The basemap outlines are downloaded and converted from Natural Earth
Cultural and Physical Vectors, release 5.1.2
(https://www.naturalearthdata.com, https://github.com/nvkelso/natural-earth-vector).

Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com.

Terms of use (https://www.naturalearthdata.com/about/terms-of-use/):

All versions of Natural Earth raster + vector map data found on this website
are in the public domain. You may use the maps in any manner, including
modifying the content and design, electronic dissemination, and offset
printing. The primary authors, Tom Patterson and Nathaniel Vaughn Kelso, and
all other contributors renounce all financial claim to the maps and invites
you to use them for personal, educational, and commercial purposes.

No permission is needed to use Natural Earth. Crediting the authors is
unnecessary.
