On June 14, 1997, at 1320 central daylight time, a Waco YMF, N820MM, exited runway 13 (3,781' x 75' dry/asphalt) at Longville Municipal Airport, near Longville, Minnesota. The airplane sustained substantial damage when it nosed over in a ditch. The pilot and one passenger reported no injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed Minneapolis, Minnesota, at 1150. The pilot stated that he conducted a landing on runway 13 due to the indication of the windsock showing the prevailing wind from 180 degrees. He said on landing that the airplane "weathervaned" to the right and he allowed it to depart the runway to avoid a ground loop. There is a ditch running parallel to the runway about 50 feet off the runway. The airplane impacted the ditch and nosed over. The pilot said that witnesses to the accident told him the windsock was not recording the actual wind which they said was from the northwest at 15 knots. A weather reporting station located 36 nautical miles to the southwest of the accident site was reporting winds of 230 degrees at 6 knots gusting to 16 knots 5 minutes prior to the accident and the same reporting station recorded winds of 170 degrees at 3 knots 15 minutes after the accident. An onsite inspection of the windsock after the accident by a Federal Aviation Administration inspector revealed that the windsock turned freely, but did not always record the wind accurately. He attributed this to the location of the wind sock in the proximity of buildings.