THIS IS what guides the Pershing II," said Paul Malebranche in a computer-filled room on Sangamore Road in Bethesda. "It's the brains of the smart missiles."

Malebranche held up a metal box the size of a small book. Hermetically sealed and hardened enough to withstand the weight of an Army truck, it contained an ordinary cassette tape. The tape was programmed not with music but a computerized map of Florence, Ala. -- an arbitrary substitute for a target in the Soviet Union.

The steel-colored oblong box had grooved sides and notches on one end, so that it could be plugged into something -- in this case, an intermediate- range ballistic missile armed with a nuclear warhead. When soldiers at missile sites in Europe choose which cassette to insert into a Pershing II -- a task not unlike picking the music to play in their off hours -- its digital code steers the missile to a target. The map on the Mylar generates a radar image of the missile's destination; the missile's own radar produces a "return image" of the ground below its trajectory; if the two don't match, the missile's guidance system makes a correction to match the tape.

Such is the software of warfare. Atomic duels waged over thousands of miles are now programmed down to tolerances smaller than a football field; modern cruise and ballistic missiles can hit their targets with an accuracy of less than a hundred meters.

Putting the targets of the nuclear holocaust on a standard tape cassette is the workaday task of the Defense Mapping Agency, one of those unsung Washington agencies that fill the background of the federal consciousness like traffic hum on the Beltway. The DMA's headquarters is a simple World War II-vintage building on a hillside behind the Naval Observatory on Massachusetts Avenue. It has hundreds of staffers in 50 locations around the world. But the real work of mapping the Earth is done in two stateside facilities: the Aerospace Center in St. Louis and the Hydrographic/Topographic Center in Bethesda.

Every workday 4,000 civilian and military employes -- scientists, cartographers, technicians -- drive past the manicured flower beds and three white flagpoles at the entrance to the Bethesda facility on Sangamore Road. They toil at the business of knowing exactly where the hills and hummocks, rivers and streams, villages and shopping centers in the world lie, especially if they are in hostile territory. The atmosphere in the working parts of the building is decidedly high-tech. No wizened mapmakers bent with sharpened styluses over ancient plates; no yellowed charts with fanciful sea monsters around the edges and the warning: "Dragons Be Here."

Picture, rather, laser scanners and computer maps drawn with light pencils on dual color monitors. Picture dimly lighted rooms faintly rumbling with the vibration of countless air conditioners. Picture state-of-the-art computers putting the world's surface into digital code, so that maps can be sent like phone calls via satellite.

Picture the cartography of terror in the nuclear age -- an era in which the mapmakers know down to the nearest foot exactly where the dragons are.

We are in the spy-in-the-sky era of modern mapping. Reconnaissance satellites cruise in the ethereal silence of extraterrestrial orbit. The pictures they take are not mere photographs of enemy entrenchments, but the intelligence-yielding data of multispectral imagery: things invisible to the naked eye that tell you, for instance, how it goes this year with the Soviet wheat crop.

Computerizing the world's maps has obvious advantages for storage and speed of delivery. Even more important to defense planners is the computer's ability quickly to update maps.

"We're in the Triptik business, like AAA," explains DMA's director, Air Force Maj. Gen. Robert A. Rosenberg. "You go to AAA if you want to know which roads are out of commission."

Telling its forces which roads are blocked, where a bridge has washed out, or where a new village has sprung up is one of DMA's primary jobs. Getting that task accomplished faster, via satellite, is its current goal.

"Wouldn't it have been neat," muses Gen. Rosenberg, "if within a few hours of the earthquake in Mexico City, you could transmit a new map to the relief people that showed which streets were blocked and which ones open? That's what we hope to be able to do."

Disaster relief is only one of DMA's jobs in the civilian sector. The National Aeronautics and Space Administration follows DMA charts of outer space, and DMA has mapped the moon. DMA's hydrographic services provide charts for naval and merchant ships. It also issues Notices to Mariners, a century-old service that warns merchantmen of sea hazards from bad buoys to marauding pirates.

All this does not mean that DMA is out of the business of making traditional paper maps with roads and contours. Indeed, it printed some 54 million sheets of various maps last year, including 3,600 new or revised ones. But DMA is digitizing the globe as fast as possible according to a five- level priority system. Potential battlegrounds such as Warsaw Pact countries are in priority one and are being digitized first; countries like Grenada, on the other hand, lie in priority five.

Grenada catapulted the DMA into the news during the American invasion of the tiny island in 1983. Ground troops complained that they had to prosecute a miniwar with the local "tourist map." In fact, with less than 24 hours notice DMA had turned out several thousand black- and-white copies of a British map of Genada; but it had a slightly different military grid than a standard American map. In addition, it contained the fanciful logo of the Grenada Tourist Board, along with such handy notations as, "Population: 120,000 warm and friendly people."

"It was a Class A, 1-to-50,000 scale British combat map, in other words, a standard combat map," says Del Malkie, DMA's public affairs officer. "The British had turned it over to the Grenadians, who let their tourist board use it. When our people pulled it from the files, they had to copy it fast and didn't bother to remove the logo. Cartographers are thinking scale and detail; they don't even see the logo." "If I had just thought to cover up that logo when we copied it," laughs Harold W. Madison, DMA's duty officer during the Grenada crisis, "nobody would have ever called it a tourist map." Within less than a week, DMA had updated and printed a color version of the map.

A useful lesson emerged from the tourist-map flap: bring DMA into operational planning at an earlier stage. "If we had had 72 hours," says Col. Ernest F. Boyer, head of DMA's crisis management team, "we could have given them a much better product." The Joint Chiefs of Staff have announced that "new procedures have been implemented so that the director (of the) Defense Intelligence Agency . . . will provide early notification to DMA . . . "

Meanwhile, the folks at the color monitors on Sangamore Road click away at their keyboards in the acronymic argot of computerized mapping: DFAD ("dee-fad") means Digital Feature Analysis Data. It is superimposed on DTED ("dee-ted"), or Digital Terrain Elevation Data to produce the map that helps the Pershing II hit Moscow, or wherever. DTED also creates a narrow-path tracking map called TERCOM ("ter- com"), the Terrain Contour Matching that makes the cruise missile go up and down valleys and around mountains. Paul Malebranche combines all this as his ODB ("Operational Data Base") to make those lead-encased cassettes he ships off to soldiers at the missile sites in Europe.

For troops on the ground -- ever the backbone of conventional fighting forces -- the digital era is close at hand, too. "There will probably never come a day when the squad leader in the field doesn't have a paper map to fold up and put in his pocket," says Rosenberg. "But I am sure that by the year 2000, we will be able to put digital products in the field commander's hands and continually push out new information to him (via satellite). When you know soil content, vegetation content, the road network, you can provide a three-dimensional digital model of the Earth and simulate the outcomes between forces."

And the obvious next step on the electronic battlefield will be the simulated soldier.