Appendix A: Terminology — Federal Geographic Data Committee

The following terminology is defined as it relates to the NSDI Framework. Longer, more complete, and more precise definitions can be found in the appropriate technical and professional literature. ¹

accuracy: the closeness of observations to true values or values accepted to be true. Accuracy relates to the quality of a result and is distinguished from precision, which relates to the quality of the operation by which the result is obtained. In common GIS practice, accuracy frequently refers to positional accuracy (&quotplus or minus X meters").
address matching: a procedure that allows two geographic data files to be related through a common address field. Address matching often refers to the assignment of an x-y coordinate based on a given street address. Attribute files with addresses can be matched to geographic base files with address ranges.
AM/FM: automated mapping and facility management, a class of geographic data applications often applied in the public works and utility industry and related applications.
arc-second: a unit of angle defined to be 1/60 of a minute, or 1/3600 of a degree, of latitude or longitude.
area: a generic term for a bounded, continuous, two-dimensional object that may or may not include its boundary.
attribute: a defined characteristic of an entity type.
attribute value: a specific quality or quantity assigned to an attribute, for a specific entity instance.
bathymetry: the art or science of determining depths of oceanic or other deep waters.
cadastral information: the geographic extent of the past, current, and future rights and interests in real property, including the spatial data necessary to describe that geographic extent. A cadastre is an official register of the location, quantity, value, and ownership of land.
cadastral reference system: in general, surveys carried out for establishing the boundaries and subdivisions of public lands or for establishing land tenure systems.
centerline: a line, imaginary or real, delineating the center of a linear feature. The centerline may be measured or derived from the real boundaries of the feature.
certification: the procedure and action by a duly authorized body of determining, verifying, and attesting in writing to the qualifications of personnel, processes, procedures, or items in accordance with applicable requirements. For the framework, it is to be certified that data comply with framework standards and are suitable for inclusion in the framework. (See data certification.)
chain: a direct nonbranching sequence of nonintersecting line segments and/or arcs bounded by nodes, not necessarily distinct, at each end.
coordinate referencing system: a set of points, lines, and/or surfaces and a set of rules whereby each point in a given space can be identified uniquely by a set of numbers.
coordinates: a set of numeric quantities that describe the location of a point in a geographic reference system. A coordinate pair describes the location of a point or node in two dimensions (usually x-y), and a coordinate triplet describes a point in three dimensions (x-y-z). A series of points (two or more) is used to describe the locations of chains and the edges of areas.
data certification: a framework term referring to the process by which potential framework data are evaluated against framework standards and deemed suitable for inclusion in the framework. (See certification.)
data model: in a database, the user's logical view of the data structure, entities, and relationships (in contrast to the physically stored data or file storage structure). A description of the organization of data in a manner that reflects the information structure of an enterprise or a theme.
database: a collection of information related by a common fact or purpose.
datum: any quantity, or set of such quantities, which may serve as a reference or basis for calculation of other quantities. In framework and GIS discussions, "datum" usually refers to a set of quantities that serve as a reference for the calculation of positions. A horizontal datum is the set of constants specifying the coordinate system to which horizontal coordinates are referred. A vertical datum is a set of constants specifying the coordinate system to which elevations are referred. In a linear referencing system, the datum serves as the basis for locating the linear referencing system in the real world and consists of a connected set of anchor sections that have anchor points at their junctions and termini.
digital orthoimage: a digital representation (raster format) of an orthoimage. The digital image is composed of pixels whose dimensions define the minimum unit of resolution (expressed in distance on the ground).
elevation: (1) the vertical distance from a datum to a point or object on the earth's surface; (2) the measurement of the height of terrain on the earth's surface, or the depth of deep waters.
ellipsoid: in geometric terms, a closed surface of which all planar sections are ellipses. In general framework, GIS, and mapping practices, an ellipsoid is a specific mathematical representation of the earth that more closely approximates the shape of the surface than a sphere does.
ellipsoid height: the distance between a reference ellipsoid and a point, measured along the ellipsoidal surface normal through the point. The Global Positioning System provides ellipsoidal heights. (See also orthometric height.)
entity: a real world phenomenon of a given type. An entity instance is an occurrence of an entity. An entity type is the definition and description of a set into which similar entity instances are classified.
feature: a defined entity and its object representation. A real world feature is used in framework discussions to emphasize the goal that framework data should be based on the original encoding of an observation, or be removed from an observation by the fewest possible generations or interpretations.
Federal Geographic Data Committee (FGDC): see appendix E for a full description.
FGDC: see Federal Geographic Data Committee.
framework: the NSDI framework is an initiative to develop a readily available set of basic geographic data. It includes the information, operational environment, and technology to provide access to these data, and the institutional setting to sustain its development.
framework data: seven themes of geospatial data (geodetic control, orthoimagery, elevation, transportation, hydrography, governmental units, and cadastral information) used by most GIS applications. These data include an encoding of the geographic extent of the features and a minimal number of attributes needed to identify and describe the features.
functional class: the characterization of a certain portion of a highway system based on its purpose. The framework employs the classification published by the American Association of State Highway and Transportation Officials (AASHTO), which is used widely in the transportation community.
generalization: reduction in detail in geographic data representation; for example, resampling elevation or image data to a larger spacing or reducing the number of points in a line.
geocode: a location identifier. Geocode also denotes the process of assigning a geocode. In common GIS practice, geocoding frequently refers to assigning an x-y location to an address (also called address matching).
geodetic control: a network of geodetic control points, or a set of known reference positions, used as a basis for obtaining positions of other features. A geodetic control point has precisely determined coordinates from which the location of other features or points can be spatially referenced. Geodetic control points are frequently called benchmarks - especially when they have a known elevation.
geographic information system (GIS): a computer system for the input, editing, storage, maintenance, management, retrieval, analysis, synthesis, and output of geographic, or location-based, information. In the most restrictive usage, GIS refers only to hardware and software. In common usage, it includes hardware, software, and data. When organizations refer to their GIS, this latter usage is usually what they mean. For some, GIS also implies the people and procedures involved in GIS operation. In this guide, the common usage - hardware, software, and data - is intended unless specifically stated as GIS hardware and software.
geoid: the equipotential surface of the earth's gravity field that best fits, in a least-squares sense, mean sea level; less formally, the figure of the earth considered as a sea-level surface extended continuously through the land masses.
geospatial data: information that identifies the geographic location and characteristics of natural or constructed features and boundaries on the earth. The information may be derived from - among other things - remote sensing, mapping, and surveying technologies.
GIS: see geographic information system.
Global Positioning System (GPS): a satellite-based navigation system deployed by the Department of Defense used to determine locations on the earth's surface.
governmental units: the geographic extent of units of government, including the nation, states, counties, incorporated places and consolidated cities, functioning and legal minor civil divisions such as towns and townships, federal- or state-recognized American Indian reservations and trustlands, and Alaskan Native regional corporations.
GPS: see Global Positioning System.
grid: (1) two sets of parallel lines intersecting at right angles in a plane coordinate system; (2) a set of cells or points arranged in a grid.
grid coordinate system: a plane-rectangular coordinate system usually based on a rigorous mathematical map projection so that easily expressed plane coordinates (x,y) can be used in place of latitude and longitude.
hydrography: surface water features, such as streams and rivers, lakes and ponds, canals and ditches, and oceans.
integration: a framework term referring to the processes required to compile a consistent set of data from different sources.
linear referencing method: a mechanism for finding and stating the location of any point along a network by referencing it to a known point. There are many kinds of linear referencing methods (e.g., mile point, reference post, and engineering stationing). All linear referencing methods consist of traversals and associated traversal reference points that together provide a set of known points, a metric, and a direction for referencing the locations of unknown points.
linear referencing system: a set of datums, networks, and linear referencing methods, whereby each point along a network can be identified uniquely by specifying the direction and distance from any known point on the network.
map projection: a systematic method of representing the whole or part of the curved surface of the earth on another, usually flat, surface. The latitude/longitude values of framework data can be converted to any map projection.
metadata: data about the content, quality, condition, and other characteristics of data.
National Geospatial Data Clearinghouse: a distributed network of geospatial data producers, managers, and users linked electronically. Building on initiatives such as the National Information Infrastructure, the clearinghouse uses a distributed, electronically connected network, such as the Internet. Each data provider describes available data in an electronic form and provides these descriptions (or metadata) using means that can be accessed over a communications network. Thus, the data for the clearinghouse are located at the sites of data producers (or, where more efficient, at the sites of intermediaries) throughout the country. Using the network, users will search these descriptions to locate data that are suitable for their applications.
National Spatial Data Infrastructure (NSDI): the technology, policies, standards, and human resources necessary to acquire, process, store, distribute, and improve utilization of geospatial data. The NSDI is an umbrella under which organizations and technology interact to foster activities for using, managing, and producing geographic data.
node: a zero-dimensional object that is the topological junction of two or more chains or an end point of a chain.
NSDI: see National Spatial Data Infrastructure.
orthoimage: an aerial photograph or satellite image from which displacements caused by terrain relief and sensor tilt have been removed. The result combines the image characteristics of a photograph with the geometric qualities of a map.
orthometric height: the distance between the geoid and a point, measured along the vertical through the point and taken positive upward from the geoid. Also called orthometric elevation. Orthometric heights are used in topographic mapping. (See also ellipsoid heights.)
parcel: a single cadastral unit, which is the spatial extent of the past, present, and future rights and interests in real property.
permanent feature identification code: a unique identification code that is assigned to a feature and does not change unless the existence of the feature does.
photogrammetry: the science and art of deducing the physical dimensions of objects from measurements on photographs. For framework purposes, the principal application is aerial photogrammetry, in which the photographs are taken of the earth from aircraft or satellites.
pixel: a two-dimensional picture element that is the smallest nondivisible element of a digital image.
PLSS: see Public Land Survey System.
point: a zero-dimensional object that specifies geometric location. One coordinate pair or triplet specifies the location.
precision: a measure of the quality of the method by which measurements are made. Precision is exemplified by the number of significant figures stated as a result of the measurement.
projection: see map projection.
Public Land Survey System (PLSS): the survey carried out by the Bureau of Land Management and its predecessors for establishing boundaries and subdivisions of public lands of the United States, using the rules embodied in the U.S. Public Land System. The system is frequently used for designating the locations of a parcel of land.
raster data model: a spatial data model in which the locations of entities are represented using an array of cells or points that hold values for attributes.
referencing system: a set of datums and rules by which the location of each point can be identified uniquely.
registration: the process of matching details in separate data sets so that corresponding data are coincident.
resolution: a measure of the finest detail distinguished in an object or phenomenon. For images, resolution also refers to the pixel size.
SDTS: see Spatial Data Transfer Standard.
shoreline: the boundary line between a body of water and the land.
Spatial Data Transfer Standard (SDTS): a standard format for transferring spatial data.
State Plane Coordinate System: the plane-rectangular coordinate systems established by the National Geodetic Survey for defining positions in terms of plane-rectangular (x-y) coordinates. There is one system for each state in the United States; however, some states have more than one projection zone.
theme: a topic or subject. The framework's data themes are geodetic control, orthoimagery, elevation, transportation, hydrography, governmental units, and cadastral information.
topology: a branch of geometrical mathematics concerned with order, contiguity, and relative position rather than actual linear dimensions. Topology is used to establish and describe spatial relationships among geographic entities.
transaction: a unit of processing activity that accomplishes a specific purpose such as a retrieval, an update, a modification, or a deletion of one or more data elements in a storage structure. In the framework, transactions are offered as the preferred means of updating holdings of framework data.
transportation: features used to move people and goods from place to place, including roads, trails, railroads, ports, airports, waterways, and related features such as bridges and tunnels.
vector data model: a spatial data model in which the location of entities is represented using points, nodes, chains and areas.