*[Most of the terms and definitions are from Department of Commerce, 1992, Spatial Data Transfer Standard (SDTS) (Federal Information Processing Standard 173): Washington: Department of Commerce, National Institute of Standards and Technology.]*

**abscissa** -- the coordinate of a point in a plane Cartesian coordinate system obtained by measuring parallel to the x-axis ("the 'x' value").

**accuracy** -- the closeness of results of observations, computations or estimates to the true values or the values accepted as being true.

**altitude** -- elevation above or below a reference datum, as defined in Federal Information Processing Standard 70-1. See also elevation.

**area** -- a generic term for a bounded, continuous, two-dimensional object that may or may not include its boundary.

**area chain** -- a chain that explicitly references left and right polygons and not start and end nodes. It is a component of a two-dimensional manifold.

**area point** -- a representative point within an area usually carrying attribute information about that area.

**arc** -- a locus of points that forms a curve that is defined by a mathematical expression.

**attribute** -- a defined characteristic of an entity type (e.g. composition).

**attribute value** -- a specific quality or quantity assigned to an attribute (e.g., steel), for a specific entity instance.

**cardinality** --the number of elements in an extended compound element

**chain** -- a directed nonbranching sequence of nonintersecting line segments and (or) arcs bounded by nodes, not necessarily distinct, at each end. Area chain, complete chain, and network chain are special cases of chain, and share all characteristics of the general case as defined above.

**child** -- the name of the data element which may occur under this data element. A child element may be an extended or a standard element.

**clearinghouse** -- see National Geospatial Data Clearinghouse.

**complete chain** -- a chain that explicitly references left and right polygons and start and end nodes. It is a component of a two-dimensional manifold.

**compound element** -- a group of data elements and other compound elements. Compound elements represent higher-level concepts that cannot be represented by individual data elements.

**coordinates** -- pairs of numbers expressing horizontal distances along orthogonal axes; alternatively, triplets of numbers measuring horizontal and vertical distances.

**data element** -- a logically primitive item of data.

**data set** -- a collection of related data.

**depth** -- perpendicular distance of an interior point from the surface of an object.

**developable surface** -- a surface that can be flattened to form a plane without compressing or stretching any part of it. Examples include cones and cylinders.

**digital image** -- a two-dimensional array of regularly spaced picture elements (pixels) constituting a picture.

**digital volume** -- a three-dimensional array of regularly spaced volume elements (voxels) constituting a volume.

**domain** -- in the definition of the elements in the metadata standard, the domain identifies valid values for a data element.

**Edge, Topology Level 0** -- VPF term for a string.

**Edge, Topology Level 1** -- VPF term for a network chain in a network (in SDTS, a "Network chain, non- planar graph").

**Edge, Topology Level 2** -- VPF term for a network chain in a planar graph (in SDTS, a "Network chain, planar graph").

**Edge, Topology Level 3** -- VPF term for a complete chain.

**elevation** -- conforming to Federal Information Processing Standard 70-1, the term "altitude" is used in this standard, rather than the common term elevation.

**entity instance** -- a spatial phenomenon of a defined type that is embedded in one or more phenomena of different type, or that has at least one key attribute value different from the corresponding attribute values of surrounding phenomena (e.g., the 10 Street Bridge).

**entity point** -- a point used for identifying the location of point features (or areal features collapsed to a point), such as towers, buoys, buildings, places, etc.

**entity type** -- the definition and description of a set into which similar entity instances are classified (e.g., bridge).

**explicit** -- method of identifying positions directly by pairs (for horizontal positions) or triplets (for horizontal and vertical positions) of numbers.

**extended element** -- a user-defined metadata element included in a metadata collection. Extended elements may be defined by a data set producer or a user community. Extended elements are elements outside the Standard, but needed by the data set producer. If extended elements are created, they must follow the guidelines in Appendix D, Guidelines for Creating Extended Elements in the Content Standard for Digital Geospatial Metadata.

**Face, Topology Level 3** -- VPF term for a GT-polygon composed of rings.

**G-polygon** -- an area consisting of an interior area, one outer G-ring and zero or more nonintersecting, nonnested inner G-rings. No ring, inner or outer, shall be collinear with or intersect any other ring of the same G-polygon.

**G-ring** --a string composed of pairs of longitude and latitude coordinates that define a closed non- intersecting boundary.

**G-ring point** -- a scalar consisting of a set of ordered pairs of floating-point numbers, separated by commas, in which the first number in each pair is the longitude of a point and the second is the latitude of the point. Longitude and latitude are specified in decimal degrees with north latitudes positive and south negative, east longitude positive and west negative.

**geospatial data** -- information that identifies the geographic location and characteristics of natural or constructed features and boundaries on the earth. This information may be derived from, among other things, remote sensing, mapping, and surveying technologies.

**graph** -- a set of topologically interrelated zero-dimensional (node), one-dimensional (link or chain), and sometimes two-dimensional (GT-polygon) objects that conform to a set of defined constraint rules. Numerous rule sets can be used to distinguish different types of graphs. Three such types, planar graph, network, and two-dimensional manifold, are used in this standard. All three share the following rules: each link or chain is bounded by an ordered pair of nodes, not necessarily distinct; a node may bound one or more links or chains; and links or chains may only intersect at nodes. Planar graphs and networks are two specialized types of graphs, and a two-dimensional manifold is an even more specific type of planar graph.

**grid** -- (1) a set of grid cells forming a regular, or nearly regular, tessellation of a surface; (2) a set of points arrayed in a pattern that forms a regular, or nearly regular, tessellation of a surface. The tessellation is regular if formed by repeating the pattern of a regular polygon, such as a square, equilateral triangle, or regular hexagon. The tessellation is nearly regular if formed by repeating the pattern of an "almost" regular polygon such as a rectangle, non-square parallelogram, or non-equilateral triangle.

**grid cell** -- a two-dimensional object that represents the smallest nondivisible element of a grid.

**GT-polygon** -- an area that is an atomic two-dimensional component of one and only one two-dimensional manifold. The boundary of a GT-polygon may be defined by GT-rings created from its bounding chains. A GT-polygon may also be associated with its chains (either the bounding set, or the complete set) by direct reference to these chains. The complete set of chains associated with a GT-polygon may also be found by examining the polygon references on the chains.

**GT-ring** -- a ring created from complete and (or) area chains.

**horizontal** -- tangent to the geoid or parallel to a plane that is tangent to the geoid.

**implicit** -- method of identifying positions by a place in an array of values.

**interior area** -- an area not including its boundary.

**label point** -- a reference point used for displaying map and chart text (e.g., feature names) to assist in feature identification.

**latitude** -- angular distance measured on a meridian north or south from the equator.

**layer** -- an integrated, areally distributed, set of spatial data usually representing entity instances within one theme, or having one common attribute or attribute value in an association of spatial objects. In the context of raster data, a layer is specifically a two-dimensional array of scalar values associated with all of part of a grid or image.

**line** -- a generic term for a one-dimensional object.

**line segment** -- a direct line between two points.

**link** -- a topological connection between two nodes. A link may be directed by ordering its nodes.

**longitude** -- angular distance between the plane of a meridian east or west from the plane of the meridian of Greenwich.

**map** -- a spatial representation, usually graphic on a flat surface, of spatial phenomena.

**media** -- the physical devices used to record, store, and (or) transmit data.

**meridian** -- a great circle on the Earth that passes through the geographic poles.

**metadata** -- data about the content, quality, condition, and other characteristics of data.

**name** -- the name of an extended element. The extended element name must not be the name of any other element in the Standard.

**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 will describe available data in an electronic form, and provide 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.

**network** -- a graph without two dimensional objects. If projected onto a two-dimensional surface, a network can have either more than one node at a point and (or) intersecting links or chains without corresponding nodes.

**network chain** -- a chain that explicitly references start and end nodes and not left and right polygons. It is a component of a network.

**node** -- a zero-dimensional object that is a topological junction of two or more links or chains, or an end point of a link or chain.

**Node, Topology Level 0** -- VPF term for a point (in SDTS, a "point").

**Node, Topology Level 1** -- VPF term for a node on a network (in SDTS, a "node, network").

**Node, Topology Level 2** -- VPF term for a node on a planar graph (in SDTS, a "node, planar graph").

**Node, Topology Level 3** -- VPF term for a point used to represent isolated features. These are topologically linked to a containing face.

**object** -- a digital representation of all or part of an entity instance.

**optionality** - The optionality of a section or compound element always takes precedence over the elements that it contains. Once a section or compound element is recognized by the data set producer as applicable, then the optionality of its subordinate elements is to be interpreted. See Production Rules section for additional interpretive guidance.

**ordinate** -- the coordinate of a point in a plane cartesian coordinate system obtained by measuring parallel to the y-axis ("the 'y' value").

**parent** -- the name of the data element under which a given data element may occur. A parent element may be an extended or a standard element.

**phenomenon** -- a fact, occurrence or circumstance. Route 10, George Washington National Forest, and Chesterfield County are all phenomena.

**pixel** -- two-dimensional picture element that is the smallest nondivisible element of a digital image.

**planar graph** -- the node and link or chain objects of the graph occur or can be represented as though they occur upon a planar surface. Not more than one node may exist at any given point on the surface. Links or chains may only intersect at nodes.

**point** -- a zero-dimensional object that specifies geometric location. One coordinate pair or triplet specifies the location. Area point, entity point, and label point are special implementations of the general case.

**primitive** -- the quality of not being subdivided; atomic.

**quality** -- an essential or distinguishing characteristic necessary for cartographic data to be fit for use.

**raster** -- one or more overlapping layers for the same grid or digital image.

**raster object** - one or more images and/or grids, each grid or image representing a layer, such that corresponding grid cells and/or pixels between layers are congruent and registered.

**rationale** - a component of an extended element. The rationale is provided by the user creating the extended element to explain the reason for its creation and its expected uses.

**repeatability** --whether or not an extended element can be repeated and optionally a minimum or maximum number of occurrences or both

**resolution**-- the minimum difference between two independently measured or computed values which can be distinguished by the measurement or analytical method being considered or used.

**ring** -- sequence of nonintersecting chains or strings and (or) arcs, with closure. A ring represents a closed boundary, but not the interior area inside the closed boundary.

**SDTS** -- the Spatial Data Transfer Standard defined by Department of Commerce, 1992, Spatial Data Transfer Standard (SDTS) (Federal Information Processing Standard 173): Washington, Department of Commerce, National Institute of Standards and Technology.

**short name** -- a unique name for each compound or primitive data element consisting of eight alphabetic characters or less. When creating extended element short names, do not duplicate an existing standard element short name.

**source** --a component of an extended element. the name of the individual or organization creating an extended element

**spatial data** -- see geospatial data.

**stratum** -- one of a series of layers, levels, or gradations in an ordered system. For this standard, the term is used in the sense of (1) a region of sea, atmosphere, or geology that is distinguished by natural or arbitrary limits; (2) a socioeconomic level of society comprised of persons of the same or similar status, especially with regard to education or culture; or (3) a layer of vegetation, usually of the same or similar height.

**string** -- a connected non-branching sequence of line segments specified as the ordered sequence of points between those line segments. Note: A string may intersect itself or other strings.

**two-dimensional manifold** -- a planar graph and its associated two dimensional objects. Each chain bounds two and only two, not necessarily distinct, GT-polygons. The GT-polygons are mutually exclusive and completely exhaust the surface.

**type** -- in the definition of the elements in the metadata standard, a compound element has the type "compound" to provide a unique way to identify compound elements. For a data element, the type identifies the kind of value that can be assigned to the data element. The choices are "integer" for integer numbers, "real" for real numbers, "text" for ASCII characters, "date" for day of the year, and "time" for time of the day.

**universe polygon** -- defines the part of the universe that is outside the perimeter of the area covered by other GT-polygons ("covered area") and completes the two-dimensional manifold. This polygon completes the adjacency relationships of the perimeter links. The boundary of the universe polygon is represented by one or more inner rings and no outer ring. Attribution of the universe polygon may not exist, or may be substantially different from the attribution of the covered area.

**vector** -- composed of directed lines.

**vertical** -- at right angles to the horizontal; includes altitude and depth.

**VPF** -- the Vector Product Format defined by Department of Defense, 1992, Vector Product Format (MIL- STD-600006): Philadelphia, Department of Defense, Defense Printing Service Detachment Office.

**void polygon** -- defines a part of the two-dimensional manifold that is bounded by other GT-polygons, but otherwise has the same characteristics as the universe polygon. The geometry and topology of a void polygon are those of a GT-polygon. Attribution of a void polygon may not exist, or may be substantially different from the attribution of the covered area.

**voxel** -- a three-dimensional element that is the smallest nondivisible element of a digital volume.