The trouble I have with scanning large documents is the "lip" around the scanner's perimeter. If you can remove the lid and bezel, you can get the map to lay much flatter, eliminating the lift at the edges and allowing you to scan the center of the page easily.
Make each scan with a little bit of overlap, then move them all into a single Photoshop document. Select all of the layers, go to edit > Auto Align layers... Experiment with the different projections to get one that works. Usually collage or reposition work well for me.
Bryan Ray, visual effects artist