I am designing a campaign world that I intend to be very large. The two things I am finding I need to do is tiling and different scales. The large scale maps give overviews and keep the tiles in alignment. I am using Gimp for my mapping. I am learning as I go how to transfer the different elements between maps so I can change the scale of elements such as roads, mountains and rivers.
I imagine it as taking a page from the google earth playbook letting me scale up to continents and down to a village and both will hold consistency.