[The Mouth of Sauron]

In the books (though not the extended versions of the films) we are told that Boromir was unsure as to the location of Rivendell and took many months finding it.

Surely however this lack of knowledge would have been unlikely. Rivendell was used as a staging post by Numenorians during the first overthrow of Sauron at the end of the Second Age. Isildur's youngest son stayed there at that time so it seems inconceivable that Rivendell was not known of in Gondor.

Further, the location of Rivendell was known by the Kings of Arnor and that information would it seems to me have certainly been shared with Gondor, then kept in Minas Tirith so that Boromir could have been given the exact location before he set off.