Boromir and his search for Rivendell

[Mithadan]

'How far is Rivendell' asked Merrry... 'I don't know that the Road has ever been measured in miles beyond the Foresaken Inn, a day's journey east of Bree,' answered Strider. 'Some say it is far, and some say otherwise. It is a strange road, and folk are glad to reach their journey's end, whether the time is long or short. But I know how long it would take me on my own feet, with fair weather and no ill fortune, twelve days from here to the Ford of Bruinen..."

A Knife in the Dark.

This is Aragorn speaking on Weathertop; Aragorn the Ranger who knew the country well and lived in Rivendell itself for much of his childhood. And what happens thereafter? After the confrontation at Weathertop, Aragorn leads the hobbits through a backcountry route, avoiding the road and, if not actually getting lost, goes astray ('We have come too far to the north... and we must find some way to turn back southwards again."). Clearly, Rivendell is hidden and difficult to find. There is an implication of some form of concealment through the power of Elrond perhaps in a manner similar to, if not as obvious and overt as, Melian's Girdle. Those who do not know where Imladris lies find it difficult to find it, and even those who have been there may encounter trouble.

To the extent that Elendil or any of his northern army visited Rivendell, many, including Elendil himself, died in the Siege of Mordor and those that survived perished in the disaster at Gladden Fields. It cannot be said that after the War any certain information regarding Rivendell's location would have been communicated to Gondor. As others have mentioned above, the fact that the road north was maintained long before the events in LoTR also does not guarantee that any such information survived the passage of time and it may simply be that no certain data on Rivendell's location ever existed in Gondor. Even it were roughly shown on a map, the difficulties in finding Rivendell that Aragorn refers to still would have delayed Boromir.

Aragorn's discussion of how the road to Rivendell is "strange" is reminiscent of Tolkien's descriptions of the Smith's journeys in Faerie in The Smith of Wootten Major. There the Smith's journeys seem longer or shorter even when he is travelling to the same place (assuming he can find it). When a mortal Man journeys to the Elves, particularly in the later Ages, perhaps he is going to a realm different from the mortal world in ways subtle and less subtle. This may be, in part, why Rivendell was hard to find.