The article discusses the course and results of the astronomical expedition headed by I.I. Islenyev. Using archival materials, an attempt to localize the Astronomical Observatory built in Yakutsk in 1768 was made. New information on the handwritten map of the Lena River compiled in the mid-XVIII century was provided. Analysis of the toponymic map load was carried out and assumptions about its application for compiling generalized maps of Yakutia at the end of the XVIII century were made.