If you have a 9 meter diameter glass ball you can paint a map on the outside to be viewed from the outside, or you can paint a map on the inside to be viewed from the inside.
(Yeah it can be painted on either side to be viewed from the opposite side but regardless.)
If viewed from the outside you can only see a small portion, but from the inside you can see the relationship between the land masses. That's what Wyld and the Mapparium were trying to accomplish. Plus it's dryer if it rains.
