The truth about maps: How cartographers distort reality

As a fascinating new exhibition shows, it's not always what they put in that matters – but what they leave out

What is a map? In effect, says Peter Barber, head of maps at the British Library, a map is a lie. "Unless you have a scale of one-to-one, every map is subjective, and always will be," he explains. "You have to select what you put on it." And selection involves rejection.

Throughout history, such lies have generally served purposes which have been political, religious or philosophical rather than scientific. Man didn't only create maps to convey a sense of physical place – here in relation to there – he made them to situate himself in society, and in relation to the powers which governed it. When God was believed to rule, his kingdom had to figure on the map as well. And if the Earth was believed to be a flat plate supported by four elephants standing on the back of a turtle – or if it was thought to be surrounded by an ocean, with the vault of the sky resting on the Earth's rim, as Homer assumed – then those images also became part of the map.

Our word "map" derives from the Latin "mappa", meaning a table napkin. The most common medieval form was circular, reflecting "the circle of the Earth" as in Isaiah 40:22 – and this became known as the "T-O" map. Here the Earth was surrounded by an ocean – the O – within which lay the three known continents, separated by waters drawn, with diagrammatic neatness, in the shape of a T. Since nearly all medieval maps had East at the top, Asia filled the upper half, while the Nile separated it from Africa and Europe; the Mediterranean formed the upright of the T. This way, Jerusalem could conveniently sit at the centre of the map, thus satisfying the Church's demands. This T-O template was used by Arab cartographers, too.

According to Barber, there is an ongoing battle for the soul of cartography – "between the Anglo-Saxon world and the German-speaking world, which tends to suggest that the only person who can pronounce on map-making is a professional surveyor". And even in the Anglo-Saxon world, the study of cartography has been skewed, he says: by geographers whose approach is relentlessly scientific, by librarians whose approach is bibliographical, and by cultural historians for whom maps are just utilitarian tools. Barber has spent decades studying the extraordinary wall maps that decorated European palaces in the 16th and 17th centuries, and which have largely escaped serious cartographical attention. He wants us to see the British Library's new exhibition of these as a challenge to the "dictatorship" of the geographers and librarians, and to the myopia of the historians. "Nobody has thought to ask why these beautiful things were made, why so much money was spent on commissioning them – and what role they played."

"Power, propaganda and art" is the exhibition's subtitle – political spin was the usual subtext. As with Hitler's schoolroom map of "Deutschland" in 1935, in which all the German-speaking areas surrounding Germany were presented – without borders – as part of the Reich: pressure from Mussolini forced him to commission a less imperialist version with the borders restored. Or as with the US government's invasion map of Europe in 1944, which was one of the most insidious pieces of map-making propaganda in history. This view, promulgated by the National Geographic, presented the Soviet Union as grossly inflated in size, and became the template for Western thinking throughout the Cold War. Only in 1988 did America adopt a modified version with an accurately scaled-down Soviet Union. As Barber points out, "when you deal with projections" – and this map was based on a projection inspired by a very partisan view of geopolitics – "you can do what the hell you like".

Some maps are propaganda pure and simple, with the most famous being Fred Rose's "Serio-comic war map for the year 1877". In this, Russia is portrayed as an octopus throttling Poland and Finland, while the other countries of Europe go about their business – Greece joining Russia to attack the Turks, France planning to avenge its defeat by Germany, Britain acquiring Cyprus. As the exhibition shows, the octopus motif proved versatile: in 1917, France portrayed Prussia as the octopus; and in 1942, Vichy France recast it as Winston Churchill.

The oldest piece in the exhibition takes us back to the dawn of map-making, in the form of a carved wall-map of Roman streets and dwellings made in AD 200. The first map-makers had limited, practical aims – they did not aspire to encompass the world. Marshall Islanders in the Pacific lashed sticks together with fibres to depict winds and wave patterns, and inserted shells to denote islands; early Mexicans indicated roads by lines of footprints; Eskimos carved coastal maps in ivory; and early Europeans drew sketch maps on the walls of their caves. Chinese cartographers – ahead of everyone else – had mapped their whole kingdom by 1125 BC, but their knowledge of the wider world was almost non-existent: they believed it was square, and they assumed their country formed the bulk of it. When Indian Taoist doctrine filtered through in the 4th century BC, announcing that China only occupied one-eightieth of the Earth's surface, Chinese maps changed accordingly.

The earliest recorded attempt at a reasoned conception of the universe was made by the 6th-century-BC Babylonians. They divided the sky into 360 degrees, and the day into hours, minutes and seconds, thus allowing time to be plotted in relation to the stars. The ancient Greeks brought logical analysis to bear on the Babylonians' astronomical observation, and it was this that led to Pythagoras' conclusion, in the 6th century BC, that the Earth was a sphere rather than flat. Three centuries later, Eratosthenes, head of the great library of Alexandria, devised a way of measuring the size of the Earth using simply the rays of the sun, a well and a vertical stone column. And it was his great Alexandrian successor Claudius Ptolemy who introduced the cartographic system we use today, with his idea of latitude and longitude, and his insistence on the use of astronomical observation as a way of calculating geographical location.

If the Alexandrian research tradition had continued after Ptolemy, such misconceptions would probably have been eliminated, but when a Christian mob destroyed the library and its contents in AD 391, Ptolemy's ideas became lost to the Western world for a thousand years as faith supplanted reason. Ptolemy's Western successors rejected scientific enquiry as pagan and irrelevant.

They were much more interested in the search for Paradise, which they believed could be physically located. Their speculations were fed by the voyage of a 6th-century Irish monk called Saint Brendan, who sailed westward on the instructions of an angel in a dream. In five years at sea, he and his crew claimed to have encountered the devil in his palace, an island belching fire, a floating crystal temple, and finally Paradise as a beautiful island. For the next thousand years, this island regularly appeared on maps, set in varying parts of the Atlantic: sometimes among the Canaries, or off the coast of Newfoundland, or near Cape Verde. The 7th-century Archbishop Isidore of Seville claimed to know both where it was and exactly what it was like; seven centuries later, the English traveller Sir John Mandeville wrote a detailed description from hearsay, while admitting that he personally had not yet been there.

In the Middle Ages, road maps and sailing charts were the only practical maps, as opposed to ideological ones. Scientific map-making was not revived until the dawning of the Renaissance urge to discover; by 1500, all educated Europeans assumed once more that the Earth was a sphere. Ptolemy's "Geography" was one of the first classical texts to be printed, and map-makers took note, even if Ptolemy's world was smaller than it is in reality, consisting of just three continents – Europe, Asia and Africa – with a relatively short stretch of water separating Europe and Asia; it was the persistence of this assumption which encouraged Columbus to sail west, in the confident expectation that he would make his landfall in Asia, rather than in an America of which Ptolemy and his disciples did not dream.

It was only in 1569, when the Dutch cartographer Gerardus Mercator devised a way of translating the sphere with relative accuracy into the flatness of a map, that map-making really took off. But his famous "projection" – on which navigation is still based – has a built-in distortion which gets worse the further you move away from the Equator: it actually suggests that Greenland is the same size as Africa – rather than being, as it is, one-14th the size. But as Barber points out: "It's impossible to depict accurately the sphere of the Earth on a piece of paper. You either get the shapes right, or you get the sizes right, and all maps compromise between these two approaches. Mercator wanted to enable sailors to draw a straight line on the map, and to have that straight line reflecting reality. And as most voyages in his time happened in the temperate zones [between the Tropics and the Polar circles], you could get away with that, but the price was distortion at the top and bottom."

In 1974, this distortion, and its traducing of geographical and economic realities, inspired the German cartographer Arno Peters to champion a 19th-century "equal area" projection which more truthfully reflected reality: aid agencies now use Peters in preference to Mercator. "This is a good propaganda tool," says Barber, "but it's just another version of the truth, not the whole truth."

So, assuming that no map is perfect, nor ever can be, which is Barber's preferred representation of the Earth? "There's a lot to be said for Google Earth," he replies, "because you can zoom in and see the street-view. And you have this beautiful central image. Until recently the globe has always been used as a symbol of power – as in The Great Dictator, in which Charlie Chaplin plays around with a globe which is really a balloon. But as a result of space flight, and concerns about ecology, the Earth has now become a symbol of fragility. I look at Google's image of the Earth, and just think how beautiful it is." He gets out his mobile and shows me his screen-saver: Google Earth. "Though this is my favourite image, it's actually not a map – or rather, it's a totally artificial one. It epitomises the trickery of maps. It's a scientifically constructed image, with false colours – but you're coaxed into believing that it's the reality. It means something to you, because it has an emotive charge." Once again, utility is sidelined, to make way for symbolism and propaganda. "All the maps in our exhibition have been chosen because they speak to the emotions, rather than to reason." All lies, beautiful lies...

Magnificent Maps: Power, Propaganda and Art is at the British Library, London NW1, from 30 April to 19 September