of note are his writings on foreign countries. He began to collect information for his books as early as 784, questioning Chinese and foreign envoys about the geography, origins and customs of foreign lands, and directed the Court of Ceremonials to question all foreigners and have their answers recorded precisely. He paid particular attention to foreign countries on the caravan and sea routes, and as a result of his effort, knowledge of countries outside China improved significantly. He produced a number of works on geography, including the 10-volume Huanghua Sida Ji (), the 40-volume Gujin Junguo Xiandao Siyi Shu () and a map Hainei Huayi Tu (海内華夷圖, "Map of Chinese and non-Chinese Territories in the World"). The map depicted China and other countries including its former colonies in Central Asia that were lost to the Uyghurs and Tibetans. Huanghua Sida Ji was presented to Emperor Dezong of Tang in 798, while both the 40-volume book and the map were presented to the emperor in 801. Upon its completion in 801, the map was 9.1 m (30 ft) in length and 10 m (33 ft) in height, mapped out on a grid scale of one inch equaling one hundred li (Chinese unit of measuring distance). The map is lost, but a later map of China from the 1136 Huayi tu map which has names of foreign places (including those to the west of China and South East Asia) inscribed on the edges that it took the names from Jia Dan's map. In the 40-volume work that Jia completed in 801, Jia wrote of two common sea trade routes in his day: one from the coast of the Bohai Sea towards Silla in Korea and another from Guangzhou through Malacca towards the Nicobar Islands, Sri Lanka and India, the eastern and northern shores of the Arabian Sea to the Euphrates River. Indeed, Korean vessels dominated the Yellow Sea trade, while most Japanese vessels were forced to venture towards the mouth of the Huai River and Yellow River, and even as far south as Hangzhou Bay. Jia's book contains some of the most accurate accounts of the Arab world in Tang Chinese sources. He wrote that the ships in the Euphrates had to anchor at the mouth of the Euphrates and transfer the trade goods on land towards the capital (Baghdad) of Dashi Guo (Abbasid). This was confirmed by the contemporary Arab merchant