On May 24, 1997, about 1045 eastern daylight time, a Cessna 182P, N20892, was substantially damaged as it flipped over during landing rollout in a field near Wentworth, New Hampshire. The certificated private pilot/owner was uninjured. Visual meteorological conditions existed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. The flight originated from the Plymouth Municipal Airport, Plymouth, New Hampshire, about 1030. The private pilot stated that he was frustrated because the Plymouth Municipal Airport was a 1 hour car drive distance away, and seemed to be useable only half of the year. The pilot stated that he wanted someplace conveniently located to his business. He asked the owner of the field for permission to land. The pilot reported that he had landed on grass runways previously, but never on an unimproved surface. He stated that he did not closely examine the field's conditions prior to the landing attempt. During the landing roll out, the pilot found the field rougher than anticipated. At 25 knots, he stated that the nose gear dug into a soft portion of the ground and the airplane flipped over, coming to rest inverted. A Federal Aviation Administration Inspector reported that the field was a non recognized airstrip, without an official designator, although pilots have used the field in the past. He described the field as 3,500 feet long, with clear approaches a each end. The private pilot had accumulated a total of 654 flight hours, with 359 hours in make and model. He stated that the airplane was functioning normally prior to the accident.