[shzhdbi09gv8ioi]

In the baltic sea, the text translation reads "This sea is not navigated with map and compass but with a sounding-lead; and throughout here there are many inhabited islands"

I wonder what a sounding-lead might be?

[dzhiurgis]

https://en.wikipedia.org/wiki/Depth_sounding

[shzhdbi09gv8ioi]

Cool, thanks!

[rzmmm]

Maybe gauging the depth of the water with a lead weight.