Map inversion is the process of returning two-dimensional paper maps to the four-dimensional space of virtual globes. Alternative globes exist - notably NASA's open-source WorldWind application - but currently only Google Earth has the added power of Google's Keyhole Markup Language (KML). Using KML, scanned maps may be draped over the terrain as ground overlays, or broken into temporally sequenced layers. Structures may be incorporated as 3-D solid models using Google SketchUp and legends may be added as screen overlays. Most importantly, field data may be embedded in placemarks at the outcrop or even the microscopic scale. Self-contained KML archives (KMZ files) may be shared over the Internet and combined with data from disparate sources.
In the process of map inversion, original mapping errors inevitably appear. There may be missing inliers and outliers and incompatible topographic relations. Technical errors also arise, including projection and contour mismatches. To be zoomable, maps must be scanned at a series of resolutions and saved as super-overlay tiles. The main challenge to virtual globe mapping in the immediate future is the incorporation of paleogeographic terrain models. Despite these hurdles, map inversion should be a research priority for the geological community. Improvements in access and analysis afforded by GES may yield major advances in understanding of regional and global geologic histories.