A Sandinista government geographer said today that a Soviet plane based here since May has shot pictures of the countryside to make basic official maps of Nicaragua's territory.

Alejandro Rodriguez, a geophysicist at the Nicaraguan Institute for Territorial Studies, asserted the Soviet An30 twin-propeller aircraft is "absolutely not used for any military purposes."

Rodriguez spoke at press conference at Managua's Sandino international airport summoned to counter charges raised last week by President Reagan and CIA Director William J. Casey as the administration battled in Congress for more aid to the anti-Sandinista guerrillas. Reagan and Casey said the plane was flying reconnaissance to aid the Sandinista Army in its battle against U.S.-backed counterrevolutionary rebels, known as contras.

The President described the plane as significantly escalating Soviet intelligence-gathering on behalf of the Sandinistas in Nicaragua.

Rodriguez responded that the U.S. leaders are "either misinformed or are lying intentionally."

The scientist said the Soviet plane arrived in Nicaragua on May 8, as part of a Sandinista government project to update its terrain and nautical maps, some of which are more than 100 years old.

"If we want to irrigate our fields, make an inventory of our forests or build a highway, we need maps," he said.

A western diplomat in Managua familiar with Soviet technology said the plane is used normally for geophysical surveys. But he pointed out that the maps and photographs could also be used for military purposes. "It helps to know where the hills and rivers are," he said.

The Sandinista government has rented the plane from the Soviet trade agency Selkhozpromexport for over two years, Rodriguez said, under a broader credit agreement with the Soviet Union. He did not specify the total rental costs.

The plane comes with a nine-person Soviet crew, including pilots and photography technicians. Only two Soviets, both mechanics, are in Nicaragua now, Rodriguez said.

He said the plane has only flown "a few" missions since its arrival, snapping pictures of Nicaragua's Pacific coast. Little contra activity has been reported in that region in the last two months.

The white plane with blue Aeroflot markings sat this morning grounded at the airport, with canvas covers over its engines, unable to fly because of the thick cloud cover during Nicaragua's seasonal rains. Rodriguez said the plane was scheduled to arrive last October to work during drier weather, but was delayed unexpectedly in the Soviet Union.

A pamphlet in English published by a Soviet enterprise named Aviaexport said the plane is equipped with a sophisticated Soviet AFA-41 topographic camera, "for cartography." The clear-plastic transparent nose of the plane appeared to carry several radars, said by the pamphlet to be for "precision radio measurement."

Rodriguez said the photographs will be developed in Nicaragua, and the first maps drawn from them will not be ready for nearly two years. He said at that time they would be routinely available to Sandinista military strategists.

"We have used rocks and machetes to defend ourselves," he said. "Why shouldn't we use maps?"

The Antonov-designed plane has an airborne time of six hours ten minutes and can photograph 3,000 square miles in one hour, Rodriguez said.