THE CONTRIBUTION OF GEODETIC DATA TO THE NATIONAL SPATIAL DATA INFRASTRUCTURE
Rear Admiral J. Austin Yeager, NOAA, Director, Coast and Geodetic Survey; Captain Lewis A. Lapine, NOAA Chief, National Geodetic Survey, C&GS; and John F. Spencer, Jr., Chief, National Geodetic Information Branch, NGS
In the 1993 National Research Council (NRC) report, Toward a Coordinated Spatial Data Infrastructure for the Nation (1), it was exemplified that geodetic control is required to systematically register all spatial information to allow their integration into Geographic Information Systems (GIS). Such GIS systems have application in wetlands delineation, mineral assessment, renewable resource management, public health, urban and regional planning, disaster response and recovery, and national defense, among others.
Geodetic data, the product of geodetic control, are essential to the development of GIS and serve as one of the primary components of the National Spatial Data Infrastructure (NSDI). The importance or value of these data to the Nation's geospatial data users has been acknowledged by various levels of government, the private surveying community, and the courts.
As part of NSDI, a model of data sharing is being established and utilized through the leadership of the National Geodetic Survey (NGS) of the Coast and Geodetic Survey (C&GS), National Oceanic and Atmospheric Administration (NOAA). Benefits of sharing data occur through cooperative efforts in collecting and maintaining data as well as through the increased transfer and use of this valuable information. The data sharing program works because the donors (private sector and local, state, and other federal governmental organizations) want to ensure the accuracy of the points they observe and earn NGS' "stamp of approval" as the Nation's highest authority for geodetic information processing and distribution.
The distribution of geodetic data leads to an increased frequency of reuse of the control points by local and regional users. In each instance the reuse potentially saves either private or public funds because of shared resources and cooperative partnerships. Responsibility for this data sharing program places an awesome challenge on the Federal Geodetic Control Subcommittee (FGCS) of the Federal Geographic Data Committee (FGDC).
On October 19, 1990, the Executive Office of the President, Office of Management and Budget (OMB) revised Circular A-16, Coordination of Surveying, Mapping, and Related Spatial Data Activities(2). The goals of the circular are to develop a national digital geographic information resource, to reduce duplication, to reduce the expense of developing geographic data, and to increase the benefits of using available data and ensuring coordination of Federal agency geographic data activities. A major objective of A-16 is the development of a NSDI with the involvement of Federal, State, and local governments, and the private sector. This national information resource, linked by partnerships and standards, will enable sharing and efficient transfer of geospatial data between producers and users.
Circular A-16 established the FGDC, chaired by the Secretary of Interior to promote the coordinated development, use, sharing, and dissemination of geographic data. The committee oversees and provides policy guidance for agency efforts to coordinate geographic data activities. Federal agencies were assigned the responsibilities of leading coordination activities for categories of data (Table I). Agency responsibilities include providing government wide leadership in developing data standards, assisting information and data exchange, and coordinating data collection. The NOAA's C&GS of the Department of Commerce is responsible for coordinating the geodetic activities.
Changes in technology, i.e. Global Positioning System, require standards for surveying, data reduction, and formatting to be reevaluated and amended as necessary. In addition, there are several aspects to data sharing that need to be considered dealing with issues such as data accuracy, completeness, timeliness, reliability, accessibility, security and liability. The FGDC has developed a metadata standard that will incorporate these aspects and expedite sharing of these valuable geodetic data. By assuring that these data comply with national standards and guidelines they contribute to and become an integral part of the NSDI.
National Spatial Reference System
NOAA, through C&GS, has a mandated responsibility to establish and maintain a geodetic reference system. NOAA's geodetic reference system was originally established using classical surveying techniques and was used primarily by federal, state, and local governments, and utility companies to maintain uniformity in land surveys. With the advent of GPS, and its availability for civilian use, it is now possible to obtain four-dimensional position data (i.e., geodetic latitude and longitude, ellipsoidal and orthometric heights, gravity and crustal motion over time) that are economical and an order of magnitude more accurate (for horizontal positioning) than data obtained from the earlier classical methods.
At the same time, the number and diversity of users of spatial data have increased to such an extent that there is a need for a NSDI to which all mapping, surveying, navigation, transportation, crustal motion studies, and geographic information systems can be referenced. With the widespread availability of low-cost GPS receivers, it is now possible to create a more accurate and useful geodetic reference system.
The United States depends on this high-accuracy national geodetic reference system for a wide array of vital environmental, commercial, defense, and civilian governmental activities. NOAA is developing this extremely accurate, geospatial framework system, the National Spatial Reference System (NSRS) in cooperation with other governmental agencies. The NSRS will have the following components:
The NSRS will provide this common geodetic positional system, the foundation for using geospatially referenced data and cost effective sharing of governmental data resources, to form the digital framework (3) of the NSDI.
NOAA has directed NGS to develop and implement the NSRS, as funds are made available, in direct support of NSDI. As the NSRS is implemented, the foundation of the NSDI will take form and begin to serve the rapidly evolving geospatial data needs of our Nation. As examples, the NSRS will improve the availability and accuracy of positional information necessary for the development of accurate parcel based components of GIS; and will enhance GPS navigation and positional activities of present and potential users.
The framework for the NSRS and the role of the NGS in implementing it are outlined in the 1994 NOAA documents, National
Geodetic Survey: Its Mission, Vision, and Strategic Goals (4), and Draft Implementation Plan for the National Spatial Reference System (5). The NOAA asked the NRC's Committee on Geodesy (COG) to review the documents, in the context of the national need for a coordinated spatial data reference system. The COG responded by organizing a forum to obtain input from the user communities and published a final report, Forum on NOAA's National Spatial Reference System (6). The COG found that the NSRS has the potential to provide enormous economic opportunities and societal benefits to the country, which includes:
National Spatial Data Infrastructure
The Clinton Administration identified creation of the NSDI as one of the initiatives necessary to "reinvent government." Within Vice President Gore's National Performance Review summary report (7), the development of the NSDI, based on partnerships with non-Federal sectors, was recognized as key to minimizing redundancy in the creation of geospatial data and to maximizing access to the geospatial data needed to solve critical environmental, economic, and social problems. The FGDC was recognized as the Federal entity responsible for helping to guide development of the NSDI. The President has signed an Executive Order 12906 - Coordinating Geographic Data Acquisition and Access: The National Spatial Data Infrastructure (8) which will accelerate the development and implementation of the NSDI. This directive addresses a variety of activities that must be carried out by the FGDC, by federal, state, and local government agencies and by members of the nonpublic sectors to fully develop the NSDI. The success of the NSDI will hinge upon the ability to build and maintain partnerships among these entities to carry out the actions of this directive (Table II).
The FGDC has described the NSDI as an umbrella under which organizations and technology interact to foster more efficient use, management, and production of geospatial data. The major objective of the NSDI is to foster enhanced use of geospatial data through better management of existing geospatial data and through more efficient collection and production of new geospatial data in ways that maximize data usefulness for multiple data users. The current elements of the NSDI include the geospatial data standards and clearinghouse partnerships in various stages of development. A plan to develop the NSDI which specifically addresses steps to develop partnerships and standards is available from the FGDC (9).
A partnership is a joint venture agreement whereby resources (and risks) are shared by the members to accomplish expected goals. Partnerships are required in order to establish the NSDI. These NSDI partnerships involve data acquisition, i.e., the geospatial data gathering, transferring, and developing of shared data bases between members. Geodetic data in standard formats form the basis for such data acquisitions. The FGDC, in accordance with the presidential directive on NSDI, will develop strategies by January 1995 for maximizing cooperative participatory efforts with state, local, and tribal governments, the private sector, and other non-Federal organizations to share costs and improve efficiencies of acquiring geospatial data.
C&GS sponsors a cost-sharing advisory program with several states, Federal-state partnerships. The program provides a liaison between C&GS and the host state, with a jointly funded C&GS employee residing in the state to guide and assist the state's charting, geodetic, and surveying programs. The program is designed to fill a need for more accurate local geodetic surveys and is in response to the states' desire to improve their surveying techniques to meet standards and specifications. The advisors primarily instruct local surveyors on how to use and maintain the NSRS. The following states have geodetic advisors: Alabama, Alaska, California, Colorado, Florida, Georgia, Idaho, Illinois, Kansas, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, Ohio, Oregon, South Carolina, South Dakota, Vermont, Washington, and Wisconsin.
Many other states have requested advisors, but present funding levels will not allow expansion of these federal-state partnerships.
The C&GS State Geodetic Advisor program has been very successful, particularly in the states that have active GPS surveying programs and state advisors with modern training. State Geodetic Advisors assist local governments and surveyors in the proper use of geodetic control and by helping users to take full advantage of the GPS-based NSRS, they indirectly bring economic benefits to the community, the end users of geodetic control. Examples of such local economic benefits include increased efficiency of local governments, prevention of waste from incorrectly implemented local GIS systems, and reduction of Federal Emergency Management Administration (FEMA) flood insurance premiums in some locales. The National Research Council report recommends that NOAA should provide a well-trained State Geodetic Advisor to each state or region. When funding is available, the State Geodetic Advisor program will be expanded so that each state has access to a state advisor with the appropriate training and ability.
An important function of NOAA and the state Geodetic Advisors is the workshop program which educates local users in the proper use of geodetic control. Generally done in cooperation with professional societies, Federal agencies, or universities, the workshops provide good technical communication exchange that strengthens Federal-state relationships. Presentations and publications in technical conferences and journals sponsored by established and potential user communities also help to establish NOAA as a technical leader, advertise its products and services, and inform the wider community about the NSRS and the its relationship to NSDI. In addition, conferences and publications are a means of promoting the use of standardized data and GIS formats, particularly in societies and organizations that have links with NOAA. Relevant professional societies and conferences with whom NOAA already participates include: American Geophysical Union (AGU), American Congress on Surveying and Mapping (ACSM), Urban and Regional Information Systems Association (URISA), Automated Mapping/Facilities Management International (AM/FM), Geographic Information Systems/Land Information Systems (GIS/LIS), Institute of Navigation (ION), American Association of Highway Transportation Officials (AASHTO), American Society of Photogrammetry and Remote Sensing (ASPRS), Society of Exploration Geophysicists (SEG), IVHS America, Transportation Research Board (TRB), Council of Governments, and Association of State Flood Plain Managers.
An example of a partnership between Federal agencies is discussed at length in an accompanying paper in the proceedings of the 1994 Federal Geographic Technology (FGT) Conference (10). This partnership involves cooperation in the establishment of GPS reference stations to support differential GPS (DGPS) applications in navigation and positioning. The need for such cooperation to prevent duplication is being actively addressed in the Federal government at the present time. The General Accounting Office (GAO) has recently completed a study of possible duplication in GPS reference stations. Also, a related GPS study funded by the Department of Transportation will be completed in September 1994. This study is aimed at determining what type of GPS reference station network might meet the majority of Federal needs. This cooperative effort described in the FGT 94 proceedings is the result of discussions that began several years ago between NGS, the U.S. Coast Guard (USCG), the U.S. Army Corps of Engineers (COE), and the Federal Aviation Administration (FAA). These discussions took place within the context of the Civil GPS Service Interface Committee established by the Department of Transportation and the FGCS of the FGDC. The result is that DGPS stations being established by NOAA, USCG, COE, and FAA to support marine, river, and air navigation have been configured so that they can also meet positioning needs of surveying, mapping, and GIS users. This meshing of navigation and positioning requirements as a partnership under the NSDI will assure geospatial data gathering commonalities in the future. The close coordination of navigation and positioning under the NSDI is exemplified by a recent exchange of letters between Secretaries Bruce Babbitt of Department of Interior, who is responsible for the develop of NSDI, and Frederico Pena of Dept of Transportation, who is responsible for the operational development of GPS for the civil user community. The federal departmental partnerships will result in the FGCS, chaired by the C&GS, NOAA of the Department of Commerce, forming a GPS Interagency Advisory Committee to report through the FGDC to the Secretaries' of Transportation, Interior, Defense and Commerce. This advisory committee will strengthen the ties between the navigation and positioning communities and provide for the implementation of a dual use system for GPS users, both military and civilian.
Secretary Babbitt in his interview with GIS World (11) emphasized the importance of standards and partnerships. He plans to have "FGDC facilitate more enduring and productive partnerships for collecting, managing, and using geospatial data to solve real problems."
Additional information concerning these and other geospatial data partnerships with NOAA's C&GS can be provided to you by writing the above address. Partnerships to implement the NSDI exist! However, they must be based on good technical communication of requirements and established (or enhanced) standards.
Geodetic Data Standard
In essence, a standard is really a model for what is acceptable to a community of users (i.e., a vendor, professional discipline, or a nation). There are two types of standards:
In order to implement the geodetic data component of the NSDI, proposed data standards must be developed and endorsed by the FGCS who represent the federal community of geodetic data users. Then, the proposed data standards must be approved by the FGDC and sent to the National Institute of Standards and Technology (NIST). NIST will evaluate the data standards and issue a Federal Information Processing Standard (FIPS) to be used by all federal agencies. A status report on the development of quality, collection, content, transfer, and metadata standards is given below:
Quality standards are the accuracy and operational standards for data collection. Quality standards for geodetic control data are presently contained in the Standards and Specifications for Geodetic Control Networks, published by the FGCS (12).
These standards were based on the classical system, the National Geodetic Reference System (NGRS). However with the transition from the NGRS to the new NSRS, new quality standards are required. Therefore, the FGCS Methodology Work Group has proposed revision of the geodetic accuracy standards of the quality standard component under the following conditions and concepts.
Collection standards are the submission, processing, and data base format standards for data sources. Collection standards for geodetic control data are presently contained in the "Blue Book," the Input Formats and Specifications of the National Geodetic Survey Data Base, published by FGCS (13). This publication describes the formats and procedures for submitting data for adjustment and assimilation into the NGS data base. Separate volumes of this publication refer to horizontal control data (Volume I), vertical control data (Volume II), and gravity control data (Volume III). Guidelines for submitting GPS positioning data are contained in Annex L of Volume I.
The NGS has determined that the value of geodetic observations for the NSRS obtained by other Federal, state, and local organizations compensates for the costs incurred by the Federal Government to provide quality assurance, archiving, and distribution functions for surveys contributing to the public good. Organizations submitting data must adhere to the Blue Book requirements and its accompanying policy statement regarding the incorporation of geodetic data of other organizations into the NGS data base.
The Blue Book has evolved over the years in response to changes in surveying technology since its first issue in 1980. For example, Volume 1, Horizontal Control Data, was revised in January 1989 to include GPS data submission and new formats for an improved, unified publication format for the descriptions that accompany published control-point values. The most recent update to the Blue Book was published in September 1994 and is presently being distributed to FGCS members and to nationwide data contributors. As circumstances arise, NGS policy requirements have been relaxed to accommodate unique situations. The US Geological Survey (USGS) in cooperation with NGS, is currently converting much of its remaining third-order leveling data to computer-readable form so that these data can be incorporated into the North American Vertical Datum of 1988 (NAVD 88). This work is being accomplished with customized NGS software. The 10-year effort will eventually incorporate about 500,000 USGS bench marks into NAVD 88, thus vastly increasing the usefulness to the surveying public both in traditional leveling applications and through improved geoid modeling of regions that would otherwise be deficient.
NGS has received data from outside users for 88,000 horizontal control points since 1980. There has been a total of 48,000 km of geodetic leveling submitted to NGS by other organizations since 1980. A similar effort to gather gravity data is currently under way. These gravity data will also be used to improve geoid height modeling, an essential requirement for accurate GPS-derived orthometric heights. The cost savings to the nation's surveying and mapping community for these horizontal and vertical control data conservatively can be estimated at about $107.2 million, whereby 88,000 points x $1,000 per horizontal point = $88,000,000 and 48,000 km x $400/km of vertical data = $19,200,000 have been incorporated into the national framework.
If we extrapolate similar savings to the Nation with regard to the above mentioned USGS benchmarks or approximately 500,000 km of leveling, the cost savings to the Nation's surveying and mapping organizations would amount to nearly $200 million.
This data sharing program works because the donors (private, county, state, and other Federal
organizations) want to ensure the accuracy of the points they observed (or had contractors
observe) and earn NGS' stamp of approval as the Nation's highest authority on geodetic control.
The data sharing program also provides the mechanism for the publication of officially sanctioned
values, the national distribution of these values, and automatic updates of the data as
computations on the NSRS are performed, increasing frequency of reuse of the control points by
local and regional users, where each instance saves either private or public funds.
The content standards are the assemblage of code types and data base elements pertaining to data holdings for storage and management. Content standards for geodetic control data are presently contained in the NGS data base data dictionary. The NGS data base presently contains approximately 10 gigabytes of stored information. The NGS data base is operational both for data processing and data distribution activities. The entire surveying and mapping community of the United States depends on its well being. Questions concerning the data contents or the data dictionary can be directed to George Frank, NGS Data Base Administrator, phone 301-713-3251, or Internet address firstname.lastname@example.org.
Transfer Standards are the importing and exporting digital data exchange standards used for data transfer. Through the use of transfer standards, data sharing will be instituted, in the worldwide market place. Trade, through growth in new informational/communications industries, causes the national economy to grow. NIST has recently published the revised Spatial Data Transfer Standard (SDTS) which will be used by the FGCS to standardize the data transfer elements of geodetic data (14).
Excluding defense systems and exceptional circumstances, the SDTS is mandatory for Federal agencies! This means that hardware/software systems procured by the Federal government for the processing of spatial data must include the capability of importing and exporting data sets which conform to the transfer standard, SDTS.
Federal agencies are also encouraged to retrofit existing digital spatial data production and processing systems so that they will also import and export data sets which conform to the standard. Existing digital spatial data sets are produced in a myriad of formats; each data product has its own format. Adoption of SDTS will enable users of conforming hardware/software systems to import and use any conforming data set without further special programming.
Geodetic data are clearly spatial and the SDTS clearly applies to this data set. Geodetic data are traditionally used by the surveying community. In the past, geodetic data has been less widely used in the cartographic and the GIS communities, often with very unfavorable results. Production of geodetic data in SDTS will enable GIS analysts to import and use geodetic data.
The geodetic data products to be produced and marketed as SDTS conforming data products
include digital data sheets, horizontal coordinate listings, and vertical coordinate listings. Other
geodetic products may be prepared as SDTS data sets, if requested by the customer. Presently,
these include geoid models and high precision satellite orbits.
Metadata standards are the catalogue and accessibility standards for data availability. Metadata help users of geospatial data find the data they seek. Metadata standards for geodetic data presently conform to the FGDC Metadata Standards (15).
The information included in the standard was selected on the basis of four characteristics that define the role of metadata: (1) availability--data needed to determine the sets of data that exist for a geographic location; (2) fitness for use--data needed to determine if a set of data meets a specific need; (3) access--data needed to acquire an identified set of data; and (4) transfer--data needed to process and use a set of data. Table III contains the major elements of the metadata standard. The NSDI Executive Order requires Federal agencies to comply with the provisions of the metadata standard by January 1995.
The final form of the Digital Geospatial Metadata Standards for Geodetic Data (16) has been submitted to FGCS for review. When approved by FGCS, these metadata for geodetic data will be submitted to the FGDC's National Geospatial Data Clearinghouse (NGDC) for nationwide access. In essence, geodetic data provide the basic framework in developing a common coordinate system reference for all other geographic features of the NSDI.
National Geospatial Data Clearinghouse
The NGDC (17) is defined in the Executive Order on NSDI as "a distributed network of geospatial data producers, managers and users linked electronically. "The National Geospatial Data Clearinghouse is a network-based information resource intended to help users find and retrieve geographically referenced data sets. As it is populated, the clearinghouse will provide access to many different types of data--address ranges, parcel and boundary information, transportation grids, geodetic control points, to name a few and eventually will include other formats of geospatial data, including vector, raster, gridded, image, geo-referenced, and analog.
The clearinghouse is a "catalogue of catalogues" linking computer nodes located throughout the country that contain information about geographic (geospatial) data. These nodes communicate with one another using the Internet, a rapidly growing, international network that connects over 2 million host computers and nearly 20 million users in over 100 countries (18).
Internet is an enormous international computer network that links government agencies, universities, corporations, and individuals. In 1988 the Internet comprised 33,000 host computers, today there are approximately 2 million host computers. The information highway is here! NGS' efforts in contributing standardized geodetic metadata and data to the NSDI are on track according the FGDC who are implementing the presidential directive.
NOAA has developed a prototype home page on the Internet World Wide Web (WWW) for online access to information on its products, services, and program activities. A starting point for locating the geodetic data and related metadata data available from NOAA's NGS is found on this home page. To date, the following NGS information has been made available on Internet using the home page: NGS' Catalog of Products and Services, NGS' Mission, Vision, and Strategic Goals, the NGS telephone directory, a diagram showing NGS' organizational structure, a geoid height model and deflection of the vertical model, geodetic software, CORS data and GPS orbital data. NGS's home page is accessible through NOAA's home page by using a WWW software browsing tool such as Mosaic. The Uniform Resource Locator (URL) address for NGS's home page is: http://www.ngs.noaa.gov (19). Why not access Internet now? Welcome aboard!
There is great diversity in approaches to developing and using geospatial data within governments
and private industries. Requirements for data may vary significantly from company to state to
nation. Means for funding these data activities are a major concern. Funding must be provided
through shared resources and partnership developments, such as shown above. Partnerships to
foster cooperative responsibilities for data production, maintenance, integration, and
dissemination must be cultivated for the overall economic good of local and global communities.
Agreed upon policies and standards for creating digital geospatial data that can be widely used,
for protecting privacy, and for providing access to data, are needed within the community to
continue to enhance the use of geospatial data for intelligent decision-making. Partnerships,
procedures, standards, policies, the required framework of geodetic control and various
dependent themes of related geographic data contribute to the NSDI.
(1) National Research Council, Toward a Coordinated Spatial Data Infrastructure for the Nation, Report of the Mapping Sciences Committee, National Academy Press, Washington, DC, 1993, 171 p.
(2) Executive Office of the President, Circular A16, Coordination of Surveying, Mapping and Related Spatial Data Activities, Office of Management and Budget, Washington, DC, 1990, 7 p.
(3) FGDC, Development of a National Digital Geospatial Data Framework, Federal Geographic Data Committee, U.S. Geological Survey, Dept of Interior, Reston, VA, 1994, 11p.
(4) Lapine, Lewis A., National Geodetic Survey: Its Mission Vision, and Strategic Goals (preprint), NOAA, Dept of Commerce, Silver Spring, MD, 1994, 22p.
(5) NOAA, National Spatial Reference System Implementation Plan, NOAA, Dept of Commerce, Silver Spring, MD 1994, 37 p.
(6) National Research Council, Forum on NOAA's National Spatial Reference System, Report of the Committee on Geodesy, National Academy Press, Washington, DC, 1994, 66p.
(7) Gore, Al, Creating a Government that Works Better and Costs Less, Report of The National Performance Review, Executive Summary, Office of Vice President, Washington, DC, 1993, 21p.
(8) Office of the President, Executive Order 12906, Coordinating Geographic Data Acquisition and Access: The National Spatial Data Infrastructure, Federal Register, Vol 59, No71, pp17671-17674, Washington, DC, 1994, 4p.
(9) FGDC, The 1994 Plan For The National Spatial Data Infrastructure: Building the Foundation of an Information Based Society, Federal Geographic Data Committee, U.S. Geological Survey, Dept of Interior, Reston, VA, 1994, 13p.
(10) Strange, William, A National Spatial Data System Framework: Continuously Operating GPS
Reference Stations, First Federal Geographic Technology Conference, Exposition, and Data Mart,
(Proceedings of FGT 94), GIS World Inc, Ft Collins, Co, 1994, 8p.
(11) Babbitt, Bruce, The GIS World Interview: Interior Secretary Bruce Babbitt, GIS World, Inc,
Vol 7 No9, Fort Collins, CO, 1994, pp31-33.
(12) Bossler, John D., Standards and Specifications for Geodetic Control Networks, Federal Geodetic Control Committee, Dept of Commerce, Rockville, MD, 1984, 34p.
(13) Yeager, J. Austin, Input Formats and Specifications of the National Geodetic Survey Data
Base, Federal Geodetic Control Subcommittee, NOAA, Dept of Commerce, Silver Spring, MD,
(14) National Institute of Standards and Technology, Spatial Data Transfer Standard (SDTS), FIPS Pub 173-1, National Technical Information Service, Dept of Commerce, Springfield, VA, 1994, 380p.
(15) FGDC, Content Standards for Digital Geospatial Metadata, Federal Geographic Data Committee, U.S. Geological Survey, Dept of Interior, Reston, VA, 1994, 78p.
(16) FGCS, Digital Geospatial Metadata Standards for Geodetic Data, Federal Geodetic Control Subcommittee, NOAA, Dept of Commerce, Silver Spring, MD, 1994, 22p.
(17) FGDC, National Geospatial Data Clearinghouse, Federal Geographic Data Committee, U.S. Geological Survey, Dept of Interior, Reston, VA, 1994, 2p.
(18) Lockwood, Millington, The National Spatial Data Infrastructure and Its Usefulness to The Dredging Community, American Society of Civil Engineers, Dredging '94 Conference, Orlando, FL, 1994, 11p.
(19) Coast and Geodetic Survey, C&GS Update, Vol 6, No. 3, Summer 1994 issue, NOAA, Dept Commerce, Silver Spring, MD, 1994, 4p.