MAPS HAVE ALWAYS shown the state of man's imagination as well as the state of the world. In the Dark Ages the mind was emotional, ignorant, and prone to delusion. The world, when drawn, usually conformed to the vision of a 6th-century Egyptian monk, Cosmas Indicopleustes, who believed the earth was rectangular and surrounded by four walls which, in turn, supported a vaulted room where God and his angels lived. Other maps showed Gog and Magog, those northern lands from which the hordes were to descend.

Reason was heresy. Better for a map to be ornamental and in line with prevailing dogma -- however ludicrous the illustrations -- than correct and be a threat to the sorcerers in power. In 391 A.d., in an act which may have started that madness, Christian mobs burned the magnificent library in Alexandria. One of the books lost in the riot was Ptolemy's Geography, which in the second century had laid the foundations of modern cartography. Practically a blueprint for an age of enlightenment with ideas about longitude and latitude, it was lost to the Western mind for more than a millennium. But not to the Moslems who held onto some copies.

Maps, as it turned out, charted a course away from the Middle Ages, and it is ironic that a mistake by Ptolemy was instrumental in the discovery of new lands. In his Geography, returned to circulation after the invention of the printing press, Ptolemy underestimated the circumference of the earth. Other mapmakers, basing their work on Ptolemy as well, put Asia close to Europe. And this errant calculation, in part, tempted Columbus to sail west, rather than east, in search of fortune.

It was easy for explorers to imagine distant lands. Marco Polo, Friar Odoric, and the Moor Ibn Batuta had all written about their fabulous travels. What was difficult for mapmakers was to define their world in relation to others, to draw maps which "embody a perspective of what which is known and a perception of what which may be worth knowing," according to John Noble Wilford, science correspondent for The New York Times and author of The Mapmakers.

In the 15th century, under the direction of Prince Henry of Portugal, navigation moved in the direction of science and away from superstition. Charts were drawn, and a compass was developed that allowed mariners to sail over the horizon without a fear that they might drop into fiery realms. There followed ideas about latitude and longitude, Mercator's projections (those long curves, or rhumb lines, that made it possible to show a round earth on a flat surface), and the invention of the chronometer.

Wilford describes with erudition the developments in geodesy (the study of the earth's size), triangulation and surveying during the 17th and 18th centuries. A Frenchman, Maupertuis, after a year in Lapland, discovered in 1737 that the earth flattens toward the poles. And the Cassini family, employing the techniques that George Washington was to use in the woods of Virginia, produced the first complete survey of a country, France.

Thus by the end of the 18th century, mapmakers, for the first time, were able to record accurately small parts of their world. But gaps in the larger picture remained. Until James Cook's voyages to the South Pacific in the 1770s, cartographers held out hope for a large southern continent, Terra Austrailis, whose name, reluctantly was given to the more modest continent later discovered. And so, as Hannah Arendt has written: "Nothing can remain immense if it can be measured."

It took Lewis and Clark three years to hike across the Louisiana Purchase and report to President Jeffrson that while the continent was bountiful, the Missouri and Columbia Rivers held no promise for a passage to India, then, still a dream. Today the same work, in greater detail, can be accomplished in hours or a few days. Aerial photogrammetry had made possible the first accurate map of the Amazon Basin, where cartographers, to their delight, found new rivers and mountains. Similar technology has been used to explore the Antarctic; sonar is used to search for oil in the Baltimore Canyon.

But no longer does mapmaking follow exploration. It precedes it. Man can no longer keep pace with his imagination. Thus, with considerable irony, much of the charter universe is unknown, awaiting either a visit from man or the development of his knowledge so that the lines on the maps can be translated into words that mean something. Maps are the outline of the future rather than compilation of the past.

In his dogged research, Wilford, in fact, found a map that projects what the world may be like in 50 million years: "At first glance," he writes, "nothing seems to have changed. No new continents. No new oceans. But closer examination reveals Los Angeles up near the Aleutian Islands, a shrunken Mediterranean Sea and an expanded Red Sea, a new gulf between East Africa and the rest of the continent, Australia overrunning much of Indonesia, Central America vanished and the Atlantic and Pacific meeting in the Caribbean Sea, and North America and Europe 1,120 kilometers farther apart."

This books begins in Babylon in the sixth century B.C. -- the oldest map extant was made there on clay tablets -- and ends with Mariner 9, which Wilford describes as "something of a flying windmill," on its way to map Mars. In between, in a graceful narrative that is as much about the contours of modernism as it is about the landscape, he is faithful to the charge Ptolemy gave to all cartographers: that is, "to survey the whole in its just proportions."