The pilot departed his home airport at night, in foggy conditions, and returned on an instrument flight rules (IFR) clearance. He stated that the return flight was "routine" and was cleared for the NDB-A approach into the airport. The pilot then canceled his IFR clearance and descended the airplane visually. He stated he entered the downwind leg for a "tighter than normal pattern" to avoid the fog at the approach end of the runway, however, GPS data revealed a spiraling descent in the vicinity of the base leg of the traffic pattern. The pilot stated, "Again, everything was routine until the base leg...As I continued, with the runway and runway environment in sight, I suddenly felt an impact...Not seeing the terrain surrounding the airport while having the airport in sight, lulled me into the belief that I was on or near glide path when, in fact, I was much lower, causing the CFIT (controlled flight into terrain)." The airplane incurred substantial damage to the left wing and left side of the fuselage. The pilot reported that there were no mechanical failures or malfunctions of the airplane. Weather reported 15 miles north included a 100-foot ceiling and a half mile of visibility due to fog. Witnesses surrounding the accident airport described the fog as "heavy" with visibility "less than 100 feet."