DEVELOPMENT OF THE NATIONAL SPATIAL REFERENCE SYSTEM
David R. Doyle
Senior Geodesist
National Geodetic Survey
Coast and Geodetic Survey
ABSTRACT
Significant improvements in Geographic/Land Information Systems (G/LIS), and surveying andmapping technologies during the last decade have dictated a reassessment of the role of theNational Geodetic Reference System (NGRS).  The National Geodetic Survey (NGS), acomponent of the Coast & Geodetic Survey (C&GS), is responding to demands from a widerange of users for improved three dimensional positional accuracy and accessibility to thereference system by developing the National Spatial Reference System (NSRS).  The NSRS willimprove the availability and accuracy of positional information necessary for the development ofaccurate parcel based G/LIS, active Global Positioning System (GPS) navigation and surveyingactivities, and improve our understanding of the geophysical dynamics of the planet we live on.
National Geodetic Reference System
The National Geodetic Reference System (NGRS) exists as a collection of discreet geodeticelements.  Horizontal positions (latitude and longitude, State Plane Coordinates) referenced to atwo-dimensional datum, the North American Datum of 1983 (NAD 83), and elevations (Helmertorthometric heights) referenced to a one-dimensional datum, the North American Vertical Datumof 1988 (NAVD 88) define the major components of the NGRS.  Both datums are the latestdefinitions of systems that began with the first geodetic surveys conducted by the Survey of theCoast, founded in 1807 (later renamed the Coast Survey-1836, U. S. Coast and GeodeticSurvey-1878, and National Ocean Service-1970).  NGS, a component of C&GS/NOS, isresponsible for most of NOS' geodetic mission.  These geodetic systems were developedindependently with little regard to being correlated.  That is, a survey monument that providedaccurate horizontal coordinates was not normally connected to the vertical network, nor werevertical points related to the horizontal reference network.  Today (1994), the NGRS containsinformation on approximately 300,000 horizontal control points and 600,000 vertical controlpoints.  
In addition, horizontal stations were often located on high mountains or hilltops to decrease theneed to construct observation towers usually required to provide line-of-sight for triangulation,traverse and trilateration measurements.  Vertical control points however, were established by thetechnique of spirit leveling which is more suited to being conducted along gradual slopes such asroads and railways that seldom scale mountain tops.  Consequently, approximately less than 10percent of the horizontal and vertical reference systems coincide.
Distribution of NGRS data has also traditionally been confined to geodesists, surveyors, andcartographers in the Federal, state, local, and private mapping communities.  While requests forgeodetic information have always been numerous, much of the mapping and surveying conductedin the United States has not been referenced to NGRS.  Federally funded topographic mapping 
(Geological Survey and Soil Conservation Service) and aeronautical and nautical charting (NOSand Defense Mapping Agency) have always been referenced to NGRS.  However, Federalcadastral surveying and mapping (Bureau of Land Management, Forest Service, and the ParkService) have only recently adopted this convention.  Inaccessibility of NGRS control points andthe expenses associated with such surveys have often been cited as prime reasons for notconnecting to the networks.
Social, ecological, and environmental changes of the last decade have increased the need for moreaccurate geospatial information.  Dramatic technological advances in computers, scanningdevices, satellite positioning/navigation systems, and remote sensing techniques have providedaccess by mapping professionals and the general public to a wide range of geo-referenced data that never existed before.  Prior to the advent of GPS, obtaining accurate three-dimensionalcoordinate information (latitude, longitude, and height) was very expensive, and could only beprovided by a small group of highly trained geodesists and geodetic surveyors.  Today, a littlespare change (approximately $500) can equip anyone with the ability to determine their location,anywhere on the planet to levels of accuracy that are better than most printed map or chartproducts.
National Spatial Reference System
To address the changing requirements for positional information, NGS began a strategic planningevaluation in 1992.  Formally adopted in March 1994 (NGS 1994), this plan details, in a series of15 strategic goals, the steps necessary to accomplish the transition from NGRS to NSRS.  
A case can be made that the change to NSRS is simply a name change, and that the NGRSalready incorporates the features necessary to support the integrity of the National Spatial DataInfrastructure.  In part this is true.  However, the term Geodetic has often been intimidating tomany data users, implying a somewhat mystic science that almost no one understood.  Changingthe name to Spatial is intended to be more consistent with the needs of the public and should notrequire data users to run for their dictionaries to find out what the term means.
The change is not just cosmetic. It represents a fundamental change by NGS to understand theneeds of geospatial data users, and to restructure its surveying operations, research anddevelopment activities, and data distribution, some of which have changed only slightly in over180 years.  These changes include development of the Federal Base Network (FBN), CooperativeBase Network (CBN), User Densification Network, Continuously Operating Reference Stations(CORS), GPS Orbits, a high accuracy geoid, and improved data access.
Federal Base Network
Development of GPS as a primary tool for geodetic surveying indicated in the late 1980s that theexisting horizontal and vertical control networks (NGRS) did not adequately support satellitetechniques.  These networks were developed using classical methods of triangulation, traverse,trilateration, and spirit leveling, where the line-of-sight direction between control points was moreimportant than the clearance above the horizon required for satellite signal acquisition. Consequently many geodetic survey points were not directly "GPSable".   
Horizontal coordinates were also a problem.  Prior to GPS, surveys designed to obtain first-order(FGCC 1984) positions were performed by very few agencies or private firms, using veryexpensive surveying equipment, and requiring significant training and education in geodesy andleast squares adjustment theory.  By 1985, GPS could easily outperform first-order horizontalaccuracy, and was becoming increasingly more affordable and accessable to the average surveyor. To accommodate the improved positional capability of GPS, new standards for horizontal datawere submitted for adoption to the Federal Geodetic Control Committee (FGCC 1989b).   A- andB-orders of accuracy specify horizontal positional tolerances of 5 mm + 1:10,000,000 of theobserved base line, and 8 mm + 1:1,000,000 of the observed base line, respectively.  There is novertical accuracy implied by A- or B-order.
Improvements to the horizontal reference system began with the first High Precision GPSNetwork (HPGN) observed under contract for the Tennessee Department of Transportation andadjusted by NGS in 1990 (Strange and Love 1991).  Subsequent surveys in Florida (1990),Delaware, Maryland, Oregon, Washington, and Wisconsin (1991) initiated the current programthat will define an improved reference frame for every state by the end of 1996.  The acronymHPGN first evolved into High Accuracy Reference Networks (HARN) (Doyle 1992) and nowFBN/CBN.
While changing names and acronyms may seem like a way to confuse the user, the intent is toprovide a definition of both positional accuracy and financial responsibility.  Stations defined asFBN will be established at the expense of NGS, at1o x 1o (75 to 125 km) nominal spacing that willprovide minimum accuracies of B-order horizontal; second-order, class I vertical (FGCC 1984);gravity at 50 microgals; and horizontal and vertical crustal motion at 1 mm/year relative accuracy. NGS will be responsible for replacing destroyed or disturbed stations and will revisit each FBNstation on a rotating 3 - 5 year basis.  A portion of the FBN nominally spaced at  3o x 3o (250 kmto 300 km) will provide A-order horizontal and first-order, class II vertical accuracies. 
COOPERATIVE BASE NETWORK
NGS encourages the densification of FBN to a nominal station spacing of approximately 15' x 15'(25 to 30 km) in cooperation with various Federal, state, local, and private sector groups.  TheCBN will provide minimum accuracies of B-order, horizontal, third-order vertical, gravity at 50microgals, and horizontal and vertical crustal motion at 1 mm/year relative accuracy.  CBNdevelopment is usually accomplished at the State level, often coordinated by NGS and usually incooperation with major state agencies such as the Departments of Transportation.  Every effort ismade to combine the interests of a wide range of users such as the Professional Land Surveyorssocieties and county or municipal agencies involved in G/LIS.  A very important differencebetween FBN and CBN is that funding and network maintenance of CBN stations must comefrom the state and local groups.
USER DENSIFICATION NETWORK
As in the past, NGS will continue to accept horizontal and vertical geodetic surveys for inclusionin the national reference system provided they are connected to the FBN/CBN.  Often referred toas "Blue Book" surveys, these projects are usually densification surveys to support G/LISactivities performed by or under contract to Federal, state, county, or municipal agencies.  Thisservice provides quality assurance, data archival, and distribution to the user community.  Allhorizontal surveys must be at least first-order (FGCC 1989b), while vertical control projects mustbe a minimum of second-order, class II (FGCC 1984) conventional leveling surveys.  All datamust be submitted to NGS in computer-readable form (FGCC 1980, FGCC 1989a) using NGSapproved software, and be adjusted and ready to be loaded in the NGS data base.  While NGSwill provide a pre-review service prior to field observations to ensure network integrity, NGS willnot provide financial or network maintenance assistance for these data.
CONTINUOUSLY OPERATING REFERENCE STATIONS
Establishment of a national network of GPS Continuously Operating Reference Stations (CORS)will make both pseudo-range and carrier-phase measurements accessible to the public for use in awide range of geodetic, cadastral, and engineering surveying applications, mapping and charting,as well as air, land, and sea navigation.  Each CORS will consist of at least one permanent GPSreceiver, a computer, and a telecommunications support system, correlated with a FBN/CBNstation located in a secured environment such as an airport, Coast Guard station, or otherprotected location.  Data from those sites supported by the Coast Guard and Federal AviationAdministration will provide position and elevation correctors for real-time pseudo-rangeobservations to improve accuracies for a variety of applications to the sub-meter level up to 200kilometers away from the station.  
Carrier-phase observables will also be provided by NGS via INTERNET for post processingobserved data to satisfy those applications requiring higher accuracies such as geodetic and landsurveying.  Availability of CORS will reduce the requirements for additional GPS receivers fordifferential positioning problems.  Connections derived from two or three CORS will ensurepositional integrity without the expense of sending additional receivers and personnel into thefield.
GPS Orbits
Access to worldwide ephemeris derived from the observations provided through the InternationalGPS Geodynamics Service (IGS) will enhance post-processed GPS data to achieve accuracies of1:107 of the orbit radius with efforts aimed at achieving 1:108.  These data are made available tousers within 72 hours.  Such orbits will be essential for effective use of CORS.
Data Access
Distribution of geodetic data transitioned from paper copy, hand edited, manually typed datasheets of the 1960s and 70s to the NGS Integrated Data Base (NGSIDB), arguably the mostsophisticated geodetic surveying data base in the world.  All NGS historical horizontal andvertical data were automated from original field observations during the 1970s and 80s to supportthe NAD 83 and NAVD 88 readjustments.  These data include positions, elevations, angles,distances, GPS vectors, elevation differences, and station descriptive text dating back to some ofthe original surveys of the Survey of the Coast (now NOS) in 1817.
Hundreds of daily telephone and mail requests for data products (including original fieldobservations as well as coordinates) are currently supported on paper, 3.5 and 5.25 inch floppydisks,  and Compact Disc - Read Only Memory (CD-ROM).  The transition to NSRS will includedirect read-only access by the public to the NGSIDB via modem connection through a bulletinboard and/or other telecommunications connections.
CONCLUSION
Redefining the NGRS is the product of an intensive 2-year study by NGS management and seniorstaff, supported and reviewed by all NGS employees using Total Quality Management techniques. This plan was reviewed by the National Academy of Science and a special committee of nationallyrecognized professionals from the surveying and mapping community (National Spatial ReferenceCommittee 1994).  The NSRS will incorporate state-of-the-art positioning and computertechnology directed at supporting a wide range of activities and users.  Crustal dynamics studies,intelligent vehicle/highway systems, air, rail and marine transportation needs, academia,governmental agencies, private industry, and families trying to find the nearest burger or tacocarry-out will all directly or indirectly benefit from current and accurate geospatial data that areeasily accessible to the public.
REFERENCES
Doyle, D., 1992, High Accuracy Reference Networks; Development, Adjustment and CoordinateTransformation, Presented at the American Congress on Surveying and Mapping AnnualConference, New Orleans, Louisiana, February 15 - 18.
Federal Geodetic Control Committee, 1980, Input Formats and
Specifications of the National Geodetic Survey Data Base,
Volume II, Vertical Control Data, October 1980.
Federal Geodetic Control Committee, 1984, Standards and Specifications for Geodetic ControlNetworks, September 1984.
Federal Geodetic Control Committee, 1989a, Input Formats and Specifications of the NationalGeodetic Survey Data Base, 
Volume I, Horizontal Control Data, January 1989.
Federal Geodetic Control Committee, 1989b, Geometric Geodetic Accuracy Standards andSpecifications for Using GPS Relative Positioning Techniques, version 5.0, reprinted withcorrections August 1, 1989.
Federal Geographic Data Committee, 1994, The 1994 Plan for the National Spatial DataInfrastructure, March 1994.
National Geodetic Survey, 1994, National Geodetic Survey Its Mission, Vision, and StrategicGoals.
National Spatial Reference System Committee, 1994, Final Report.
Strange, W. and Love, J., 1991, High Accuracy Reference Networks; A National Perspective,Presented at the American Society of Civil Engineers Specialty Conference - TransportationApplications of GPS Positioning Strategy, Sacramento, California, September 18 - 21.