The digital revolution in geologic mapping

Geologic field data collection, analysis, and map compilation are undergoing a revolution in methods, largely precipitated by global positioning system (GPS) and geographic information system (GIS) equipped mobile computers paired with virtual globe visualizations. Modern, ruggedized personal digital assistants (PDAs) and tablet PCs can record a wide spectrum of geologic data and facilitate iterative geologic map construction and evaluation on location in the field. Spatial data, maps, and interpretations can be presented in a variety of formats on virtual globes, such as Google Earth and NASA World Wind, given only a basic knowledge of scripting languages. As a case study, we present geologic maps assembled in Google Earth that are based on digital field data. Interactive features of these maps include (1) the ability to zoom, pan, and tilt the terrain and map to any desired viewpoint; (2) selectable, draped polygons representing the spatial extent of geologic units that can be rendered semi-transparent, allowing the viewer to examine the underlying terrain; (3) vertical cross sections that emerge from the subsurface in their proper location and orientation; (4) structural symbols (e.g., strike and dip), positioned at outcrop locations, that can display associated metadata; and (5) other data, such as digital photos or sketches, as clickable objects in their correct field locations. Google Earth–based interactive geologic maps communicate data and interpretations in a format that is more intuitive and easy to grasp than the traditional format of paper maps and cross sections. The virtual three-dimensional (3-D) interface removes much of the cognitive barrier of attempting to visualize 3-D features from a two-dimensional map or cross section. Thus, the digital revolution in geologic mapping is finally providing geoscientists with tools to present important concepts in an intuitive format understandable to the expert and layperson alike. INTRODUCTION The presentation of spatial geologic data as maps and cross sections essentially began with Cuvier and Brongniart’s (1808) map of the Paris Basin and Smith’s (1815) geologic map of England and Wales. Their approach to presenting field data as color-coded units in recognizable map formats became the de facto standard for geologists around the globe (e.g., Griffith, 1838; Hitchcock, 1878). These geologic maps were based on GSA Today, v. 20, no. 4/5, doi: 10.1130/GSATG70A.1 *E-mails: [email protected], [email protected], [email protected]. countless hours of classifying, measuring, cataloguing, and interpolating rock units across field areas at outcrop to continental scales. Over the years, the means of transportation for fieldwork advanced from horseback to four-wheel drive vehicles, but the basic methods of field data collection and presentation remained largely unchanged. The first years of the twenty-first century saw a major advance in the methods of field data acquisition and geologic map presentation. This was facilitated by three main technological advances: (1) the descrambling of global positioning system (GPS) satellite signals, (2) the advent of affordable mobile computers capable of running geographic information system (GIS) software in the field, and (3) the universal availability of free, Web-based virtual globes (also known as digital globes or geobrowsers). Although GPS had been operational since the mid-1990s, it wasn’t until 2000, when Selective Availability was discontinued, that GPS became effective as a precise positioning tool for the general populace (White House Office of Science and Technology Policy [OSTP], 2000). Manufacturers began producing inexpensive handheld and car-mounted GPS devices as backpacking and motorized travel aids. At the same time, computer manufacturers started marketing portable personal digital assistants (PDAs) and tablet PCs capable of integrating GPS with GIS software for mobile digital data collection. Today, advanced mobile computing systems, such as the Trimble Explorer series and xPlore tablet PCs, are ruggedized and water-resistant enough to handle the typical bumps, scratches, and rain showers common to geologic field mapping and research. Techniques for the digital presentation of geologic maps advanced during the late twentieth century using an assortment of proprietary platforms (e.g., Selner and Taylor, 1993; Condit, 1995). Recent developments utilize free and almost universally accessible global terrain models within Web-based virtual globes, such as NASA World Wind, Google Earth, and Microsoft Virtual Earth. These “geobrowsers” generally support the open-source scripting language KML (Keyhole Markup Language), an XML-derived language that facilitates user manipulation of a geobrowser environment. KML scripting has enabled geoscientists, as well as other producers of spatial data sets, to display field data and maps in a virtual three-dimensional (3-D) interface. Provided the user has an active, reasonably fast Internet connection, the possibilities for exploring an ever-expanding collection of geospatial data sets from locations around the globe are almost unlimited. Users can bypass the requirement for an active Internet connection by loading pertinent data and maps into the cache of a geobrowser, thereby enabling access to a virtual 3-D interface on location in the field. This article documents an approach to geologic field mapping and presentation that utilizes recent technological and