Angkor, in Kampuchea Cambodia , one of the great, prolonged building enterprises in history and also, at the height of its vitality, one of the world's astounding cityscapes, makes a romantic picture these days, its man-made lakes all but gone, its magnificent temples shrouded in green. Angkor is slowly returning to jungle.

For the moment it seems little can be done about this sorry state but it is important to keep the issue alive, to make sure the image of this extraordinary world treasure is not forgotten. That is the intention and the effect of the photographic exhibition, "Angkor: A Heritage in Need of Help," which went on view yesterday at the National Geographic Society.

The society dispatched a three-man team (writer, photographer, editor) to Angkor last year and published the results in the May issue of its magazine. The exhibition consists of enlargements of color photographs culled from this trip along with photographs taken at the site 14 years ago.

In a way the story that these comparisons tell is surprising. Angkor literally disappeared from the world's view a decade ago when a long-entrenched French-Cambodian archeological team was finally driven from the site by war. Despite some dramatic evidence of theft and trigger-happy vandalism, fears concerning the safety of the vast Angkor treasures prove to have been greatly exaggerated.

On the other hand the results of the nearly 70 years of work by the Angkor Conservancy, one of the triumphant episodes of modern archeology and restoration, are seriously threatened by omnivorous tropical vegetation. Indeed, whole temples are disappearing into the jungle.

Wilbur Garrett, the Geographic editor who traveled to the site in 1968 and returned last year, recalls walking past a pavillion of the Preah Khan--one of the latest and largest of the Angkor temples--on the recent visit without even knowing it was there. In 1968 the building, carefully restored, stood free in the sun.

When the Preah Khan was constructed in the late 12th century under the rule of Javaraman VII, one of the more fabulous builders of all time, Angkor was one of the living wonders of the world, an unrivaled exercise in town planning on a huge scale. Javaraman VII was the last in the line of building-obssessed Khmer kings that stretches back in time at Angkor, with but few interruptions, to the late 9th century.

The city that they built was a work of art based upon two things: an impressive system of hydraulic engineering that irrigated the plain and provided the means for the elaborate setting of moats, canals and lakes; and a theory of divine kingship, based upon Hindu theology, that inspired literally everything in Angkor from the layout of the city to the form of the temples to the wondrous profusion of stone sculptures that ornaments practically every nook and cranny of the place.

Angkor Wat, constructed in the 12th century, is the largest and best-known of the temple complexes, but its religious purpose and its basic form, with a series of rectangular walls and terraces gradually ascending to a high central shrine surrounded by subsidiary towers, is an elaboration of principles established at the very beginning of the Khmer empire.

Like its predecessors, Angkor Wat was conceived as a "temple mountain," a symbolic rendering in stone of Mount Meru, the Hindu throne of the gods, and like them it was devoted not only to worship of the central Hindu deities but also to celebrating the union of the gods and the Khmer king. This dual impulse, at once political and religious, was the driving force behind the immense building programs of successive Khmer rulers, each, it would seem, more ambitious than the last.

This prolonged period of medieval Cambodian art was, as scholar of the Orient Sherman Lee affirms, "one of the really unique manifestations of art in Asia." Over a period of four centuries the great, if anonymous, Khmer carvers, stone-workers and architects created a noble body of work that is remarkable both for its consistency and variety.

It is the ensemble, even more than the individual masterpieces, that especially kindles the imagination today, although both are in danger. The temples were conceived as inseparable spatial and sculptural wholes--even in fuzzy, 50-foot-long reproduction at the Geographic the relief carving of the myth of the "Churning of the Sea of Milk" along a wall at Angkor Wat can take the breath away--and then they were linked by that incredible system of waterways.

What a sight it must have been! Inhabited by more than a million people, it was, in the evocative description of Bernard Phillipe Groslier, who headed the Angkor Conservancy at the time of its demise, "a forest of gilt temples reflected in the waters of canals and lakes" on the broad Cambodian plain.

When Groslier and his aides left in 1972 the archeological enterprise was already but a shadow of its former size, which at its height numbered more than a thousand workers and quantities of sophisticated equipment. As Geographic writer Peter White records, upon their return they found Angkor in the care of Pich Keo, a former Groslier assistant, who had "one small truck, a bicycle and a hundred unskilled men, not even enough to keep all the greenery cut that sprouts among the stones . . . "

Unfortunately Angkor is still hostage to the warring factions of contemporary Kampuchea, the Vietnamese-dominated central government and Khmer Rouge guerillas still loyal to the ignominious Pol Pot. Even though Amadon-Mahtar M'Bow, director-general of the United Nations Educational, Scientific and Cultural Organization (UNESCO) recently called for a neutral zone to protect Angkor, chances for a resolution seem slim.

One can, of course, take the long view. After all, Angkor was shrouded by the jungle for more than three centuries before serious archeological efforts got underway at the beginning of this century. But what a shameful waste.

For now, the Geographic show, co-sponsored by UNESCO, is as close as we will get to the glorious ancient relic. The show continues through Aug. 29.