Deconstructing Classical Geologic Maps Using Google Earth's Keyhole Markup Language

In previous work, we have scanned classical geological maps such as William Smith's map of England and Wales, as well as our own paper-based maps, and draped them over the surface of Google Earth as "ground overlays." This is a very effective way of bringing old maps back to life and visualizing the geological structures they portray. However, just as it is difficult to search for content in scanned JPEG images on the world wide web (as opposed to text in HTML documents, for example), so a scanned ground overlay provides limited access to the data from which a map was originally constructed. To deconstruct a map, we first remove and discard the north arrow and scale bar, which have no meaning in the zoomable, rotatable space of a geobrowser. We restore insets to their correct geographic position, and we transfer legend and memoir information to a free-floating screen overlay or a pop-up window. We next scan each lithological division on the map into a separate layer with a transparent (alpha-layer) mask, and we isolate major one-dimensional features such as fault traces. Finally, we project cross sections onto vertical planes and locate them along their corresponding lines of section. The result is a "GIS-ification" of the classical maps, which become interactive maps where different lithologies and structures can be viewed in a truly 4-D visualization.

The map deconstruction process can be tedious and time-consuming. It requires a working knowledge of image processing and modeling software (Adobe Photoshop, Google SketchUp) and the ability to write scripts in Keyhole Markup Language, a dialect of XML. However, the effort is well rewarded as the resulting interactive maps make research data readily accessible and easy to visualize. The process facilitates a new level of user-control over the presentation and evaluation of geological maps.