The calculated distance (air line) between Seongnam-si and Gyeongsan-si is approximately 143 miles respectively 229 kilometers.
By car or train, the actual journey to Gyeongsan-si is certainly longer, as only the direct route (as the crow flies) between Seongnam-si and Gyeongsan-si has been calculated here.
Air line (approximately)
229 kilometers
124 nautical miles