This is a diagram of the Nine Worlds on the Yggdrasil world tree. It shows the relationship of the various dimensional planes/realms to each other.
The diagram is not drawn to scale, nor do the positioning of the worlds on the diagram have any real significance because they are not in the same dimensional plane. The only reason for placing Asgard at the top, is that the world-tree Yggdrasil has its trunk and branches there. Yggdrasil’s roots manifest on most of the other worlds. Please see http://www.myconfinedspace.com/2009/03/23/nine-worlds-of-asgard/ for more information.