This report appeared in a special issue of the International Journal of Geographic Information Sciences, issue 4 of volume 13, in 1999.
GeoVRML techniques have the potential to provide functional and transparent communication between geographic information and 3D Web visualization tools. This report outlines recommended practices and modifications to the VRML 97 standard to consider pre-existing cartographic projections and georeferenced data. The concepts outlined for incorporating georeferenced coordinate systems in VRML worlds have generic applicability to 3D Web technologies like MPEG-4, Java3D and Chrome.
The interactive three dimensional (3D) representation of georeferenced data on the World Wide Web (Web) is achieved with tools like the Virtual Reality Modeling Language (VRML). VRML97 is the approved International Standard (ISO/IEC 14772) file format for describing interactive multimedia on the Internet. In general, a VRML file is also called a "world". Users explore these "worlds" with Web browsers that support the viewing of VRML files. More information on VRML can be found at the Web3D Consortium Web pages, see: (http://www.web3d.org).
The VRML97 standard was designed primarily by the computer graphics community. Typical computer graphics imagery focuses on locally bounded regions and small screen sizes where maximum pixel ranges are approximately 1600 by 1280 pixels. As a result, VRML97 relies on single-precision (32 bit) IEEE floating point data values. The coordinate system for VRML97 is based on the simple Cartesian local (X,Y,Z) coordinate system with the orgin being at (0,0,0) and Y representing up. This coordinate system is often sufficient for many computer graphics problems.
These two parameters of the VRML 97 standard provide limitations for the representation of geographic and cartographic data as well as georeferenced computational modeling simulations in VRML. For example, since the earth's diameter approximates 12 million meters, it is not possible to present geographic data resolutions greater than 10 to 100 meters with single-precision data values. This means that data obtained from global positioning systems (GPS) with absolute locations within 1 meter resolution cannot be accurately presented in VRML97. The heavy reliance on Cartesian coordinates also poses difficulties with data in Geodetic (GDC or lattitude/longitude), Universal Transverse Mercator (UTM), Lambert Conformal Conic (LCC) or other pre-existing cartographic projections. In February 1998, the VRML Consortium approved the formation of the GeoVRML Working Group to discuss and develop tools, recommended practices and standards necessary to generate, display and exchange georeferenced data in VRML, (Iverson & GeoVRML, 1998). In December 1998, the VRML Consortium expanded its charter and renamed itself as the "Web 3D Consortium".
This report reviews the major recommended practices and modifications to the VRML standard under consideration and development by the GeoVRML Working Group. Additional emerging 3D Web technologies and their relation to geospatial data visualization will also be highlighted. GeoVRML techniques have the potential to provide functional and transparent communication between geographic information and 3D visualization tools, (Rhyne, 1997).
Figure #1: Example VRML world with a TIFF image of a USGS map drapped over the 3D surface is shown on the left. On the right is a similar image made with a visualization toolkit package. Notice how the map is inverted in the VRML browser. We hope to improve this situation with GeoVRML Coordinate systems. Images developed by Theresa-Marie Rhyne and Thomas Fowler of Lockheed Martin Technical Services at the United States Environmental Protection Agency's Scientific Visualization Center. See: (http://www.epa.gov/gisvis).
Moving towards GeoVRML Coordinate Systems:
The geographic information systems, cartographic and military simulation communities have developed a number of standards for the representation of geospatial information and georeferencing of arbitrary data, (Rhyne, 1998). The Open GIS Consortium is presently moving forward with efforts to support the full integration of geospatial data into mainstream computing and the widespread usage of interoperable commercial geoprocessing software, see: (http://www.opengis.org/). There are also International Organization for Standardization (ISO) efforts in the Geographic information/Geomatics arenas, see: (http://www.iso.ch/meme/TC211.html).
In order to attempt to include a methodology for supporting georeferenced data in the upcoming (1999) revision to the VRML97 standard, the GeoVRML Working Group decided to base its efforts on a currently existing reference model and software package entitled the SEDRIS Geographic Reference Model (GRM). The Synthetic Environment Data Representation & Interchange Specification (SEDRIS) is a project funded by the United States' Defense Modeling and Simulation Office. The SEDRIS GRM supports twelve different coordinate systems and provides tools to automatically convert reference marks between them. The software source for GRM is publically available and is currently implemented in the C programming language. More information on the SEDRIS GRM can be found on the SEDRIS Web site at: (http://www.sedris.org/).
The GeoVRML Working Group is thus recommending a Level 1 practice whereby geographical coordinates based on the SEDRIS GRM are converted into a local Cartesian coordinate system for improved level of detail in GeoVRML visualizations. The GeoVRML Working Group is also exploring a Level 2 practice whereby geo-referenced data is transparently and seamlessly converted from a wider and multiple variety of sources, (Iverson & GeoVRML, 1998).
Researchers at the SRI International - Artificial Intelligence Center, have recently developed, for public release, the GeoTransform Java class file hierarchy based on the SEDRIS GRM. With the GeoTransform Java package, it is possible to perform efficient and accurate geographic coordinate transformations for the Geodetic Coordinate System (GDC), GeoCentric Coordinate System (GCC), and Universal Transverse Mercator (UTM) System. GeoTransform allows for authoring VRML worlds that read coordinates in any of these systems and tranparently convert the geographic data into Cartesian Coordinates for display in a VRML browser. More information on the GeoTranform Java package can be found at: (http://www.ai.sri.com/~reddy/geovrml/geotransform/) .
Defining the GeoOrigin Node:
In order to build a georeferenced VRML world, a GeoOrigin node is defined in the VRML file. This GeoOrigin allows for converting coordinates from cartographic earth-based coordinate systems into the existing VRML97 Cartesian reference frame, (Iverson & GeoVRML, 1998). A single GeoOrigin node, representing a single georeferenced point, becomes the reference frame identified with the VRML world's zero-based (0,0,0) origin.
EXTERNPROTO GeoOrigin [ field MFString geoSystem ["GDC"] field SFString geoCoords "" ] "urn:geovrml:protos#GeoOrigin"
The geoSystem field selects a geographic reference system from the naming conventions based on the SEDRIS GRM. Some of these georeference coordinate systems require additional arguments to fully designate the coordinates. As an example, the Geodetic (GDC) system involves the selection of ellipsoid, geoid, and datum references. Additional strings in the geoSystem field support this requirement.
The geoCoords field is a sequence of 64-bit precision values seperated by spaces that define an absolute location using the coordinate system selected in the geoSystem field. Optional strings in the geoSystem field determine the interpretation of the geoCoords field. As an example, "DMS" can specify that the geoCoords string will include degree, minute and second fields for each latitute and longitude value in a GDC coordinate. Every geospatial location determined by a geoSystem and geoCoords pair defines an implicit orthogonal Cartesian reference frame indexed by x,y,z in meters with the designated geospatial location at the origin and with y being the up direction. This allows for conformance with the VRML97 standard.
A more detailed discussion about the GeoOrigin node can be found in the Request for Comment document on GeoVRML Coordinate Systems. This discussion is located at the GeoVRML Working Group's Web site at: (http://www.ai.sri.com/~leei/geovrml/).
During the past year, SRI International developed a series of VRML97 nodes for improved support of terrain visualization. These contributions were developed as part of the GeoVRML Working Group and are in the public domain. A comprehensive discussion of these efforts can be found in the SRI International - Artificial Intelligence Center Report No. 559, which is cited the the references below, (Reddy, et. al., 1998). These new GeoVRML nodes can be accessed on the Web at: (http://www.ai.sri.com/geovrml/protos) .
Integrating Spatial Data Repositories and GeoVRML Visualizations:
There are a number of efforts underway to examine the use of the Virtual Reality Modeling Language (VRML) for the interactive exploration of geospatial data repositories (Rhyne & Fowler, 1996). In the United States, some of this work is being done in conjunction with the Federal Geographic Data Committee (FGDC) 's National Geospatial Data Clearinghouse (see: (http://fgdc.er.usgs.gov/)). In addition to the use of intelligent agents, data mining techniques are being employed to assist with the retrieval of spatial data. The development of GeoOrgin nodes in VRML will support the use of agent and data mining technology for rapid creation of interactive web-based visualizations. This will greatly facilitate visual information retrieval of geospatial data.
Reaching out to other Interactive 3D Web Technologies:
In addition to VRML, there are other 3D Web technologies under development. Three examples include (a) the development of the MPEG-4 standard; (b) Java 3D and (c) Chrome. In early 1998, the International Organization for Standardization (ISO) announced that it will use Apple Computer's QuickTime file format as the basis for a unified digital media storage format for the MPEG-4 standard for graphics content on the Web. The VRML Consortium has established a Working Group to examine MPEG-4 and VRML integration. Java3D, from Sun Microsystems, supports the development of 3D computer graphics applications in the Java programming language. This includes the development of VRML browsers with Java 3D. Another emerging 3D Web technology is Chrome from Microsoft Corporation. Chrome is a Windows 98 add-on that uses the Extensible Markup Language (XML) to access Windows 98 multimedia capabilities for creating 3D content on the Web. The concepts outlined above for incorporating georeferenced coordinate systems in VRML worlds have generic applicability to 3D Web technologies like MPEG-4, Java3D and Chrome. Details about the QuickTime file format and its adoption by ISO as the starting point for MPEG-4 can be found at the Apple Computer web site, see: (http://www.apple.com/quicktime/). More information on Java and Java 3D can be found at the Javasoft Web site, see: (http://www.javasoft.com/products/java-media/3D/). Additional information on Chrome can be found by searching the Microsoft web site at: (http://www.microsoft.com).
The use of VRML for cartographic and geographic presentation is currently being examined by research groups participating in the International Cartographic Association's Commission on Visualization, (Fairburn and Parsley, 1997). Preliminary definitions of the needs for geofunctions in virtual reality and VRML were done at Leicester University in July 1997, (Moore, et. al.). The Commission has also explored other multimedia and web-based technologies for developing mapping products, (Cartwright, 1998) and (Andrienko & Andrienko, 1998). The Association for Computing Machinery's Special Interest Group on Graphics (ACM - SIGGRAPH)'s collaboration with the ICA Commission on Visualization has attempted to examine how computer graphics technology can be effectively adapted to meet cartographic needs and requirements. This project, entitled the ACM SIGGRAPH Carto Project, is pleased that the VRML Consortium chose to create the GeoVRML Working Group to actualize effective exchange of georeferenced data in VRML. We anticipate GeoVRML techniques expanding to address many 3D Web Technologies as the VRML Consortium redefines itself as the Web 3D Consortium. The issues discussed here are important steps toward functional integration of geographic information and 3D visualization tools. We hope similar efforts will continue to emerge in the future.
We would like to acknowledge the efforts of Lee Iverson, founding Chair of the GeoVRML Working Group of the Web 3D Consortium, Don Brutzman, Vice President for Technology of the Web 3D Consortium, and Martin Reddy (who built many of the new GeoVRML nodes for VRML97). We are also appreciative to Judy Brown, Past Chair of Special Projects for ACM SIGGRAPH, for all the encouragement she provided during the first two years of the ACM SIGGRAPH Carto Project.
ACM SIGGRAPH Carto Project Web Site: (http://www.siggraph.org/~rhyne/carto/).
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Iverson, Lee & the GeoVRML Working Group of the VRML Consortium. 1998, GeoVRML RFC1: Coordinate Systems, (http://www.ai.sri.com/geovrml/rfc1.html).
ICA Commission on Visualization Web Site: (http://www.geog.psu.edu/ica/ICAvis.html).
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Biography of the Author:
Theresa-Marie Rhyne is a Director at Large of the ACM SIGGRAPH Executive Committee and is the Project Director of the ACM SIGGRAPH Carto Project. She is a lead scientific visualization researcher for Lockheed Martin Technical Services at the United States Environmental Protection Agency's Scientific Visualization Center.