<p>Owing to the increasing demand for research on large-scale wildfires, it is crucial to accumulate investigation records, with fire marks being a key focus in postfire investigations. In this research, we have measured the spatial distribution of char directions on stembark surfaces and the heights of fire marks at 1019 points across the wildfire burned area in Japan covering 413 hectares. The aims were to (i) characterize the spatial distributions of fire marks within a large wildfire; (ii) identify and distinguish terrain-related determinants of stembark char directions from those influenced by other factors; and (iii) elucidate the fire spread process, highlighting the advantages of ground-based surveys. The findings revealed that (A) stembark char directions exhibited contrasting trends, aligning with the upslope direction on slopes and the eastward direction on ridges, typically within 60 degrees. The eastward alignment on ridges corresponded to the leeward direction of strong regional background winds during the first day, when most of the area burned; and (B) the surface fire moved upslope from the estimated ignition point, intensified on the windy ridge, and transitioned into a crown fire. While strong westerly winds lasted only about six hours after ignition, the fire likely reached the eastern coastline.</p>