Please look at the map (click the map to enlarge). There is Ulleungdo "欝陵" at off the coast of 江陵 with three tiny islets around it, and you may notice a larger island at the north of Ulleungdo and it labelled as "Usando". This Usando is apparently different from Liancourt Rocks in the location, shape and size. It maybe an imaginary island which Korean people believed to be present.
The map resembles the Map of Joseon which Kaneganese introduced before in the following post:
It seems possible that the mapmaker for the 江戸大節用海内蔵 referenced the map. According to Kaneganese's investigation the woodblock encyclopedia had been revised for many times and the oldest version was published in 1704. So it may have been a common knowledge among Japanese people that Usando was a larger island at the north of Ulleungdo.
By the way, Korean scholars insisted that Usando (Josando) in the map in the formar post was Liancourt Rocks. But this can't be true if one thinks with a commonsense interpretation.