Maps and their makers: an introduction to the history of cartography

much more accurate measurement of a number of short lines, or bases.Similarly, instead of determining direction by observing the position of a shadow at midday, or of a constellation in the night sky, or even of a steady wind, use was made of terrestrial magnetism through the magnetic compass, and instruments were evolved which enabled horizontal angles to be measured with great accuracy.Meanwhile the astronomers showed that the earth is spherical, and that the position of any place on its surface could be expressed by its angular distances from the Equator (latitude) and a prime meridian (longitude), though for many centuries an accurate and practical method of finding longitude baffled the scientists.The application of these astronomical conceptions, and the extension of the knowledge of the world through exploration and intercourse, encouraged attempts to map the known world-but this introduced another problem: how to map a spherical surface on a flat sheet.The mathematician came to the rescue again-with his system of projections, by which some, but not all, spatial properties of the earth's surface can be preserved on a single map.Then the astronomers discovered that the earth is not a perfect sphere, but is flattened slightly at the Poles; this introduced further refinements, such as the conception of geodetic as opposed to astronomical latitudes, into the mapping of large areas, and great lines of triangulation were run north and south across the continents to determine the true 'figure of the earth', and to provide bases for their accurate mapping.Meanwhile, increasing demands were being made on the map maker.The traveller or the merchant ceased to be the sole user of maps.The soldier, especially after the introduction of artillery, and the problems of range, field of fire, and dead ground which it raised, demanded an accurate representation of the surface features, in place of the earlier conventional or pictorial delineation, and a solution in any degree satisfactory was not reached until the contour was invented.This again adds to the task of the surveyor who must run lines of levels and sometimes go to the extent of pegging out the contour lines on the ground.Then the archaeologist, the historian, and much later, the modern geographer had their own special the booksellers and engravers, ""who copied and compiled what they wanted quite uncritically, using any old maps and plates that came to hand''.Clearly the maps, many thousands in number, which have MAPS AND THEIR MAKERS practical purposes, which corresponded approximately with what the Greeks called 'chorography', there developed very slowly the science of 'geography', that is, the mapping of the whole known world on scientific methods, what we should call cartography.Unlike the determination of latitude, for which the axis of the earth provides an established reference datum, the problem of longitude long bafHed the astronomers, for no meridian is marked out as an initial one, in the way that the Equator serves as an initial parallel.Since the earth makes one revolution in a day, more or less, it was early recognized that simultaneous observations of a celestial phenomenon such as a lunar eclipse would, through the difference in the local times at the moment of observation, give a value for the difference of longitude (1 hour=15° of longitude).Without the requisite astronomical tables or accurate portable time-keepers, the method was impracticable, though a few attempts were made to observe eclipses for this purpose.On all early maps, until the seventeenth century, longitudes were arrived at by transforming distances into their angular values in relation to the circumference of the globe.For this it was necessary to arrive at a value for the circumference, which, divided by 360, would give the length of a degree.This was done with considerable accuracy, perhaps owing to the cancelling out of errors, by theGreek astronomer Eratosthenes, who measured the meridian arc between Alexandria and Syene.The figure he arrived at for the circumference of the earth was 252,000 stadia, which assuming he employed the short stade, was the equivalent of 24,662 miles, a result only some fifty miles short of the reality.From this result it followed that a degree was equivalent to 68.5 miles.Unfortunately, this quite accurate figure was not accepted by his successors, with important results for the history of cartography.The Greeks had also attacked the problem of the projection of the earth's surface on to a plane in order to arrive at an orderly arrangement, or graticule, of parallels and latitudes, with reference to which positions could be located.The drawing of parallels was relatively simple, at least in the restricted area for which observations were available.Eratosthenes CLASSICAL AND EARLY MEDIEVAL HERITAGE 19attempted to extend two parallels eastwards on the basis of the relative directions between important places noted by travellers and also on the assumption that districts with similar climates and products would lie on or near the same parallel.In this way he established two main parallels, one running along the assumed axis of the Mediterranean (Gibraltar-Messina-Rhodes) continued through the Taurus, the Caspian Gates and along the Imaus mountains.For another he assumed that Meroe in Egypt lay on the same parallel as south India.The establishment of meridians, for reasons already stated, presented still greater difficulties.Without the aid of the magnetic compass, it was extremely difficult to determine the bearing of one place from another.This knowledge was derived from approximate astronomical observations, such as the position of sunrise at the equinoxes, or of the constellations in the night sky.From these Eratosthenes established an initial -meridian, which assumed that the mouth of the Don, Lysimachia on the Dardanelles, Rhodes, Alexandria, Aswan and Meroe all lay on a direct north to south line.These attempts to provide a fixed framework for the world map were criticized by his successors on the ground that the available data were insufficient.Hipparchus, the best of the Greek astronomers, corrected him in detail, and laid the foundation for further progress by compiling a table of latitudes.With the progress of more detailed knowledge and the expansion of the known world through the achievements of Alexander the Great and the Romans, a vast mass of detail accumulated for the use of later cartographers, who could thus take up the task outlined by Eratosthenes and Hipparchus with greater assurance of success.In the second century A.D., two names are outstanding, those of Marinus of Tyre and Claudius Ptolemy of Alexandria.The work of Marinus is known to us almost entirely from Ptolemy's references to him in his 'Geographical exposition'.tMarinus developed earlier ideas to construct a network of meridians and parallels, but in his world map he drew them as straight 1For convenience I refer to this as his 'Geography'.It was essentially a guide to drawing the world map.