GeoVRML 1.0 Recommended Practice
GeoVRML 1.0 provides a suite of solutions for representing and visualizing geographic data using a standard VRML97 (Virtual Reality Modeling Language) browser. VRML is an ISO standard file format for representing 3-D data over the web [VRML97]. This document contains descriptions of various issues related to the representation of geographic data in VRML97 and then defines a number of new nodes to provide solutions to these issues using VRML's extensibility features. It will be assumed that the reader is familiar with the VRML97 specification.
In addition to this document, an accompanying open source sample
implementation of the GeoVRML 1.0 nodes is provided so that users may
easily incorporate these capabilities into their own VRML scenes.
The following list provides a high-level list of capabilities that are specifically addressed by GeoVRML 1.0.
Coordinate Systems - GeoVRML provides the ability to embed latitude/longitude or UTM coordinates directly into a VRML file and have the browser transparently fuse these into a global context for visualization. GeoVRML 1.0 supports 3 coordinate systems, 21 ellipsoids, and 1 geoid.
Precision - VRML97 provides only single-precision floating point values. This is insufficient to represent data on a planetary scale down to around 10 m resolution or beyond. GeoVRML provides solutions to extend this precision and enable sub-millimeter positional accuracies.
Scalability - GeoVRML provides various scalability features to manage the streaming of large, multi-resolution models over the web.
Metadata - GeoVRML provides the ability to specify a generic subset of metadata describing geographic objects, including the ability to link to a full metadata description.
Animation - The ability to interpolate within the supported geographic coordinate systems is provided so that animations can be defined with respect to key points on the surface of the planet.
Introspection - Functionality is provided to be able to query a GeoVRML scene and discover the geographic coordinate of any georeferenced point.
Navigation - GeoVRML 1.0 provides some basic support for navigation schemes that are specific to geographic applications. Specifically, the issue of elevation scaled velocity is addressed.
There is a large number of issues that GeoVRML could address with respect to representing geographic data in VRML. We have already listed those areas that we do address; however, it may also be useful to specify explicitly a number of important areas that GeoVRML 1.0 does not address. This is done so that the reader may better ascertain the relevance of this work to their own application area.
GeoVRML 1.0 does not address issues of time, other than the ability to specify date metadata. Certain issues of navigating planetary models are similarly not addressed, e.g. terrain following. GeoVRML 1.0 supports the use of only a single, static local coordinate system for dealing with floating point limitations. With respect to geographic coordinate systems, we currently only support the latitude/longitude (GDC), Universal Transverse Mercator (UTM), and geocentric (GCC) coordinate systems, and we only support the WGS84 geoid. Finally, GeoVRML does not directly address the issue of ortho-rectification. It is assumed that existing tools will be employed in order to correctly warp unrectified imagery to an elevation grid before translation to GeoVRML. These issues are prime candidates for addressing in a future version of GeoVRML.
A number of design decisions were taken during the GeoVRML 1.0 development that were necessary due to constraints of the VRML97 specification. For example, in order to represent double-precision values we encode these using strings. Also, we are unable to use many exposedFields in the interfaces to the GeoVRML nodes because most of these fields are passed into Script nodes, which do not support exposedFields. It is hoped that a future revision of the VRML standard will address these and other issues so that the GeoVRML interface specification can be made more clean.
Questions or comments.
Copyright 2000, SRI International.