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FROM FOUNDATION TO FRAMEWORK

The purpose of establishing a foundation is to ensure that spatialdata collection from different sources at multiple resolutions canbe integrated through time to create a framework upon which the NSDIdata needs may rely. But which spatial data themes should be includedin a framework? The assignment of priorities calls for careful judgmentof needs and demands for spatial data. Many public policy decisionsas well as daily operations and other applications of governmentrely on spatial data. All spatial data users can identify data themesthat are critical to their applications. Organizations recognizetheir own data priorities, as demonstrated by their willingness tosupport the needed data collection and conversion activities. Thesedifferent applications and missions of individual organizations haveevolved into the currently dispersed nature of data collection anddata stewardship, particularly among federal, state, and local agencies.

For example, the Ohio Geographic Referenced Information Program recentlycompleted a survey of users in urban and near-urban areas of Ohioabout their specific spatial data needs. The compilation showed thatthe spatial elements most frequently identified included geodeticcontrol, parcel boundaries, parcel attributes (e.g., ownership, assessment),municipal boundaries, rights of way, bridges, street centerlinesand attributes (including addresses), land use, and hydrography.The largest number of respondents indicated



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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE 3 FROM FOUNDATION TO FRAMEWORK The purpose of establishing a foundation is to ensure that spatialdata collection from different sources at multiple resolutions canbe integrated through time to create a framework upon which the NSDIdata needs may rely. But which spatial data themes should be includedin a framework? The assignment of priorities calls for careful judgmentof needs and demands for spatial data. Many public policy decisionsas well as daily operations and other applications of governmentrely on spatial data. All spatial data users can identify data themesthat are critical to their applications. Organizations recognizetheir own data priorities, as demonstrated by their willingness tosupport the needed data collection and conversion activities. Thesedifferent applications and missions of individual organizations haveevolved into the currently dispersed nature of data collection anddata stewardship, particularly among federal, state, and local agencies. For example, the Ohio Geographic Referenced Information Program recentlycompleted a survey of users in urban and near-urban areas of Ohioabout their specific spatial data needs. The compilation showed thatthe spatial elements most frequently identified included geodeticcontrol, parcel boundaries, parcel attributes (e.g., ownership, assessment),municipal boundaries, rights of way, bridges, street centerlinesand attributes (including addresses), land use, and hydrography.The largest number of respondents indicated

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE that they desired locational accuracy for their data elements of2.5 feet or better. Other states have also been active in tryingto define their spatial data needs. Lists of needed spatial datawill be different, reflecting the mandates of local, state, or federalorganizations, and the applications (e.g., growth management, resourceuse, environmental protection, or provision of social services) forwhich the information is collected. Framework data can be referred to as those sets of data, integrated with the foundation,that form the basis for spatial information and analyses. There mightbe different data sets that would form distinct frameworks; for instance,a framework for natural resource analysis could consist of differentdata sets than a framework that might be used for urban infrastructureissues. Just as there are multiple profiles within the Spatial DataTransfer Standard, multiple frameworks could be established. In addition to the foundation, there may very well be some datasets that are important in many different frameworks. Going backto the building construction metaphor, although foundation constructionwould be similar, the specific framework might differ markedly betweena house or a factory or an office building, even though they probablyhave similarities in engineering design. Just as there might be frameworks that differ by type ofanalysis,the framework data requirements also could differ by geographic regions. There is no expectation that the same level of accuracy or eventhe same framework layers will be compiled (or needed) in the nearfuture over the entire United States. The focus should be on theability to integrate different levels of spatial data details andaccuracy. The purpose of designating data within a framework is so that spatialdata can be compiled and maintained for the benefit of all NSDI users.The MSC believes that some rationale is necessary for selecting andestablishing framework data. The committee sug-

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE gests that the following criteria are necessary for data to be consideredas framework data within the NSDI (additional specifications areaddressed later in this chapter): broadest national constituency of users—spanning the largest geographic area and supporting the greatestnumber of applications; significant return on investment—in the form of increased productivity and efficiency; needed to manage critical resources, for developing policies, oradministering programs for preservation and use of resources; and serves as a fundamental source to create or leverage other spatialdata. IDENTIFYING FRAMEWORK DATA A commonly held view about spatial data has emerged among a largeand diverse group of spatial data users in federal, state, and localgovernments as well as in private enterprises. This view1 states that the productivity and effectiveness of their organizationswill be significantly enhanced if their access to spatial data isimproved. The MSC believes that it is appropriate for the federalgovernment to play a lead role in making these data available forpublic use. It is very difficult (and in some cases impossible) forindividual agencies, groups, or enterprises to provide all the neededdata by themselves. An impetus behind the 1990 revision of Office of Management and BudgetCircular A-16 and the formation of the FGDC was the recognition thatcooperation among different agencies would be required if the nationis to have access to the reliable spatial data that

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE are needed. The goal of such cooperation is the minimization of redundancyin data collection and single-purpose spatial data systems (oftencalled “stove-pipe” systems). The FGDC currently has several subcommittees organized around broadspatial data themes (Table 2). These subcommittees are tasked by Executive Order 12906 to formulateplans for the development of content standards (e.g., definitions,conventions) for their respective data themes. Responsibility formany of these themes is delegated among several federal agencies.The establishment of these subcommittees can be considered as recognitionof priority areas. With careful examination, one would be hard pressedto identify one of these themes that does not meet most of the criteriaspecified above. As discussed in the MSC’s companion report, Promoting theNational Spatial Data Infrastructure Through Partnerships,2 ideally there should be one data steward for any standard data themein any geographic area. In many cases, federal agencies will becomethe de facto custodians (stewards) of certain data themes and dataproducts at the national level. In other cases, state or local agencieswill need to be the stewards of certain subsets of data themes. Itis not necessary for the federal government to oversee all of thesedata. It must, however, be able to know where the data reside andbe able to quickly obtain the data and integrate them to satisfyvarious objectives. A stewardship certification program along withdirected funding and coordination with regional and local expertsshould be a vital component of the NSDI. SPECIFICATIONS FOR FRAMEWORK DATA As stated above, the purpose of designating spatial data within aframework is to enable the data to be compiled and main

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE TABLE 2 Spatial Data Themes of FGDC Subcommittees Data Theme/Subcommittee Agency Chairing Subcommittee Base cartographic USGS Bathymetric NOAA Cadastral Bureau of Land Management Cultural and demographic Bureau of the Census Geodetic National Geodetic Survey, NOAA Geologic USGS Ground transportation Federal Highway Administration Hydrologic USGS Portrayal of certain international boundaries Department of State Soils Soil Conservation Service Vegetation Forest Service Wetlands Fish and Wildlife Service maintained for the benefit of all NSDI users. If that is the case,the data must meet minimum specifications, including reliability,currentness and other metadata, integration with the foundation,and availability. All of the data themes shown in Table 2 may not meet these specifications. To meet them, some data themeswill need increased production activities, additional research, andintegration with the foundation. The MSC suggests the following minimum specifications for frameworkdata and their integration with the foundation: Framework data must be compiled, archived, and maintained in digitalform. Digital form permits the data to be adaptable to differentor evolving hardware and software

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE technologies and facilitates use and exchange of the data. It encouragesdecentralization of data collection and archiving and exchange ofdata via telecommunications or other electronic means. That dataare digital does not mean they exist in fixed or uniform resolutionnationwide. With improvements in data collection technology, resolutionwill improve. As partnerships are put in place, some local or regionaldata collection efforts will proceed at different resolutions thanin neighboring regions. The MSC accepts this as a realistic and desirableview of early generations of the NSDI. Framework data must include metadata descriptions. Metadata descriptions should be in an accepted standard exchangeformat, itemizing accuracy, currentness, consistency, and completeness.Metadata descriptions at a minimum must include the procedures thathave been applied in processing the data, the date(s) of processing,the region of coverage, and the agency or agent. Metadata reportsmust be integrated with data in a way that facilitates digital additionsas subsequent procedures are applied. The FGDC recently approvedan NSDI metadata content standard that embodies these descriptionsand incorporates a detailed explanation of what is considered asmetadata. Framework data must be mathematically and semantically integratedwith the NSDI foundation, with details of the integration procedureincluded in the metadata description. Requirements for accuracy andprecision dictate a need for a geographical reference system (Earthreferences) as a basic foundation for the NSDI. Features should beintegrated with the foundation to ensure consistency for frameworktheme integration. Some data represent features whose boundariesare nondiscrete (e.g., gradients), such as soils, vegetation, wetlands,and other natural resource features. While these data do not adhereexplicitly to the minimum specifications, they should be compiledusing a base that adheres to

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE minimum specifications. Metadata descriptions need to identify thelocational accuracies and the nature of the delineations being describedby these data. Framework data must be available in an accepted, openly publicized,standard data exchange format. The Spatial Data Transfer Standard(SDTS) is the current Federal Information Processing Standard (FIPSPublication 173). This specification does not preclude exchangingdata in other formats. Framework data must be accessible to the public. Mechanisms for public access to the data must ensure that currentnessand reliability of both data and metadata are preserved and maintained.They must also protect against unauthorized modification of the datasource. Users frequently are demanding access to spatial data throughelectronic media (e.g., tapes, CD-ROMs) or computer networks. Asdata networks (such as the Internet) proliferate and increase incapability, many people will use them to either search for data throughthe metadata catalog or obtain the data directly on-line. PRIORITIES FOR FRAMEWORK The assignment of priorities is often fraught with disagreement amongdifferent parties, which have their own sense of what is most important.The highest priority should be to ensure that the foundation (geodeticcontrol, terrain, and orthoimagery) is adequate to meet the needsof the NSDI, particularly for the integration of other spatial data. Although production of some of the data that make up the foundationare under way or are planned, most of it will result from other “product-specific” data collection efforts, often driven by specific mandates. Beyondthis, priorities should follow the criteria listed for identifyingframework themes

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE (broad national constituency, significant return on investment, etc.)as well as the immediacy of the societal demand. For example, ExecutiveOrder 12906 (April 11, 1994) gave framework designation and immediatepriority to transportation, boundaries, and hydrology data. Thesethree themes are common to many, if not most, framework considerations.Their availability, probably at a variety of different resolutionsor scales, will be applicable to the widest-possible sectors of thenation and will provide a basis for strengthening the concept ofthe NSDI. Discussion of these three data themes follows. Transportation The transportation data network transcends the utility of a commonmap because it is a base for defining, organizing, and accessingplaces (and their associated information) within both complex urbanenvironments and rural areas. In a digital format the importanceof the transportation network is magnified and its uses are expandedmanyfold. What might have served as a guide or descriptor of pathwaysthrough a particular geographic area can now be used in routing ofcommerce and navigation (vehicles, rail, air, and nautical). Thenetwork can also serve as a basis for the indexing, analysis, anddisplay of large volumes of tabular and statistical data (e.g., usingCensus polygons that are defined by transportation features). One of the more prevalent forms of transportation data are streetcenterline spatial data (SCSD),3 and most of the discussion here will focus on these data. SCSD arebasically computerized street maps, where streets are representedas centerlines to which attributes of the streets are appended. Almostthree decades of practice have proven the value of differentiatingbetween the left and right sides of each street segment and encodingattributes to them

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE such as street names, address ranges, ZIP codes, census and politicalboundaries, school districts, traffic zones, and congressional districts.Practice also demonstrates the value of including and reconcilingnonstreet features in these data bases to form topologically consistentblocks (e.g., water bodies, railroads, political boundaries). TheBureau of the Census has incorporated some of these street featuresinto its TIGER data bases. Street centerline spatial data should include coordinate referencesat common, well-defined, line segment endpoints as well as coordinatedescriptions of the path of the street (if not straight) betweenintersections. The SCSD structure accommodates use of coordinatesat varying levels of accuracy, allowing economical and pragmaticdata-set development with (expensive) high spatial accuracy whereneeded in urban areas but generally lower accuracy (where acceptable)in rural areas. Although SCSD are often thought of as a data basefor urban applications, they support a myriad of natural resourceapplications in rural areas. SCSD provide a good example of a framework spatial data theme byvirtue of their extensive current use in facility site selection,census operations, socioeconomic planning studies, legislative redistricting,and logistical operations management. Present and near-term developmentsin personal computers and consumer electronics may expand SCSD useto virtually all citizens in trip planning, route guidance, and electronicatlas applications. The digital data requirements for the IntelligentVehicle Highway System will likely dictate higher accuracy and morecomprehensive attribution than most SCSD developed for GIS applications. SCSD are important because they express fundamental relationshipsbetween street addresses (the most common spatial reference for thebuilt environment) and coordinates and other locational links (“geocodes”) mentioned above. SCSD are widely used to link

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE street-addressed data to geographic references for GIS and otherdesktop mapping applications. The TIGER files, particularly the attribute data, serve as a majorstepping stone toward a mature SCSD for the nation. However, TIGERfiles lack accurate coordinates registered to the foundation, completestreet addressing, and an ongoing maintenance program. The TIGER files could be integrated with the foundation by the following actions: improving coordinate accuracy using ortho-rectified imagery that is tied to the geodeticcontrol network; completing and improving street and addresscoverage in partnerships with the U.S. PostalService, 911 emergency agencies, state and localgovernments, and the private sector; and establishing an ongoing update facility employinglocal government partnerships for timely information (transactional updates) about new streets. These deficiencies of the TIGER files are recognized by the Bureauof the Census. The importance and timeliness of the societal demandfor accurate transportation data are reflected in Executive Order12906, which calls for provision of such data in order to supportthe decennial census for the year 2000. Funds should be identifiedto allow these improvements by January 1998, which is the targetdate specified in the executive order. Political, Administrative, and Census Boundaries In many cases, data on political, administrative, and census boundariescan be partially built from the basic elements comprising

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE the SCSD. As such, much of their geographic accuracy and currentnesswill depend on the quality of the SCSD data. For this reason, integrationof the transportation data theme should precede or accompany integrationof the boundary theme. This sequence will reduce error and increaseefficiency, thus reducing costs. Boundaries are of great interest to the federal government becauseany program that has a local revenue-sharing component depends onan authoritative definition of city/town/place-level geography. State,local, and private interests are also increasingly involved as statespass legislation addressing all sorts of issues from funding of municipalpensions for firemen and policemen to taxation on homeowners to carinsurance rates. While the Bureau of the Census conducts an annualBoundary and Annexation Survey (BAS), it performs the minimum surveyto support that year’s programs, and the results of the BAS are notpublished except in conjunction with the decennial population andhousing census. The line segments in an SCSD data base can be combined to createa number of important jurisdictional or areal data bases. These datacan be distinguished as being governmental units, census statisticalareas, administrative areas, or some other classification. Governmentalunits include states, counties, minor civil divisions, incorporatedplaces, and consolidated cities. Census statistical areas includeblocks, block groups, tracts, and block numbering areas. Some examplesof administrative areas are school districts, police precincts, votingwards, and ZIP code delineations. The Bureau of the Census determines changes to census statisticalareas and limits these changes to once every 10 years. However, politicaland administrative boundaries might change at any time. For example,a rapidly growing city might annex several towns at its periphery.Since these changes are reported every 10 years for the census, thetemporal accuracy of some boundaries cannot be guaranteed. Partnershiparrangements, primarily with

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE local and county governments, are essential to maintaining currentnessof boundary data. Hydrology Hydrologic data include the location, geometry, and flow characteristicsof the nation’s river and stream network, lakes, and other surfacewaters. Hydrologic data provide a wealth of information that supporta variety of uses—for example, prevention of flood damage, allocationof surface water (including dated agricultural, riparian, and otherwater rights), sources of nonpoint pollution, minimum stream flows,wetlands preservation, urban water requirements, and recreationalinterests. Use of hydrologic data with other spatial data sets permitsrapid decisions on the advisability of reconstructing or removingvarious artificial barriers to natural flow by forecasting the effectsof each perturbation of flow within a flood setting. With accurateand timely hydrologic data, the Federal Emergency Management Agency(FEMA) could more accurately forecast needs for evacuation, emergencyassistance, and long-term mitigation of damages. Many hydrologic features can be derived directly from the orthorectifiedimagery and digital terrain (parts of the foundation), to providethe control for identifying stream courses and their flood-plainsand other surface waters. Surficial geologic and soil data providethe basis for estimation of both the ground water contribution tostreams and the runoff and absorption of precipitation. Important existing hydrological data that need to be integrated withthe foundation include the Environmental Protection Agency’s RiverReach Files, USGS’s Hydraulic Units Files, and FEMA’s Flood Fringe and Ways. Several other hydrologicdata types also merit consideration. These include (1) stream gaugingand sediment load data, (2) water quality data, and (3) data on aquifers.

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE All of these data are important in national and state water supplyplanning and quality management. As far as the framework specifications given above are concerned,current hydrological data products need to be tied directly to thefoundation. However, more effort has gone into preserving the topologyof the data rather than their accurate positional attributes. Inaddition, efforts are needed to make these data available in an openexchange format (much of the data are currently only available ina vendor-specific format). OTHER FRAMEWORK THEMES The purpose of the specifications for integrating framework datathemes to the foundation is to strengthen the NSDI by promoting thebenefits resulting from data sharing and the formation of partnerships.There are a large number of spatial data themes that are importantin a wide variety of applications. All spatial data users can identifyadditional data themes that are critical to their own applications.From a national perspective these closely correspond to the datathemes of the FGDC subcommittees. Many of the data sets within thesethemes meet most, if not all, of the suggested criteria for frameworkdata; however, most do not yet meet the recommended framework specifications. Many of these themes have not yet been compiled in digital form,nor integrated with foundation data. Nonetheless, there is a societalneed for these data themes. Examples of such spatial data themesinclude wetlands, soils, geology, demography, ecosystems, land use,and cultural relations. The wetlands theme has been discussed indetail in a previous MSC report4 and has obvious importance in land-use planning and environmentalconsiderations. Two broad data themes—cadastres and natural resources—are briefly

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE discussed below as examples of important data that need to be integratedwith the foundation and meet other framework specifications to maximizetheir utility within the NSDI. Cadastral Data Cadastral systems are those activities and data associated with landownership. A single cadastral system incorporates land title andevidence, rights and interests in land, and the spatial extent oftitle, rights, and interests. Most of the responsibility for the collection, management, and maintenanceof cadastral information lies with the states. Most states pass thisresponsibility to local governments, such as counties. The federalgovernment has the authority and responsibility for cadastral informationon federally held lands. In 30 states the primary system for defining the spatial extent ofcadastral information is the Public Land Survey System (PLSS). Thissystem was originally created by the federal government to facilitatethe orderly inventory, settlement, and privatization of western lands.As the territories were granted statehood and the PLSS system wascompleted, responsibility for maintenance was turned over to thestates. A cadastral digital data framework is needed to describeand define interests and rights in real property. Acceleration ofa computer-readable file of PLSS data would be a critical step towardachieving this objective. This was recommended by the National ResearchCouncil in 1982,5 and the recommendation has been repeated many times by diverse organizations.6 The federal government is responsible for a diverse group of mandatesand functions that require cadastral information. These include supportingNative American land tenure, managing land resources on federal holdings(both surface and subsurface), acquiring property for specific projects,regulating real estate financing,

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE agricultural assessment and support programs, environmental assessment,and other public safety and welfare programs. The federal governmentshould develop a flexible SDTS profile for cadastral data and provideincentives to the states to establish digital cadastral data in formsthat are easily shared and integrated. Cadastral data should be compiled and maintained in digital form,should include metadata descriptions, and should be referenced tothe foundation. A single formatting system for cadastral informationshould be chosen for the nation as a whole, in the form of an SDTSprofile. Cadastral boundary data should be made accessible. However, because cadastral attribute information is often subject to personal privacy concerns, it shouldcontinue to be controlled locally. Natural Resources A variety of data themes describe natural resource features, whichare vital to land management, environmental management and protection,and economic development of public and private lands. All of thesedata themes (e.g., geology, water, ecosystem distribution, soils,wetlands) are valuable and are used in a variety of applications. Wetlands are a natural resource of critical societal importance fora variety of environmental, biological, and aesthetic reasons, includingbiological diversity, water quality, wildlife, and fishery production.Completion of the National Wetland Inventory by 1998 (conterminousUnited States, Alaska by 2000) and its automation by 2004 has beenmandated by Congress. There is a diverse user community for wetlandsdata—federal and state land management agencies, regulatory agencies(all governmental levels), and private sector development groups.Data describing wetland conditions are actually made up of otherdata, including the presence

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE of hydrophytic vegetation, hydric soils, and wetlands hydrology.7 The various soil data sets (provided through the Soil ConservationService (SCS) not only are critical in delineating wetlands but alsohave a broader utility in agricultural resource management, conservation,water quality, and erosion control. One ongoing program of the SCSis the National Resource Inventory (NRI), which provides a comprehensivedata survey of nonpublic lands conducted on a five-year schedule.There are about 25 data elements (e.g., physical and chemical characteristicsof the soil, soil moisture, hydrology, biology) included in the NRI. Geologic data have well-established value8 as a basic framework for the management of numerous natural resources,including groundwater, mineral resources, energy resources, and,to a certain extent, soils and related biological resources. Geologicmaps are the fundamental source for creating many other kinds ofmap data, such as landslide hazard maps, earthquake hazards maps,aquifer maps, groundwater vulnerability maps, mineral resource maps,and, in combination with other data sources, soil maps. The utility of all these natural resource data themes would be greatlyenhanced if they were more closely tied to foundational control;this would allow the integration that is needed by resource and landmanagers. The current availability of these natural resource datathemes in digital form is somewhat limited, with wetlands and soilsdigital data more available than geologic data. Wetlands and geologicdata lack agreed-upon content (definitional) standards; however,the FGDC standards efforts could make significant progress on contentstandards in the next few years. NOTES 1. See, for example, K. Brown (1990), Local Government Benefits from GIS, PlanGraphics, Inc., Frankfort, Kentucky; and S. R. Gillespie (1992),The

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A DATA FOUNDATION FOR THE NATIONAL SPATIAL DATA INFRASTRUCTURE value of GIS to the federal government, in Proceedings: GIS/LIS’92 AnnualConference and Exposition, Anaheim, California, pp. 256-264. 2. Promoting the National Spatial Data Infrastructure through PartnershipsDC, 114 pp. 3. A discussion of SCSD and their use within the urban infrastructureis also given in a previous MSC report, Toward a Coordinated Spatial Data Infrastructure for the Nation (1993), National Academy Press, Washington, DC, 171 pp. 4. Ibid. Wetlands data and their associated issues were discussedexplicitly in Chapter 5 and an Appendix. 5. Need for a Multipurpose Cadastre (1980). National Research Council, National Academy Press, Washington,DC, 112 pp. Modernization of the PublicLand Survey System (1982). National Research Council, National Academy Press, Washington,DC, 74 pp. At the time of these reports, GPS positioning was comparativelyexpensive; however, GPS probably is now the least expensive and mostaccurate way of determining position. 6. As an example, see A Study of Land Information, Department of the Interior, Washington, D.C., 61 pp. plus appendixes.This study was mandated by the Federal Land Exchange and FacilitationAct of 1988 (P.L. 100-409). 7. For further information on the status of wetland mapping and delineation,see Toward a Coordinated Spatial Data Infrastructure for the Nation, National Academy Press, Washington, D.C. 8. National Geologic Mapping Act of 1992 (P.L. 102-285); also seeU.S. Geological Survey Circular 1111, Societal Value of Geologic Maps (1993) and National Research Council report, Solid-Earth Sciences and Society (1993), National Academy Press, Washington, DC.