Surface Estimation of an Ancient City Using Spatial Interpolation: The Case of the Capital of Silla, an Ancient State in Korea

Abstract. Silla was an ancient state in Korea, whose capital was Gyeongju. The city began to develop in earnest around 6C A.D. and was the leading city in Northeast Asia until 935. More than 600 archaeological excavations have revealed the city's existence. In this study, we use the survey information from these sites to estimate the surface that was once inhabited by its citizens. This has the advantage of reconstructing the topographic landscape of the ancient city, identifying the land use patterns of the time, and predicting the elevation of the ruins below the current surface. The IDW and Kriging interpolation methods were utilized to estimate the historical land surface. The data used here basically uses spatial information provided by WebGIS, which was built and operated by the Korean government since 2009 to catalog and digitally document all archaeological survey information. Based on this, the elevation of features identified in the excavation reports was extracted to create a database. The total number of sites is 292, and the elevation of 985 points during the Silla period was obtained. This is a statistically reliable sample size. IDW and Kriging interpolation was performed by dividing the searching neighborhood type into standard and smooth, and cross-validation was performed for a total of 18 models. The most accurate model was Kriging interpolation, and the model with circular semi-variogram with smooth factor set to 1 had the lowest RMSE. Based on the cross-validation results of these models, the DEM of the ancient city of Silla was finally created. Comparing it to the current surface, it was found that the surface was highly curved in areas close to rivers at an elevation of less than 40 meters above sea level. These topographical conditions likely prevented a grid-like road network from being laid out here. Furthermore, the comparison of the present and past topographical cross-sections showed that the surface had been cut and embanked in the past. As more information on excavated sites is added in the future, the accuracy of the interpolation model is expected to improve. This will be a fundamental study for reconstructing the landscape of the Silla capital.