Correction: the boundary is not a municipal one. I don't know what it is.
Anyway, good maps are available from
http://www.euskal-eurohiria.org/euskalhiria/ingl/eurocite/eurocite.html
Click on a village (e.g. Hendaye), and then 'locate on the map'.
You can zoom in up to 1:5000 and switch from map to aerial photo. It is not without mistakes,
though: the municipal (here =international) boundary runs straight through the Ile de la
Conférence...
Peter S.