Although Zhang Heng had a map in his hand, there were no satellites in this era. Besides, the accuracy of the hand-drawn maps was limited as well. Basically, Zhang Heng only saw the rough location of the library on the map. Other than that, the dunes in the desert would constantly move due to the strong desert winds. Hence, he could not find any landmark for him to refer to in this vast desert.
In the end, Zhang Heng spent an entire month in the desert before finding the stele that Lucilla mentioned to him. Once Zhang Heng spotted the stele, he would be able to see the library's entrance. While he was traveling in the desert, he accidentally ran into a group of robbers. Although he managed to eliminate the group of robbers, he still lost two camels and a local guide.
