Appendix A: Terminology
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
- 1 The definitions of many terms here are taken from work by
Guptill (1988) and the Federal Geographic Data Committee (1994). Other
reference sources include The International GIS Dictionary (McDonnell and
Kemp 1995) and work by professional associations (ASCE et al. 1994).
- 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 ("plus 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.
- automated mapping and facility management, a class of geographic data
applications often applied in the public works and utility industry and
- a unit of angle defined to be 1/60 of a minute, or 1/3600 of a
degree, of latitude or longitude.
- a generic term for a bounded, continuous, two-dimensional object that may
or may not include its boundary.
- a defined characteristic of an entity type.
- attribute value
- a specific quality or quantity assigned to an
attribute, for a specific
- the art or science of determining depths of oceanic or other deep
- 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.
- 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
- 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
- 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.
- 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
- 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
- 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
- a collection of information related by a common fact or purpose.
- 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).
- (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.
- 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
- 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.
- 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
- Federal Geographic Data Committee (FGDC)
- see appendix E for a full description.
- see Federal Geographic Data
- 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
- framework data
- seven themes of geospatial data
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
- 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.
- 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
- 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
- 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
- see geographic information
- 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
Global Positioning System.
- (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 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.
- surface water features, such as streams and rivers, lakes and
ponds, canals and ditches, and oceans.
- 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
- 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.
- a zero-dimensional object that is the topological junction of two or
more chains or an end point of a
- see National Spatial Data
- 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
- 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 heights are used in topographic mapping. (See also
- 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.
- 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.
- a two-dimensional picture element that is the smallest nondivisible
element of a digital image.
- see Public Land Survey
- a zero-dimensional object that specifies geometric location. One
coordinate pair or triplet specifies the location.
- 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.
- 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
- the process of matching details in separate data sets so that
corresponding data are coincident.
- a measure of the finest detail distinguished in an object or
phenomenon. For images, resolution also refers to the
- see Spatial Data Transfer
- 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
- a topic or subject. The framework's data themes are
governmental units, and
- 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.
- 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
- 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.