Looking down on it from a helicopter, East Germany's border with the West is like a poorly planned superhighway - a wide swath of cleared land cut from timbered hills and marshy flats that zig-zags along the border's craggy contours as far as one can see.

Closer up, it is much grimmer: a diabolically designed, steel-mesh fence laced with automatically triggered guns and backed up by mine-fields. It runs for about 840 miles, from the Baltic Sea in the north to the Czechosvakian border in the south.

For three decades now, the postwar boundary between East and West Germany has come to symbolize the contrasts between two societies and idwologies.

For most Westerners, the focus of division has been the Berlin Wall, built around the western sector of that divided city in 1961 by the Communists to stem the flow of defectors from East Germany into West Berlin.

But West Berlin, 125 miles deep inside East Germany, is already isolated. Although thousands of Western visitors view Berlin's high cement wall, it is only a small segment of what has become one of the most unusual construction projects in modern history.

Indeed, relatively few people, East or West, have seen enough of the main border between the two Germanies to grasp the dimensions to East German's physical effort to effectively seal off its 17 million inhabitants from the West.

Motorists passing through the relatively few highway entry points from West to East get a glimpse of sections of the border and its array of traps. So do soldiers on both sides who patrol portions of it, and farmers who still cultivate fields, at least in the West, right up to the border's edge.

But in its entirety, the border undertaking is awesome.

Between 1949 and 1961, an estimated 2.5 million East Germans defected to the West, most of them in the immediat postwar years. The border in those days was lightly guarded, with spotty, makeshift fences and occasional mined areas. In 1961, when the Berlin Wall went up, the border installations also began to be improved.

But it was in the early 1970s that the East Germans began a massive modernizations effort along the border that U.S. Army officers estimate is costing $650,000 a mile.

The keystone is the fence that now runs almost the entire length of the border. Construction crews are still working at the rate of about 100 yards a day to plug remaining gaps and Westerners estimate it will probably be finished this year.

The new fence is three yards high and goes another yard into the ground to prevent tumbling. The mesh is flexible and the wire itself is sharpened along its edges so that the metal collapses around and cuts any hand that grabs it.

Beginning in 1972, the East Germans started mounting antipersonnel mines on the fence posts, which are a few yards apart. These mines - basically miniature shotguns that spray pellets and shrapnel - are interconnected by trip wires that cuase the mines to fire a crisscross pattern. The mines will fire even if the trip wire is cut rather than just grabbed.

Behind the fence is a four-yard wide ditch with concrete slabs sticking up at an angle to trap any vehicles that might try to break out.

Behind the ditch is a plowed strip designed to show footprints to border guards of anybody who gets that far.

Then there is another bulldozed strip between 10 and 50 yards deep that is a minefield.

Behind that is a narrow roadway along whigh pairs of border guards, usually on motorcycles, patrol.

Behind that are watchtowers, many of them tall and new, about two miles apart.

The emplacements have been very effective. The number of people who make it across has been cut to a trickle. According to soldiers of the U.S. 11th Armored Cavalry Regiment who patro a 230-mile sector of the western side, only about a dozen people have made it across in their sector so far this year, and that is a higher figure than last year at this time.

In 1975, with the new fence at an earlier stage of construction, 600 people reportedly fled across the border, according to West German estimates. In recent years, however, several thousand others reportedly have been captured and perhaps as many as 200 killed.

At the U.S. Army's observation post Alpha several miles south of here, West Germn villagers keep a wreath near the foot of the fence where an East German father and son were shot only a few yards from the U.S. watchtower last Christmas Eve.

For the most part, however, East Germans now come out by different ways, either through other East European countries or by being ransomed about $15,000 per person. Mine explosions are still heard at night in border villages like Philippstal, but mostly they are caused by hapless rabbits.

Philippstal, northeast of the city of Fulda, lies right up against the border and in many ways reflects the contrasts and ironies of border life.

In the postwar years, many West German border villages slipped into a gradual, melancholy decline as the younger generation sought more secure farms and homes. Here, three new ranch-style homes are being built, possibly a small vote of confidence in the future.

The famous halfway house is also here, the home of the Hossfeld family that turned out to be right in the middle of the postwar German dividing line, with its main entrance in the East. On New Year's evening of 1952, the family packed its belongings, moved to the western wing, and sealed up the connecting archway with bricks.

Today, busloads of American GIs from bases further back in West Germany arrive here in dress uniforms. An Army tour guide leads them along the banks of the Werra river for a close look at the East German border and fence on the other side.

Although it is the potential front line for any new war in Europe, the border actually does not look like a military zone and very few troops of either side are very close too.

Yet, the border has become a major public relations tool for the U.S. Army in Europe. It is the only prospective front line the American military has left where you can look across and see "the enemy."

Only elements of the 3,600-man 11th Armored Cavalry Regiment, who patrol this section, actually get to see the border very often. So the Army brings up soldiers from the rear. Some 16,000 GIs from the U.S. Fifth Corps, stationed at Frankfurt, have been based here in the past year for a look.

From a watchtower halfway across a bridge that spans the river, East German border guards watch an assortment of U.S. soliders stream out of buses. Each group stares at the other through field glasses.

"We've got a real live training aide here called East Germany," says Maj. Henry Lowe of the 11th Cavalry.

"You appreciate your training a little bit more when you see this. They hear about it at Ft. Benning or Ft. Knox. But here it is real, not abstract. They can see the East German guards, maybe a Soviet tank, look at the border."

For the cavalry troops, the only ones with actual border duty, officers and enlisted men all seem to feel this is good duty, with a sense of realism and activity.

But Lowe and others feel it is important to keep busing in the soldiers from the rear areas. "A lot of them are shook by what they see here, especially those who live along the Rhine" in West Germany's heartland.