How to Draw a Map. Malcolm Swanston
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to destroy that threat, Caesar’s own ships were burnt and, according to some descriptions, the resulting conflagration spread to the Great Library, destroying much of its invaluable contents.
Ptolemy sat among the patched-up halls of the library compiling his Geography in around 150 CE. Much of his writing relied on earlier work, especially that of Marinus of Tyre (70–130 CE). His study was written in Greek – as were most of its precursors – on a papyrus roll over eight sections or ‘books’, and in it he reviewed the combined total research of the classical world to date. The outcome of this monumental work was to define mapmaking, at least in the West, for the next 2,000 years.
Ptolemy regarded himself firstly as a mathematician, astronomer and philosopher. He did not refer to himself as a geographer. Indeed, the Greek spoken by Ptolemy had no word for geography. What we call a map, he called a pinax, or he may have used the phrase periodos ges, a ‘circuit of the earth’. In time, these terms would be replaced by the Latin mappa.
Ptolemy had established his credentials, first in astronomy, writing a work called Ho megas astronomos (‘The Mathematical Collection’), later known as the Almagest (Arab astronomers used the Greek superlative term Megiste for this work, and when the definite article in Arabic, ‘al’, was prefixed it became Almagest).
The Almagest’s 13 books deal in detail with astronomical concepts, the stars, the solar system and other observable objects. In them, Ptolemy produced his geocentric theory that placed the earth at the centre of the universe, often known as the Ptolemaic Cosmology, and this view was held by the majority of observers and thinkers until the heliocentric, sun-centred theory developed by Copernicus 1,300 years later.
In the Almagest Ptolemy wrote:
I know that I am mortal by nature, and ephemeral; but when I trace at my pleasure the windings to and fro of the heavenly bodies, I no longer touch earth with my feet, I stand in the presence of Zeus himself and take my fill of ambrosia.
He obviously felt some passion for his work. Now we come to Ptolemy’s Geography. He approached this work by first gathering the learning from the Babylonian, Greek, Roman and Persian worlds and applying this accumulated knowledge into a mathematical framework.
A large part of Book 1 describes how to draw the maps using his ‘projection’ – his coordinates are in degrees: 360 to complete the circle, 60 minutes to a degree, just as we still use. His east–west longitudes are measured eastwards beginning with 0 degrees at a point just west of the Canary Islands, known to him as the ‘Blessed Isles’. The north–south latitudes are measured heading north from the equator, again like our own today. He did, however, mark his degree sign with an asterisk: 10*, not 10° as we now use.
His skill was in devising this mathematical formula that allows the rendering of the earth’s sphere onto a flat surface. This was an improvement on past ‘world’ map projections, given that all maps are some kind of compromise when rendering a sphere onto a flat sheet of papyrus or paper. He also placed north at the top, an orientation once again familiar today. This was the first example of a conical map projection. Ptolemy had succeeded in providing a simple, reliable method of drawing a world map.
The complete text drew on all the works available to Ptolemy in Alexandria, such as Tacitus with his descriptions of Gaul. Ptolemy laid down rules for mapping local features such as cities, harbours and farms. He stressed the importance of astronomy and mathematics to geography, creating a system based on the unchanging features of the sun and the stars, and demanding that longitude and latitude be used to fix locations of geographical features: mountains, estuaries, settlements, etc. By adhering to this system, mapmakers ensured their maps were accurate and could be systematically reproduced. That is, of course, as long as the information supplied was accurate – a problem the modern mapmaker still encounters.
The completed text of Geography is more than a list of geographical coordinates. Ptolemy chose to review and carefully analyse the descriptions available to him, then selected the most reliable, but he still left a warning as to the reliability of some descriptions. The mapmaker is only as good as the information he has to hand. This gazetteer was eventually to list some 8,000 locations, according to their latitude and longitude, beginning in the west with the British Isles, moving eastwards across Europe towards Asia Minor and ending with India in the east, his known world. It is not known if Ptolemy produced maps himself to go with his geographic gazetteer; if he did, none have survived.
Ptolemy divided the globe’s circumference into 360 degrees (the Babylonian sexagismal system); every degree was measured in units of 60. He estimated each degree at 500 stades (2,700 kilometres), a total of 180,000 stades. He envisaged his world as smaller than Eratosthenes’ calculations; however, he did argue that the world’s inhabited regions were larger than many believed, reaching from the Fortunate Isles in the west to Cattiagara in the east, believed to be somewhere in modern north Vietnam. From north to south was measured at 40,000 stades from Thule at 63 degrees north to Agisymta in sub-Saharan Africa at 16 degrees south. Ptolemy’s world had blurred edges based on theories, stories and guesswork. For instance, he believed that a large, unknown continent existed in the southern hemisphere in order to balance Europe and Asia in the northern hemisphere and that the Indian Ocean was enclosed by Africa, extending eastwards to join up with Asia. This last feature appeared on Ptolemaic maps even after the Portuguese had sailed around Africa into the Indian Ocean.
After Ptolemy’s time, whatever was left of the Great Library of Alexandria was lost through invasion and war, a fact that haunts the enquiring mind to this day. However, some works survived and Geography was one of them. The earliest known copy of Geography that exists is in Arabic and dates from the 12th century, preserved by Islamic scholars who reintroduced it into Europe, where it was translated from Arabic into Byzantine Greek in the 13th century and then into Latin in the 15th century. The work made it into print in 1475. The Ulm edition, printed in 1482 in Germany, included woodcut maps of new discoveries like Greenland, which was placed using the Ptolemaic principles of its longitude and latitude. Now, 1,300 years after his death, Ptolemy was a bestseller as his books poured off the printing presses.
Map 9. Ptolemy’s maps were the first to use longitudinal and latitudinal lines as well as specifying terrestrial locations by celestial observations.
Ptolemy’s mathematical scientific system made the chaotic world understandable by the application of methodical geometric order. Thousands of descriptions made by sailors, travellers and generals, with all their sense of wonder at the almost endless diversity of landscapes and peoples, came together as an understandable whole. Ptolemy’s achievement lasted into and past the Renaissance. I still plot my maps in degrees and minutes. I still check gazetteers of verified locations – now online, of course.
These things belong to the loftiest and loveliest of intellectual pursuits, namely to exhibit to human understanding through mathematics both the heavens themselves in their physical nature (since they can be seen in their revolution about us), and the nature of the earth through a portrait (since the real earth, being enormous and not surrounding us, cannot be inspected by any one person either as a whole or part by part).
Ptolemy’s Geography, Book I
The classical world in which this achievement was created, though, was now under threat.
By the beginning of the 4th century CE, the empire in the West was perhaps less ‘Roman’: many of its provinces were in the ownership of federated tribes – semi-independent kingdoms concerned with their own politics. The Roman state was a less cohesive entity. In the East, Alexandria, still a centre of learning, though much reduced, also became a centre of revolt. The museum buildings, by now over 500 years old and not in the best of condition, were finally destroyed,