With a suitable infrastructure in place to index georeferenced
data, we need to discuss the details of how these data can be
represented and generated. Accordingly, this section will describe
the creation and maintenance of georeferenced data for the Digital
Earth. Particular emphasis is placed upon the adoption of open
standards and the ease with which new data can be generated and
embedded into the global infrastructure.
Figure 2 (a) A tiled, pyramid representation for terrain, and
(b) Illustrating the use of this structure to produce
viewpoint-dependent, multi-resolution imagery.
One of the principal goals of this proposal is to allow multiple
types of georeferenced data to be embedded into the global
infrastructure. For example, we could have 100 km resolution data for
the entire globe but recursively insert higher-resolution datasets for
smaller regions of interest, for example, a 1 km resolution dataset
for the conterminous United States and a 1 m resolution dataset for
Yosemite Valley, CA. Then we could insert 3-D models for buildings in
Yosemite Valley, some of which could contain hyperlinks to various
multimedia presentations about the National Park. Our data
representation must be able to support this level of flexibility.
We propose to implement the above Digital Earth representation by
introducing three primary types of VRML files: Tree files, Terrain
Tile files, and Data files. These are the basic building blocks of
our VRML terrain representation.
- Tree files
- These implement part of the multi-resolution
structure for the entire globe. In effect, these files are the glue
that holds the Terrain Tile and Data files into a global quad-tree
structure. The Tree files enable us to distribute the entire
hierarchy over multiple servers and to abstract the multi-resolution
structure from the actual terrain and other data. The tree files
contain descriptions and links to all available data for a single
region at one level of detail.
- Terrain Tile files
- These contain the actual terrain data for
a single tile of a pyramid. This includes the elevation geometry and
links to the texture-map imagery for that specific tile of terrain.
There will exist a separate pyramid of Tile files for each terrain
dataset. Using the previous example, the 1 km U.S. dataset and the 1
m Yosemite dataset will each have its own pyramid of Terrain Tile
- Data files
- These are arbitrary georeferenced data embedded into the
Digital Earth structure. For example, these could be building models,
roads, weather simulations, and terrain annotations. Any of these
might also contain hyperlinks to other types of information such as
audio, video, or textual presentations. This is where the richness of
the Digital Earth is enabled.
Data Production Tools
For the Digital Earth to be a useful and widely adopted facility, it
must be extremely simple for users to introduce their own data into
the global infrastructure. This requires the existence of several
tools to help manage the contents of the Digital Earth. We propose to
develop software that will aid users to produce multi-resolution
terrain in VRML format and insert these data into Digital Earth, to
embed new georeferenced objects into the structure, to update existing
objects, and other useful operations. We propose to utilize SRI's
tsmApi library to fulfill these capabilities.
The tsmApi (Tile Set Manager Application Programming Interface) is an
open, freely available, C library provided by SRI International. The
library offers functions for reading, writing, and managing
terrain-related data. This includes functions for generating
multi-resolution, tiled, terrain using VRML. Currently, these datasets
can be created from supported input formats such as raw imagery, PBM
images, and LAS bitmaps.
The tsmApi library also includes a fully re-entrant VRML parser
written at SRI International. This can be used to parse VRML files
efficiently into memory and to write these structures back out to a
VRML file. Functions are also provided to perform various geographic
coordinate transformations, for example, UTM to Lat/Long and Lat/Long
to Geocentric. (These are based upon code from the National Imagery
and Mapping Agency's NIMAMUSE product.)
Pre-compiled tsmApi distributions are available for several
platforms as well as the full C source code and extensive Web-based
documentation. Using all of these materials, users can develop their
own tools for managing the content of the Digital Earth. We propose
to deliver several such tools as part of this project, for example,
software to perform the following operations:
- Produce multi-resolution tiled terrain pyramids in VRML.
- Insert these terrain pyramids into the main infrastructure.
- Embed georeferenced objects into the main infrastructure.
- Remove selected components from the Digital Earth.
- Modify existing components, for example, update metadata or
- Query the contents of the Digital Earth.
Next: Vizualization System
SRI's Digital Earth Project
SRI's Digital Earth Proposal
Fri Apr 23 17:52:37 1999