Inference on the Surveying Methods at the 8th Century in Japan

Ancient regional records on geography named Fudoki were compiled in Japan at the 8th century. There were about 60 provinces at that time, but only 5 Fudoki (transcripts) have remained at present. Among them, Izumo Fudoki (edited in 733) is not only almost complete, but also contains detailed geographic data (surveying data) not found in the others. The surveying data are distances between major points, heights and sizes of mountains, sizes of lakes and isles etc. However, there are many differences when comparing these surveying data with actual sites. The Fudoki researchers had tried many times to interpret the relationships between the two, but many unclear points have remained. During the time, we civil engineers had little interest in the Fudoki. In this paper, we estimated the surveying methods by comparing the Fudoki data with map data and confirming the sites, then obtained the following conclusions. The distances were almost correct, the heights of the mountains were converted using the number of steps, the perimeters of the mountains were calculated with the trails as diameters, and the sizes of the lakes and isles were the lengths of the waterways. The surveying methods at that time were simple, but the results obtained were practical. We think that these ancient surveying methods were widely used, not in one region or one period.