Robert D. Hodgson, 56, the geographer of the State Department and an expert on maritime boundaries, died of cardiac arrest Tuesday at Commonwealth Doctors Hospital in Fairfax. He had a heart ailment in recent years and was stricken at his home in Oakton.

Dr. Hodgson was born in New York City and grew up there and in up-state New York. He graduated from the University of Michigan in 1944 and at the same time was commissioned a second lieutenant in the Marine Corps. He was wounded in the Okinawa campaign in World War II.

He began his career as a geographer in 1946 with the old War Department, but then returned to Michigan, where he earned master's and doctoral degrees in geography.

In 1951, he joined the State Department and for the next six years was stationed in Bonn, Germany.

In 1957, he became assistant geographer in the State Department in Washington and later was named geographer. From 1971 until his death, he was director of the Office of Geographic as well as holding the position of geographer.

For the past 10 years, Dr. Hodgson had become increasingly involved in geographical and cartographical problems in connection with various United Nations Law of the Sea conferences. He was principal technical adviser to the U.S. delegations to several international meetings on maritime boundaries.

In the course of his career, Dr. Hodgson also testified as a witness in cases involving disputes between the federal government and state governments on offshore oil claims. He also was a member of the U.S. board of Georgraphic Names from 1957 to 1977 and was chairman of the board's foreign names committee from 1972 to 1977.

He was the author or coauthor of numerous papers published in professional journals.

In 1973, Dr. Hodgson received the State Department's Superior Honor award and the Distinguished Service Award from the Law of the Sea Institute.

He was a member of the Association of American Geographers. He was a member of the committee of geography of the National Academy of Sciences from 1967 to 1977 and was vice chairman of the International Geographical Union from 1970 to 1977.

Survivors include his wife, Margaret, of Oakton; four sons, David W., of Durham, N.C., Peter S., of Fairfax, Mark L. of Falls Church, and Luke A., of Oakton; three daughters, Laura A. Johansen, of Charlotte, N.C., Amy E., a student at Virginia Tech in Blacksburg, Va., and Susan D. Sommerfeld, of Chicago, Ill., a brother, Charles I., of San Diego, Calif.; a sister, Mary Maloney of Batavia, N.Y., and seven grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Robert D. Hodgson Memorial Fund, c/o Robert W. Smith, 800 Villa Ridge Rd., Falls Church, 22046, to aid young geographers through scholarship assistance or prizes for excellence in writing. g