For the past few years, the USGS has been experimenting with
digital revision methods. The first DLG-3 revision software was
introduced by the USGS in 1995: the RevPG product. RevPG is
the Arc/Info-based Revision and Product Generation program
developed by the USGS in cooperation with the Environmental
Systems Research Institute (ESRI) as part of a Cooperative
Research and Development Agreement (CRADA). RevPG has
been developed for the revision of DLG files using digital
orthophoto quarter-quadrangles (DOQQ) as the source material.
As indicated by Decker (1986), “the DOQQ is treated as the
ground truth and the DLG is updated to match the features
identified on the DOQQ.”
Revision with RevPG is a highly interactive process. The
operator displays a digital orthophoto quarter-quadrangle and
visually compares the DLG-3 features with the terrain shown in
the raster image. Changes in the geometric centerline or the
centerline of new features are collected by a heads-up digitizing
process. Appropriate attributes are attached to each geometric
centerline. Besides the interactive tools, RevPG provides some
automatic tools. For example, it checks for consistency between
two layers of the same quadrangle (for example, hydrography and
hypsography).
Besides RevPG, the USGS has developed some additional
products and tools for the revision process. The Digital Raster
Graphics (DRG) files is one of these products. DRGs are color
raster images of analog 7.5-minute quadrangles. DRGs can be
merged with DOQQs in order to generate an image with the
existing features in the DLG data and the most current terrain
representation. A program developed by the USGS: DRG_DOQ
MERGE performs this operation. These computer-based tools
are being tested by the USGS and by a few other agencies and
universities; their efficiency and performance are still unknown.
3. THE GISOM PROJECT AND SPATIAL DATA
REVISION
3.1 Background
The Ohio State University Center for Mapping started a study of
analog-to-digital map conversion technique in 1991. This effort
turned into the GISOM project. The GISOM project started in
October 1993, with two goals: (1) developing a cost-efficient
methodology for conversion of 7.5-minute USGS quadrangles
into Digital Line Graphic-3 (DLG-3) files (the USGS digital
format), and (2) testing this methodology by converting all 793
7.5-minute quadrangles covering the state of Ohio. The GISOM
project will last four years, with an expenditure of five million
dollars. The GISOM project is in its third year and has completed
the conversion of 500 quadrangles. All the DLG files generated
need revision and we are developing such capability.
The GISOM project is a cooperative effort among the federal
government (USGS), the state government (Ohio Departments of
Administrative Services, Development, Natural Resources, and
Transportation, the Ohio Environmental Protection Agency, and
the Ohio Geographically Referenced Program), the university
(The Ohio State University Center for Mapping), and the private
sector (independent contractors).
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3.2 Technical Aspects
As part of the GISOM project, five of the nine cartographic
layers of the 7.5-minute quadrangles are converted into seven
DLG-3 files: boundaries, hydrography, hypsography, public land
survey system (PLSS), and transportation (three files: roads,
railroads, and miscellaneous transportation).
Conversion includes the collection of the geometric centerline of
the elements of each cartographic layer and the corresponding
attributes. The geometric centerline is expressed by four
topological elements: nodes, lines, degenerated lines (points),
and areas. Attributes are classified as major attributes (common
for a given layer) and minor attributes (with specific meaning to
describe the characteristics of the elements of a layer). A
maximum of one major and ten minor attributes can be attached
to a topological element.
Conversion is done by a combination of heads-up digitizing and
interactive and automatic line following. A commercial program
has been used with reasonable results for automatic vectorizing.
Software developed by the Center for Mapping is used for
attribute collection, heads-up digitizing and/or interactive line
following digitizing. The heads-up software was modeled after
the USGS heads-down digitizing software.
After the attributing and digitizing is completed, files are
checked by a quality control program (PROSYS) developed by
the USGS. This program evaluates the internal consistency of
these files. After that, interactive quality control for geometric
accuracy, attribute consistency, and completeness is performed
by Center for Mapping staff, and then, files are automatically
transferred to the USGS mainframe via FTP.
4. SPATIAL DATA REVISION AT THE
CENTER FOR MAPPING
4.1 Background
The Ohio State University Center for Mapping has been
researching the topic of spatial data collection and revision
(especially DLG files) for the last five years. This is part of a
project on analytical study of spatial data and spatial data
representation (Ramirez, 1989, 1991; Ramirez and Lee, 1991;
Ramirez and Fernandez-Falcon, 1992). A major topic of
research has been the development of a conceptual framework for
spatial data collection and revision.
The Center for Mapping is interested in the “total revision”
approach as defined by the USGS. GIS analysis and the
computer-based design of man-made features (bridges, highways,
shopping malls, reservoirs, etc.) is better served by a consistent,
accurate, and up-to-date terrain representation. Therefore, our
research of revision includes the revision of spatial data
representing the relief as well as “planimetric” features (natural
and man-made).
4.2 Understanding Changes on the Surface of the Earth
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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