The map of Arda shown here is based on Maps IV and (especially) V reproduced by Christopher Tolkien in volume IV of The History of Middle-earth. These maps belong to an early phase of Tolkien's mythology, and in a few cases the nomenclature of the originals has been updated (for example, Map V refers to the region known in the published Silmarillion as Avathar by its older name Arvalin).

The main point of conjecture on the map is the placing of Ered Luin and the Orocarni. In fact, Map V does not name these mountain ranges, but Map IV shows ranges in these locations, and labels them the 'Blue Mountains' and 'Red Mountains' respectively. Equally, the range stretching southwest from Helcar into the (unnamed) southern continent might possibly correspond to an older conception of a range known as the Grey Mountains (quite unrelated to the northern range of the same name).

It's important to remember that the original maps on which this one is based predate the stories of the Third Age, and aren't entirely compatible with the later geography of Middle-earth. In particular, the relatively small northern continent would probably need to be rather larger than is shown here to accommodate all the lands that Tolkien later devised.