I got this information from a presentation by the Tartu University researcher Jürgen Karvak. When pressing him for sources. I got this one: https://www.ra.ee/kaardid/index.php/et/map/view?id=6606&_xr=eNpdkG0OgjAMhu/SA+i6L0o9jFlgIgkwBdRE490VKQb9ta3PnjdtAyM/ala7wGjm28CoGNpw2g4x9MURpopmKPoYxljuD31qp5plwFwpEGHBY/pC+4GeYbjtmzBOD2JwtDGEhCuWOmHabzLrnRHWxX/PWLtiv15uF6/4epiJqIlyYz1ahX71a07AJYLI68zlqDKnSea4p3ne9wpm1TFc21RG4VVfl58QhrOcKG01oasuoYqix3dLz+cLkGllHQ==

Another public source he mentioned was https://kitsendused.maaamet.ee/#/avalik while only displaying layer 4 -> 40.9.

He also claimed to have a private source from Tartu Veevärk which presumably cannot be used here.

If these are not adequate enough sources, perhaps he can publish some research that we could then cite as a proper source.

Sorry about the shared nodes. I guess I can separate them again if need be.