There were two important innovations for supporting map making during Yi Dynasty. One was an establishment of astronomical observatory system in the 18th year of King Seojong(1437), and the other was an invention of plane table and siting instrument in the 13th year of King Sejo(1467). The observatory system called 'Kan-ui' made available for measuring altitudes of stars, and also exact time which were able to get accurate latitudes and longitudes. However, this observations were primarily made for improving calendar system rather than measuring latitudes and longitudes for map making. Grid system of map was very old in China, and eventually Korean cartographers were aware of the system long before the actual indroduction. The common use of grid system for map making in Korea was after the introduction of sirhak around 17th century. It is certain that, at the time of Chong Sang-gi(1678–1752), grid system was used in various ways. King Chong-jo ordered in 1791 to measure latitudes and longitudes for each provincial capitals and to use 4-ri(about 1.6 km) grid, 154 longitude lines and 280 latitude lines. The grid was the same interval lines, not for real latitudes and longitudes. King Chong-jo's order for using map grid system may be understood as an improvement and standardizing grid system rather than the first introduction of the system. Manuscript copies of Chong Sang-gi's map usually do not have grid lines with a few exceptions. It is believed that the most of grid lines were eliminated in the process of duplication by hand into half-transparent Korean mulberry paper. It is hardly believed that Chong's 8-sheets of provincial map in the same scale fit together so well into a whole Korean map without using any grid system. Map-4 shows a rare copy of Chong's map with grid lines. Large scale atlas sheets made for military and administrative purposes applied varied grid intervals as large as 1-ri(about 400 meters) grid in about one-to-foty-five thousand in scale. The modern latitude and longitude grid system was introduced rather late. It is probably the Daehan Chon-do(Map of Korea) published by the Editorial Bureau of Ministry of Education in 1899. It was the next year after the establishment of Land Survey Office (Ch'uk-chi-a-mun) in 1898. The late adoption of modern longitude grid system may be explained due partly to isolation policy in the last part of Yi Dynasty.