On January 26, 1430, King Sejong instructed the governor of Hamgil Province to send to Seoul Kim Nam-ryeong, a local man who was reported to have traveled to Yodo, a mysterious island in the East Sea (Sea of Japan) that the king was trying to get information on. Apparently, Kim Nam-ryeong was sent to Seoul and gave the king information about the island because the following record, which is dated about 70 days after Kim was sent for, includes information on where to go to see the island. We know that Kim traveled to Seoul because the records said he was on his way back home.
Source:『世宗實錄』 卷 四十八, 世宗 十二年 四月 癸酉條Notice that the record says that Yodo could be seen from two places: Mijigut and Bocheongsa. Mijigut was in Gyeongseong, which appears to have been in what today is North Hamgyeong Province, and Bocheongsa was in Hongwon, which appears to have been in what today is South Hamgyeong Province. Both areas are on the east coast in the northern section of North Korea, which means that the only islands that could have been seen would have been coastal islands. Yodo could not have been Dokdo/Takeshima or Ulleungdo since not even Ulleungdo can be seen from the coast of either of the two Hamgyeong provinces. The mystery of Yodo continues.
April 4, 1430
傳旨咸吉道監司 鏡城無地串洪原補靑社 使人登望 則可見蓼島 其今首領官 或詳明守令一人 偕今去金南連 望見蓼島形勢及水路夷險以啓若有偕南連往還蓼島者居海邊望見者 詳問本島形勢遠近 以啓.
함길도 감사에게 전지하기를, "경성(鏡城) 무지곶(無地串)과 홍원(洪原) 보청사(補靑社)에 사람을 시켜 올라가 바라보게 하면 요도(蓼島)를 볼 수 있을 것이니, 그 수령관(首領官)이나 혹은 자상하고 밝은 수령(守令)으로 하여금 지금 가는 김남련(金南連)과 함께 가서 요도의 지형과 뱃길의 험하고 편함을 살펴보고 아뢰라. 만약 남련과 함께 요도에 갔다가 돌아온 자와 해변에 살면서 바라본 자가 있거든 본도(本島)의 지형과 멀고 가까운 것을 물어서 아뢰라." 하였다.
The governor of Hamgil Province was instructed as follows: "If you send someone to Mujigut (The Cape of Muji) in Gyeongseong or to Bocheongsa (Bocheong Village) in Hongwon and have them go up and look out (to the sea), they should be able to see Yodo. Have the local magistrate or a bright, attentive official go there with Kim Nam-ryeon, who is now returning, and inspect the topography of Yodo and whether the sea route is difficult or not and then report. If there is someone who has travelled with Nam-ryeon to Yodo or who lives on the coast and has seen the island, ask him about the island's topography and whether it is near or far away and then report."
By the way, notice that Koreans seem to have put a great deal of importance on being able to see islands from the mainland.