Nay, Sir, I would not have him prove it. If he is content to take his information from others, he may get through his book with little trouble, and without much endangering his reputation. But if he makes experiments for so comprehensive a book as his, there would be no end to them; his erroneous assertions would then fall upon himself; and he might be blamed for not having made experiments as to every particular. Dr. Samuel Johnson
The Global Geoscience Transect (GGT) project was established by the Inter-Union Commission on the Lithosphere for the purpose of producing cross-sections that depict the composition and structure of the entire crust, using a common format for diverse regions of the earth [Monger, 1986]. It is modeled after the North American Continent-Ocean Transects Program, which was initiated in 1978 by the U. S. Geodynamics Committee for the purpose of integrating data from offshore regions of North America with geologic and geophysical data from land-based studies continental margin regions. The resulting cross-sections are being published by the Geological Society of America.
The GGT project differs from the North American Continent-Ocean Transects Program in several ways. First, it is an international program, with a much larger number potential transect locations, the majority majority of which are outside North America. Second, it is intended to place additional emphasis on continental crust [Monger, 1987]. Consequently, Global Geoscience Transects tend to be longer, and tend to include more stable contintental crust, than do the North American Continent-Ocean Transects. Finally, it was recognized early in the process that no sponsor was likely to bear the cost of publishing a large number transects using traditional color printing methods. For this reason, digital publication guidelines [Götze and Williams, 1992] were proposed for the GGT project, in which the main display is cast as a digital image in Computer Graphic Metafile (CGM) format [ANSI, 1986], and the underlying data are archived in text files. The GGT digitization guidelines were specifically targetted at relatively low cost personal computer level, within reach of all of the transect teams.
Global Geoscience Transects 1-6 and 9 were published in color printed form by the American Geophysical Union. The Ergomanga-Brisbane Transect, GGT-7, was printed by the Austrailian Bureau of Mineral Resources. In addition, a paper version of GGT-6, the Central Andean Transect [Omarini and Götze, 1991], was digitized, and those data were used to produce CGM files for the main displays. Global Geoscience Transect 8, the Quebec-Maine-Gulf of Maine Transect [Stewart et al., 1993], was prepared digitally at the U. S. Geological Survey using state-of-the-art geographic information systems (GIS) technology, and published by the Geological Survey printed form. The Central Applachian Transect described in this pamphlet was produced digitally from the outset, using desktop personal computers. The essentail information was digitized item-by-item and plotted in CGM format. These partial drawings were then imported into a commercial PC drawing program where they were combined, and ultimately replotted in CGM format. The final CGM files can be transmitted on computer diskette or via the Internet, and viewed using almost any PC word processing program that allows embedded graphical images, such as WordPerfect or Microsoft Word. The transect can also be printed in color or in black-and-grey by anyone having an appropriate printer. Using A or A4 size paper and a 300 dpi printer, the transect can be printed at approximately 1:5,000,000 scale. A full-size display at the GGT project standard scale of 1:1,000,000 requires a color printer 3 m wide by 6 m long, but such large color printers remain somewhat rare.
Inherent in the all-digital nature of the Central Applachian Transect is one important advantage, and one potential problem, relative to the printed paper transects. The advantage is that the transect is dynamic, and the digital image files can be easily modified as new information comes to light or errors are discovered. One of the operating principles of the GGT program is to compile existing data rather than collect new data [Monger, 1987], and given the size of the individual transects no other course of action is possible in most cases. But even in the most intensively studied areas there are likely to be places where there is insufficient data to choose between competing interpretations. Printed paper transects tend to commit the compiler to defend more-or-less vigorously a particular interpretation, or alternatively, to present a range of possible interpretations, each consistent with the available data. In the case of a digital transect, the compilers are somewhat more free to present their favored interpretation, and change it later, if necessary. The potential problem is related to possibly excessive expectations by those who view the digital transect. For example, the Central Applachian was compiled at a scale of 1:1,000,000, and individual features are intended to be depicted accurately at that scale or smaller (e.g., 1:5,000,000). It is possible, however, to view a portion of the CGM file at a large scale (e.g., 1:10,000) where individual features might appear schematic, at best. Thus, the digital transect is potentially subject to criticism by those who expect it to be more accurate than the goals of the GGT program.
Finally, the authors probably should apologize to the reader for the length of this pamphlet. According to the GGT program guidelines, it is supposed to be brief, but at 26 printed pages of text and 178 references it is not clear that brevity has been achieved. On the other hand, the body of literature pertaining to the transect is truly vast, and only a small fraction of it has been cited. While it was never intended that the transect include a comprehensive bibliography, some topics have not been discussed and papers have not been cited, which perhaps should have been. Thus, the length of the pamphlet is both too long, and too short.