David Goodwin Knapp, a federal geophysicist for more than 40 years, died of cancer Feb. 8 at Boulder (Colo.) Memorial Hospital. He was 69.

A native of Indianapolis, Mr. Knapp was a graduate of Butler University there. He began with the National Bureau of Standards and then worked at the departments of Commerce and the Interior.

He served with the former U.S. Coast and Geodetic Survey, the electromagnetism and geomagnetism branch of the U.S. Geological Survey, and the National Oceanic and Atmospheric Administration, which was moved to Boulder in 1971. Mr. Knapp retired in 1975.

He contributed to many scientific journals and belonged to the Washington Academy of Sciences, the Amecian Geophysical Union, and Sigma Xi, the Scientific Research Society of North America. He was history reporter for the American area of the International Association of Geomagnetism and Aeronomy.

Mr. Knapp was also active in the PTA and Boy Scouts in Silver Spring, where he was committeman for Troop 275.

Survivors include his wife, Doris White Knapp, of the home in Boulder; two sons, Barry G., of Boulder, and Allen C., of Washington; two daughters, Louise E., of Boulder, and Emily S., of Redwood City, Calif.; a brother, Wallace B., and a sister, Getrude both of Bethesda.

The family suggests that memorial contributions be made to the American Cancer Society.