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Digital Mapping Techniques '98 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 98-487

Digital Conversion of the Austin West Quadrangle, Texas

By Thomas A. Tremblay, Sarah B. Dale, and John R. Andrews

Texas Bureau of Economic Geology
The University of Texas at Austin
Box X University Station
Austin, TX 78713
Telephone: (512) 471-1534
Fax: (512) 471-0140


The Geologic Map of the Austin West Quadrangle is one of a series of digital data bases of geologic maps being produced by the Bureau of Economic Geology (BEG), The University of Texas at Austin. The goal of this effort is to make traditional BEG map products (previously only accessible as paper products) available in a digital geographic information systems (GIS) format. These digital GIS files are designed both for internal research by BEG staff and for use by other public and private entities.

This paper is intended for two audiences. First, it is directed toward experienced GIS analysts who will be using the Austin West files for GIS analysis and who wish to know the technical details about how these files were converted to digital format. This report is also intended for those people who are not experienced in GIS but who may become involved in a similar conversion project. We include a discussion of basic concepts as a possible model for future conversions.


The conversion from paper maps to digital GIS format was accomplished using generally accepted cartographic standards. Film positives of original maps were scanned, automatically vectorized, and then coded with attribute information. Arc/Info 7.1 and ArcView 3.0 are the GIS programs used throughout the project to automate and construct tables for the maps. This document serves as the basic metadata summary and describes the methods used during digital conversion.

The files discussed in this report include Arc/Info coverages for geology, faults, points, roads, and cities from the quadrangle map. The digital files are arranged in layers that cover the Austin West Quadrangle, identical to the original map publication. The files are large scale and are best suited for analysis at the local level. The scale of the Austin West Quadrangle Map is 1:24,000. Although the digital GIS files can easily be combined with more precise, larger scale information, it is important to note that interpretation or analysis using these maps should always reference 1:24,000 scale or smaller.


The digital files for the Austin West Map are based on the 1:24,000 scale maps printed by the BEG (Rodda and others, 1979). The files reflect the 1979 map publication with no systematic update of features. A companion text by the same authors is available from the BEG that explains the geologic interpretation and provides a complete listing of data sources used in map compilation. To order the original paper map, consult the BEG's Internet site (


A four-step process was used to convert the map data to digital form:

  1. Automation -- A scanned image of the original film positive was vectorized, edited, and processed into arc, polygon, and point formats.
  2. Projection -- Digital files were transformed from digitizer coordinates to real-world coordinates and converted to a cartographic projection.
  3. Tables -- Tables were created to hold label information for polygons, arcs, and points.
  4. Symbology -- A symbol variable was added into the coverage tables and a shading keyfile developed for map plots.

The resulting products include eight digital GIS files and intermediate work files and an ArcView project configured for easy GIS viewing of these data.


The scanned film positive was automatically vectorized using ArcTools editing software. Line features were coded according to feature type, and polygon features were coded on the basis of their geologic description. Polygons were then labeled using an automated look-up table. All coverages were created using Arc/Info 7.1 in a UNIX operating system environment.

A paper check plot was printed and compared with the original map for accuracy and completeness. Any necessary edits to line placement or labeling were accomplished and noted. Node errors and label errors were corrected during the editing stage of work.

Projection Information

The original base map used to compile the geology data was a U.S. Geological Survey map of the Austin West Quadrangle, the original projection of the printed map is Polyconic. Digitizer coordinates were transformed into Polyconic and reprojected into UTM.


Fly-throughs are popular media for visualizing many forms of spatial information. The Austin West fly-through uses GIS technology and 3-D visualization to illustrate one of the basic principles of geologic mapping -- topographic relief is commonly influenced by underlying geology. Color-coded geologic units (GIS coverage) are graphically combined with computer-derived hill shading and draped over a digital elevation model (DEM). The viewer is then placed in different positions relative to the geologic surface and "flown" over the terrain. The "fly-through" is presented as a tool for investigating spatial relationships between the surface expression of geologic units and topographic relief.

The digitized Austin West Quadrangle provides a thematic overlay for 3-D visualization of geologic-topographic relationships in the rolling Hill Country of central Texas. Meandering through the center of the quadrangle, the Colorado River and its tributaries dissect the Edwards Plateau to the west, providing a suitable setting for this type of terrain model.

The Austin West fly-through has two principal components. In addition to the DEM, an image of the thematic information is created as an overlay for the DEM. The overlay consists of a grid composite that combines a computer-generated hill shade effect with the geologic data. Hill shading enhances the 3-D quality of the surface by illuminating sunlit surfaces and darkening shaded areas. Care must be taken to avoid the loss of geologic information which may be obscured by shadows. The grid composite is then overlaid on top of the DEM to create the final surface.

A suite of GIS modeling programs are used to position the viewer relative to the surface. Once the "flight path" has been determined, multiple parameters must be considered and tested. Through trial and error, parameters such as height above the surface, distance between successive frames, and viewer orientation are determined. Screen captures are recorded as the viewer is moved through the surface to create a series of frames similar to the frames on a roll of video film. When the desired series of screen captures has been recorded, the animation video is assembled and edited. Arc/Info version 7.1.1 supports fly-through animation but has limited editing capabilities.

Future applications of animation and visualization will incorporate virtual reality technology. Virtual reality applications provide more information than a fly-through in that the viewer interacts freely with the data set and is not limited to a predetermined course of investigation. Sophisticated algorithms run on a powerful computer calculate the view model in near realtime as the viewer navigates through the data set. Advanced information technologies provide new models for understanding the complex spatial relationships inherent in geologic mapping.


Dent, Borden, 1985, Principles of Thematic Map Design: Addison-Wesley Publishing Company, Reading, Massachusetts.

Environmental Systems Research Institute, 1994, Understanding GIS: The ARC/INFO Method: Environmental Systems Research Institute, Inc., Redlands, California.

Greenhood, David, 1964, Mapping: The University of Chicago Press, Chicago.

Rodda, P.U., Garner, L.E., and Dawe, G.L., 1979, Geologic Map of the Austin West Quadrangle: Bureau of Economic Geology, the University of Texas at Austin, scale 1:24,000.

Texas Geographic Information Council, 1997, Statewide Geographic Information Systems Implementation Plan: United Through a Common Geography: Texas Geographic Information Council, Austin.

Texas Geographic Information Systems Standards Committee, 1992, Standards and Guidelines for Geographic Information Systems in the State of Texas: Department of Information Resources, Austin.

U.S. Geological Survey, 1995, Draft Cartographic and Digital Standard for Geologic Map Information: U.S. Geological Survey Open-File Report 95-525.

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