
Chronometer
Improvements in surveying
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Problems of projection: 16th century
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The European discovery of America and of the Pacific coincides with an increase in ocean travel and with the new printing techniques of woodcut and engraving. The result is a great demand for maps which can be cheaply produced and which, unlike a globe, will take little space - lying flat, and capable of being folded or even bound into book form.
The printed map is in its vigorous infancy during the 16th century. But a globe remains the only accurate way of representing the land masses on the surface of the spherical earth. How are the newly discovered facts of world geography to be represented on a flat surface?
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The problem is real, and in a real sense insoluble. Imagine a rubber globe, hollow like a football. The information on its surface is accurate. But try cutting the globe in half and laying each half out flat, as on a page. It is impossible to do so. Distortion is inevitable. The particular distortion chosen is known as the map's projection. One of the best known is that used by Gerardus Mercator.
His framework is far from new. The grid system of latitude and longitude dates back to Hipparchus in the 2nd century BC, and the prime meridian (or 0° longitude) has run through the Canaries since the second century AD, placed there by Ptolemy. But Mercator's projection is based on new scientific principles.
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Mercator's projection and atlas: 1569-1595
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Mercator publishes in 1569 a map of the world specifically stated, in its title, to be intended as an aid to navigation. It is laid out on the projection now known by Mercator's name, though it has been used by one or two others before him.
Mercator's projection has the effect of greatly enlarging territories as they recede from the equator. India, for example, appears smaller than Tierra del Fuego. The Moghul emperor Jahangir is understandably displeased at the diminutive size of his empire when the British ambassador, Thomas Roe, presents him with a copy of Mercator's world map.
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The distortion of Mercator's projection is a benefit to navigators. By gradually lengthening the lines of longitude towards the poles, Mercator achieves a matching scale for longitude and latitude in every section of the map (the northern degrees of latitude, being shorter in reality, are exaggerated on a regular grid). A compass course can be plotted at the same angle on any part of Mercator's map. As a result marine charts still use this projection.
From 1569 Mercator devotes himself to a vast project, producing a series of maps of Europe which compare Ptolemy's version with improvements based on modern knowledge (much as Vesalius has to measure his own anatomical discoveries against the yardstick of Galen).
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By the time of his death Mercator has either published or prepared large engraved maps, designed for binding into volume form, of France, Germany, Italy, the Balkans and the British Isles.
A year after his death, in 1595, Mercator's son issues the entire series under the title Atlas sive Cosmographicae Meditationes ('Atlas, or cosmographic meditations'). It is the first collection to bear the title 'atlas'. Probably based on the Greek mythological character Atlas, whose task is to support the heavens, the name becomes the standard European word for a volume of maps.
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Chronometer: 1714-1766
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Two centuries of ocean travel, since the first European voyages of discovery, have made it increasingly important for ships' captains - whether on naval or merchant business - to be able to calculate their position accurately in any of the world's seas. With the help of the simple and ancient astrolabe, the stars will reveal latitude. But on a revolving planet, longitude is harder. You need to know what time it is, before you can discover what place it is.
The importance of this is made evident when the British government, in 1714, sets up a Board of Longitude and offers a massive £20,000 prize to any inventor who can produce a clock capable of keeping accurate time at sea.
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The terms are demanding. To win the prize a chronometer (a solemnly scientific term for a clock, first used in a document of this year) must be sufficiently accurate to calculate longitude within thirty nautical miles at the end of a journey to the West Indies. This means that in rough seas, damp salty conditions and sudden changes of temperature the instrument must lose or gain not more than three seconds a day - a level of accuracy unmatched at this time by the best clocks in the calmest London drawing rooms.
The challenge appeals to John Harrison, at the time of the announcement a 21-year-old Lincolnshire carpenter with an interest in clocks. It is nearly sixty years before he wins the money. Luckily he lives long enough to collect it.
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By 1735 Harrison has built the first chronometer which he believes approaches the necessary standard. Over the next quarter-century he replaces it with three improved models before formally undergoing the government's test. His innovations include bearings which reduce friction, weighted balances interconnected by coiled springs to minimize the effects of movement, and the use of two metals in the balance spring to cope with expansion and contraction caused by changes of temperature.
Harrison's first 'sea clock', in 1735, weighs 72 pounds and is 3 feet in all dimensions. His fourth, in 1759, is more like a watch - circular and 5 inches in diameter. It is this machine which undergoes the sea trials.
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Harrison is now sixty-seven, so his son takes the chronometer on its test journey to Jamaica in 1761. It is five seconds slow at the end of the voyage. The government argues that this may be a fluke and offers Harrison only £2500. After further trials, and the successful building of a Harrison chronometer by another craftsman (at the huge cost of £450), the inventor is finally paid the full prize money in 1773.
He has proved in 1761 what is possible, but his chronometer is an elaborate and expensive way of achieving the purpose. It is in France, where a large prize is also on offer from the Académie des Sciences, that the practical chronometer of the future is developed.
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The French trial, open to all comers, takes place in 1766 on a voyage from Le Havre in a specially commissioned yacht, the Aurore. The only chronometer ready for the test is designed by Pierre Le Roy. At the end of forty-six days, his machine is accurate to within eight seconds.
Le Roy's timepiece is larger than Harrison's final model, but it is very much easier to construct. It provides the pattern of the future. With further modifications from various sources over the next two decades, the marine chronometer in its lasting form emerges before the end of the 18th century. Using it in combination with the sextant, explorers travelling the world's oceans can now bring back accurate information of immense value to the makers of maps and charts.
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Improvements: 18th - 20th century
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Once the basic information about the world's landscape is broadly known, improvements in mapmaking become those of greater accuracy, clarity and detail.
A strong impulse towards better surveying and mapmaking comes from the demands of the military. Britain's national cartographic agency admits as much in its title. It is established in 1791 as the Ordnance Survey ('ordnance' being an old-fashioned word for artillery). With a threat of invasion from France during the 1790s, it is not surprising that detailed maps of Kent are the first to be printed.
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Advances in surveying make it possible to calculate with accuracy the height of mountains and later even the depth of oceans. Places of the same altitude can be plotted, and recorded in the form of contour lines. By the late 20th century satellites add a new dimension. Powerful lenses in orbit above the earth record the tiniest details on the planet's surface, plotting even the changing patterns of weather or vegetation.
Improvements in colour printing make it increasingly possible to publish this wealth of information in complex form. And digital technology brings an added flexibility in the use of maps on computer screens.
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