In the era before radar, plane had to fly according to what are called Visual Flight Rules. These rules, referred to as VFR, have been greatly expanded in the eighty years since this map was made.
Japanese Air Transport (Wikipedia article-approach with caution)
Korea was under Japanese rule from 1910-1945
Under Japanese rule
The terminus of the Japan Air Transportation Map
Shows the locations indicated along the Japanese Aviation Map
View Japanese Visual Flight Map, 1933 in a larger map