Glossary

for Exploring GIS, by Nicholas Chrisman


Index to glossary entries (in alphabetic order)
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V| W

The book has a running glossary with many terms defined at the bottom of the page. This resource collects all of those definitions and adds other definitions that appear in the body of the text.
Page numbers refer to the printed version of the text.

These terms are © Copyright, 1997 John Wiley and Sons. Use of these definitions should credit Exploring Geographic Information Systems by Nicholas Chrisman.

A

Absolute measurement p.16
a level of measurement higher than ratio where the unit of measurement is not an arbitrary decision, so the numbers cannot be rescaled and retain their meaning. Probability is an example of an absolute scale.
Accuracy p. 27
closeness of a measurement to a value thought to be true; repeatability can be estimated by repeated measurement, measured by variance for continuous measures; accuracy of classification for categories can be summarized by a misclassification matrix when compared to a survey of greater accuracy.
Areal interpolation p. 222
transformation of an attribute of one set of choropleth zones to another set of choropleth zones.
Attribute p. 6
the range of possible values of a characteristic; an attribute value is a specific instance of the characteristic associated with a geographic feature.
axiom p. 24
a proposition accepted as true without proof; an assumption that is formally recognized. (component of data model)
AVHRR p. 229
Advanced Very High Resolution Radiometer, a satellite sensing system operated by the US National Oceanic and Atmospheric Administration (NOAA) with a resolution of about 1.1 km, twice-daily repeat cycle, broad scenes (2400 km), and four bands of spectral data.

B

Bézier curve p. 62
a smooth curve that passes through specified points with a given direction (tangent) at those points.
bilinear interpolation p. 214
interpolation method where the value is obtained by linear interpolation on the two axes (row and column). Uses four neighbors from a raster representation of the surface, and averages out the inconsistency between the four possible triangles.
bit map p. 46
a spatial data structure that records a binary value (presence or absence of some attribute value) for a set of cells in a compact representation.
blunders p. 73
errors, usually due to human operators, whose magnitude cannot be modelled by regular statistical models.
Boolean algebra p. 109
system of operations applied to sets (and logical propositions); Boolean variables are zero or one, hence strongly connected to modern computing; originated by George Boole in 1847.
breadth-first p. 200
algorithms to traverse a tree that explore the structure by enumerating each level totally before moving to a finer level; as opposed to enumerating each path from the root to the 'leaves' (depth-first).
buffer p. 142
a zone constructed outwards from an isolated object to a specific distance.

C

CAD p. 31
computer-aided design, software packages designed to automate drafting of mechanical drawings.
cadastral p. 264
pertaining to the legal register of land parcels, particularly for land taxation or property transfer; more loosely, related to property parcels.
cartographic scale p. 82
formally, the ratio between distance measurements on a map and the same distance on the ground (for example 1:24000). In a digital environment, this term refers to the complex (and often unwritten) rules for selection, generalization and representation used for a particular map series. [Not to be confused with a scale of measurement.]
cartographic spaghetti p. 78
a cartographic data structure that represents each feature as a string of coordinate pairs, and imposes no rules for relationships or logical consistency.
case p. 24
In statistics, an individual unit of observation. Selecting the unit of analysis (discrete unit of control) establishes the measurement framework for statistically based studies.
categorical coverage p. 38
an exhaustive partitioning of a two-dimensional region into arbitrarily shaped zones that are defined by membership in a particular category of a classification system.
cell p. 44
a regular unit of spatial sampling. Usually applied to area-based measurement frameworks that treat a system of cells as an exhaustive coverage, rather than a set of points. Often functionally identical to pixel.
CGIS p. 71
Canada Geographical Information System, perhaps the earliest, certainly one of the most ambitious, prototypes of a modern GIS. (Tomlinson 1967) CGIS created a digital coverage of land suitability maps for the areas of Canada with agricultural potential.
chain p. 34
a directed set of non-intersecting line segments with nodes at each end and reference to left and right polygons.
choropleth framework p. 52
measurement framework whose spatial units (derived from a categorical coverage of named objects) serve as control for attribute measurement (e.g. census tabulation).
choropleth map p. 13
a thematic mapping technique that displays a quantitative attribute using ordinal classes applied as uniform symbolism over a whole areal feature. Sometimes extended to include any thematic map based on symbolism applied to areal objects.
cluster analysis p. 203
a procedure that groups points in a multidimensional space (attribute measurements) into clusters that minimize the distance from cluster centers (or some other objective function).
Cohen's kappa p. 122
a measure of agreement between two classifications. Defined as (observed accuracy - change agreement) / (1- chance agreement) where the chance agreement is estimated by the cross-product of marginal frequencies (statistical independence model).
compression p. 67
a software procedure that encodes a data structure so that its storage occupies less space (under certain conditions); may preserve all the information (loss-less) or deliberately simplify.
connected coverage frameworks p. 31
a general term for measurement frameworks that involve relationships between distinct spatial objects; includes network and categorical coverage frameworks.
contour p. 30
a line connecting points of equal elevation on a topographic surface. An control p. 27
a mechanism of restraint on the variation of a system to permit measurement of one component of a phenomenon while other components only vary within the limits of the control.
control point p. 72
a feature whose location can be established in an external spatial reference system (ideally, the Geodetic Reference System) and on the source material to be digitized.
coordinates p. 18
a structured set of measurements related to a specific spatial reference system; usually applied to pairs of distance measurements (X,Y) on independent axes of a planar reference system or to angular measurements such as latitude-longitude pairs on a spherical reference model.
counts p. 16
numerical measurements that aggregates the number of some objects within a collection unit (for example population). Counts depend on the definition of the objects counted, so they cannot be rescaled arbitrarily.
course p. 161
line on a surface where slopes converge from two directions. (See Topology of Topography.)
cubic convolution p. 214
interpolation method where the value is interpolated by fitting a third-order equation to the sixteen grid points surrounding the desired location.
custodian p. 265
an organization that takes responsibility to generate a particular kind of information for a defined geographic region and agrees to make it available to others.

D

dasymetric p. 224
a method proposed by John K. Wright to estimate densities using areal interpolation and quantitative puzzle-solving; more generally, a type of thematic map whose boundaries are conditioned by some other distribution.

database p. 24
structured collection of data with software to provide access in different ways; has a data model, a data structure and an implementation (representation).
database schema p. 245
logical arrangement of tables, attributes, and integrity rules to structure a database. Involves definitions of entities and their relationships.
data dictionary p. 246
detailed definitions of the codes employed for identifying objects and for attribute values.
data model p. 23
In the database literature
1) general description of sets of entities and the relationships between these sets of entities (Ullman 1982)
2) collection of object types, collection of operators on those object types and a collection of integrity constraints (Codd 1981).
[Integrity constaints are the axioms of a data model.]
In a GIS, composed of a measurement framework and a scheme for representation.
data structure p. 57
arrangement of data entities that permits the construction of relationships through software operations; implements a data model.
Delaunay triangulation p. 153
a network that connects each point in a set of points to its nearest neighbors; topological 'dual' of the Voronoi network.
DEM p. 41
a framework for recording spot elevations in a regular rectangular grid (matrix); an acronym originally created from Digital Elevation Model at US Geological Survey. To avoid ambiguity, DEM will be used exclusively for a grid framework, so it can be read matrix.
derived measurement p. 8
numerical measurements constructed from the relationship of other measurements (for example, a density is a ratio of weight by volume). [As opposed to extensive measurements.]
diagonal p. 121
cells in a square matrix whose row and column indices are the same. In a transition matrix, they represent no change; in an error matrix they represent no error.
Digital Line Graph (DLG) p. 82
a data format developed by US Geological Survey National Mapping Division that uses a topological vector model.
digitizer p. 70
a manually controlled machine that records a spatial measurement usually on the surface of a tablet.
DIME p. 86
Dual Independent Map Encoding, a digital database of streets and other census boundaries developed to conduct the 1970 US Decennial Census; an early and prominent implementation of a topological data structure.
discount rate p. 240
economic factor that deflates a future sum to make it comparable with current expenses. Reflects expectations of interest rates and inflation.
dynamic segmentation p. 55
a method for referencing attribute information along a network that does not divide each segment of the network wherever any attribute changes.

E

easement p. 236
a legal agreement that grants partial rights over a portion of a property, such as a utility easement to install overhead wires or underground pipes. Also applied to rights-of-way for roads.
efficiency p. 235
compares the resources expended to attain a given product
effectiveness p. 235
compares the nature of the products and the process of creating them
ellipsoid p. 60
three dimensional object formed by rotating an ellipse around its minor axis; an oblate ellipsoid approximates the shape of the Earth (geoid), computed by the best fit to geodetic observations. See Table 3-2 p. 76
equal interval p.93
a classification procedure that divides the total range of attribute values by the number of classes; breakpoints are spaced at equal intervals, whether the class has any member or not.
extensive measurement p. 8
numerical measurements along an axis where addition provides the basic rule; the base units of SI are extensive, as are units of monetary value (dollars, pounds sterling, francs, euros).

F

feature p. 31
cartographic feature: an instance of a defined class of objects that cannot be divided into objects of the same type. 31
field p. 29
an abstract construct of a mathematical relationship viewed as a spatial structure. In physics: a region of space subject to a force (as a magnetic field). In GIS, often used as a synonym for surface.
flexible production p. 257
a system of economic organization characterized by rapid changes in production plans, "just in time" delivery, and other techniques that break from the economy of scale, mass production approach characterized by the assembly line.
foreign key p. 97
item in a relational table that contains a value identifying rows in another table; represents a relationship between two elements of a relational database.
fuzzy set theory p. 16
an extension to set theory that permits an object to have a degree of membership (usually represented as a number between zero and one). Fuzzy membership values do not have to follow the rules of probability.
fuzzy tolerance p. 114
a distance within which intersections and points will be treated as coincident. To be processed correctly, the fuzzy tolerance cannot be handled immediately (otherwise a point might be moved twice and beyond its original tolerance). A 'cluster' of points must be grouped so that no point is moved more than the tolerance.

G

generalization p. 83
In cartography, conversion of a geographic representation to one with less resolution and less information content; traditionally associated with a change in scale.
geodetic p. 76
related to the science of earth measurement (geodesy).
geodetic control p. 29
a set of surveyed monuments used to define a spatial reference system for a particular project.
geoid p. 6
three-dimensional shape of the Earth defined by the surface where gravity has the value associated with Mean Sea Level. [The geoid is lumpy, while an ellipsoid is smooth, representing the average elevation of the geoid to some level of approximation.]
geographic information system (GIS) p. 5
1) "a system of hardware, software, data, people, organizations and institutional arrangements for collecting, storing, analyzing and disseminating information about areas of the earth." (Dueker and Kjerne, 1989, p. 7-8) [Commentary on this accepted definition]
2) the organized activity by which people measure aspects of geographic phenomena and processes; represent these measurements, usually in the form of a computer database, to emphasize spatial themes, entities and relationships; operate upon these representations to produce more measurements and to discover new relationships by integrating disparate sources; and transform these representations to conform to other frameworks of entities and relationships. These activities reflect the larger context (institutions and cultures) in which these people carry out their work. In turn, the GIS may influence these structures. [This definition in more graphic form.]
geostatistics p. 181
branch of statistical estimation concentrating on the incorporation of spatial measures, particularly distance and neighborhood, into models.
GIRAS p. 155
Geographic Information Retrieval and Analysis System, a project conducted by US Geological Survey in the 1970s; produced vector interpretations of land use/land cover stored in an early topological data structure.
GPS p. 77
Global Positioning System, a constellation of communications satellites that broadcast timing signals that can be converted into a distance measurement, permitting trilateration (surveying by knowing the sides of triangles).
graphic elements p. 15
the characteristics of a symbol system that can be manipulated to encode information (also called 'visual variables', graphic dimensions, etc.). For cartography, these include size, shape, hue, saturation, brightness, orientation and pattern. (See Robinson and others, 1995).
ground truth p. 121
a determination of geographic attributes judged to be of higher accuracy; usually applied to a 'point' classified into a land use/land cover category.

H

'heads-up' digitizing p. 71
a digitizing station that provides a graphical user interface on the screen of a workstation (hence sometimes called on-screen digitizing). The operator uses a pointing device (mouse or trackball) to navigate on the scanned image of the original source, without having to look down at a digitizing tablet.
heuristic p. 201
a procedure that attacks a problem in a way directed towards the goal, but not guaranteed to attain it exactly.

I

indirect measurement p. 50
a procedure that assigns attributes to spatial objects through some attribute of those spatial objects (such as a soil class or a political subdivision), not through direct measurement.
information p. 8
data (observations, measurements, etc.) placed in context of a system of meaning (a set of relationships and assumptions about those relationships). Information, built into larger context, constructs knowledge. [Loosely defined as a difference that makes a difference.]
integrated survey p. 137
an approach to land evaluation that combines the opinions of many disciplines in producing a common representation of the processes that form a landscape.
interval p. 12
a level of measurement that assigns numerical values to objects, but does not provide a 'zero' value to serve as a reference. Differences between interval values have meaning as ratio measures.
invariance p. 10
properties that remain unchanged despite transformations of the numbers used to represent a measurement.
isolated object frameworks p. 30
a general term covering those measurement frameworks that use a specific attribute value as a control to obtain spatial measurements; includes spatial object and isoline frameworks.
isoline p. 30
a line connecting points of equal elevation on a surface (See contour)
isoline framework p. 30
a measurement framework that establishes control by a systematic set of slices through an attribute to obtain lines that represent the surface.

J

join p. 97
procedure that attaches values from a database table to another table based on matching a foreign key to its primary instance.

K

kriging p. 212
a geostatistical technique for interpolation that uses information about the spatial autocorrelation in the vicinity of each point to provide 'optimal' interpolation (in the sense of greater use of the information provided by the spatial arrangement).

L

Landsat p. 29
a series of Earth-observing satellites. The early ones (first launched in 1972) had 80m resolution; the more recent ones include the 30m TM sensor.
land system p. 247
a unit of an integrated survey where common processes (involving soils, vegetation and other factors) have created a combination of features that will support a particular group of uses.
layer p. 18
a collection of related geographic data. Depending on context, a layer may have some specific relationship to other layers. For raster imagery, a layer can represent one sensor in a multispectral array. In some cases, a layer may contain a coverage, hence provide an attribute value at all locations. In some cases, a layer simply collects all features with a common graphic symbolism.
least-squares p. 73
an estimation procedure that minimizes the sum of squared deviations between observations and a numerical model for those observations; used in ordinary regression analysis and many other statistical procedures.
level of measurement p. 9
a grouping of measurement scales based on the invariance of certain properties. Measurement scales at a common level of measurement can be transformed into another scale at the same level without reducing the information content.
LLRW p. 126
Low Level Radioactive Waste; Low-level waste is generated (on the order of 160,000 m3 for the US per year) by nuclear power plants, hospitals and various other industries; excludes waste from weapons construction and the spent fuel from nuclear reactors (high-level waste).

M

mandates p. 265
organizing principles of purpose that drive an organization; in a bureaucracy, the laws, administrative rules and regulations that define the purpose and content of actions.
measurement framework p. 23
a scheme that establishes rules for control of other components of a phenomenon that permit the measurement of one component. In a geographic information system, there are three components: time, space, and attribute.
metadata
information describing a collection of data; includes source, access issues, format, schema and data quality.

N

national grid p. 74
a spatial reference system usually adopted by legal or administrative procedures for use through a given country. Typically consists of a projection or a system of projection zones plus a geodetic datum and ellipsoid.
NDVI p. 220
difference between two bands (near infrared minus visible red) divided by the sum of the two bands; high values indicate active vegetation growth; often applied to NOAA AVHRR data.
network framework p. 35
a measurement framework based on a set of distinct spatial objects (usually instances of linear objects like roads or rivers) that connect to form a network.
node p. 34
a zero-dimensional object that is a topological junction (or end point) and a geometric location. (Definitions for topological objects simplified from (Morrison 1987)).
nominal p. 10
a level of measurement based assigning objects into discrete categories.
North American Datum p. 76
an adjustment of geodetic measurements that provides the horizontal reference for North America. The 1927 Datum held Mead's Ranch, Kansas as a fixed point, while the 1983 Datum performed a simultaneous adjustment of all measurements. 1927 uses the Clarke's ellipsoid, while 1983 uses the 1980 Geodetic Reference System. The plural of a geodetic datum is 'datums', despite the Latin origins.
NP-Complete p. 203
class of algorithms conjectured to exhibit worst case performance that cannot be written as a polynomial equation of the number of objects processed.

O

objective function p. 201
the formula giving the goal in an optimization problem.
ordinal p. 10
a level of measurement that orders objects or categories, but does not provide further information regarding the distance between items.

P

parcel p. 226
contiguous unit of the earth's surface defined by a common collection of legal rights.
peak p. 161
point of local divergence on a surface; all neighboring points are lower. (Topology of topography)
pixel p. 29
smallest resolvable unit in an image; an area (usually rectangular) forming a part of a systematic, uniform division of a study area. Contraction of picture element.
planar graph p. 37
an arrangement of nodes and lines such that the lines intersect only at nodes, thus remaining embedded in a plane (or a surface topologically transformable onto a plane).
point-in-polygon p. 116
procedure to determine which points fall into which polygons; can attach attributes to either the points or the polygons.
polygon p. 34
an area (bounded continuous two-dimensional object) consisting of an interior area, one outer ring and zero or more non- intersecting non-nested inner rings.
polygon overlay p. 108
a procedure that calculates the geometric relationships between two geographic representations (usually applied to vector models); used to merge attribute information .
primitives p. 31
basic components that are sufficient to build a larger system; the primitives of two-dimensional geometry are points, lines, and areas.
projection p. 60
coordinate transformation that converts latitude longitude measurements into planar coordinates. Projections can be based on a developable surface (such as a plane, cylinder, or cone) or on a mathematical function.
proportional symbols p. 13
a thematic mapping technique that displays a quantitative attribute by varying the size of a symbol. Typically, proportional symbols use simple shapes such as circles, and are scaled so that the area of the symbol is proportional to the attribute value. Proportional symbols are located at a point, even when they represent information collected for an area.
prototype p. 16
an approach to categorization that defines a category by identifying a particular object as the typical example. Other objects assigned to this category may not not share all characteristics with the prototype object. The degree of resemblance can be treated as a distance in a taxonomic space.

Q

quadtree p. 67
a spatial data structure that organizes a hierarchical structure of square cells through iterative division into four daughter cells (Samet 1990).
quantile p. 93
a classification procedure that assigns an equal number of objects into each class. The interval of each class will vary unless the distribution is completely uniform.

R

raster p. 65
a spatial data model based upon a regular tessellation of a surface into pixels or grid cells.
ratio p. 12
a level of measurement that includes both extensive and derived measurements. The numerical values of the measurement have a meaningful zero reference and a fixed unit of measure.
ray-casting p. 187
computational technique used to simulate a visual scene with optical effects, variations in light sources and other effects; usually developed for arbitrary 3D objects, not just a single surface.
recursive p. 83
a programming procedure that invokes itself to subdivide a problem.
registration p. 72
the process of connecting a spatial representation to a broader spatial reference system.
relational database p. 97
a data model based on set theory. Each set has elements that can be uniquely defined by a primary key. A table (relation) stores all records for a set. Each record in a table has the same columns for attribute values. Relationships between tables are constructed by storing the key to a record in the other table (a foreign key).
representation p. 57
a symbolic mechanism to encode data. Using computers, involves a data model, a data structure and its implementation.
representationalism p. 9
a philosophy of measurement that defines measurement as the connection of numbers with entities which are not numbers. The numbers are not seen as inherent but as a representation of a defined aspect of the entity. [This philosophy is opposed to the classical theory that measurement is the numerical expression of one quantity relative to another; quantities were seen as inherent.]
resampling p. 214
a transformation usually applied to convert raster data into another arrangement of raster cells. May require interpolation to determine the attribute at a position not sampled by the original grid.
resolution p. 27
least detectable difference in a measurement; in a geographic context, resolution applies to all components (time, space and attribute) according to the measurement framework. Not identical to accuracy.
retraced line p. 63
a technique applied to simple vector data structures to embed inner rings within outer rings. A line that is repeated (drawn twice) should not appear graphically.
ridge p. 161
line along which a surface diverges in two different directions. (Topology of topography)
ring p. 35
a sequence of non-intersecting chains that close.

S

scalar field p. 158
a surface whose value can be represented by a single number.
scale (measurement) p. 10
a system used to encode the results of a measurement; typically a number line, but generalized to include a list of categories. [Distinct from cartographic scale.]
setback p. 142
a zone inside a polygon constructed by a fixed distance from the edge of the polygon; typically used to restrict building or activities too close to the edge of a property parcel.
SI p. 9
Système International d'Unités; the system of weights and measures established by international agreement in 1875. The International Bureau of Weights and Measures in Sèvres, France oversees the measurement standards. SI defines seven base units from which many others can be derived: meter-length, kilogram-mass, second-time, kelvin-temperature, ampere- electric current, mole-chemical quantity, candela- intensity of light.
sliver p. 114
an artifact of polygon overlay; usually created by overlay of two sources with different accuracy, different sources or different interpretations.
slope p. 158
a multicomponent property associated with a surface at each point; the vertical component (gradient) expresses the local rate of change, the horizontal component (aspect) gives the direction.
soil inclusions p. 52
a category of soil expected to occur inside the units mapped as another category.
spatial autocorrelation p. 181
degree of correlation between a surface value and the values of its neighbors; propensity of spatial data to vary smoothly. (Core element of geostatistics.)
Spatial Data Transfer Standard (SDTS) p. 34
a specification of generic data formats and metadata content to promote interchange of geographic information. Adopted by the US Government as Federal Information Processing Standard (FIPS) 173.
spatial object framework p. 30
a measurement framework that identifies a particular category, then maps the location of that object (as a point, a line or an area).
spatial reference system p. 18
a mechanism to situate measurements on a geometric body, such as the Earth; establishes a point of origin, orientation of reference axes, and geometric meaning of measurements, as well as units of measure. [basis of coordinates.]
spline p. 62
a smooth curve that models the behavior of a thin spring (with a given modulus of elasticity) constrained to pass through specified points.
SQL p. 93
Structured Query Language; a standard interface for access to a relational database through queries that select records matching logical expressions.
stand p. 237
a contiguous area of a forest considered to be homogeneous in its ability to support the intended forest crop. Used to direct forest practices such as planting, thinning, and harvest.
surface p. 29
a spatial distribution which associates a single value with each position in a plane; (technically a field of a single-valued function) usually associated with continuous attributes.
SYMAP p. 211
SYnagraphic MAPping package, developed under the direction of Howard Fisher at Harvard Laboratory for Computer Graphics beginning in 1966.

T

temporal reference system p. 17
an agreed measurement scheme for time; involves a time to start counting (an origin) and a unit. International conventions establish a calendar and synchronized clocks based on Greenwich Mean Time.
tensor p. 158
a multi-component surface of higher degree than a vector. The theory of relativity requires tensors to handle space-time and electromagnetic fields.
TIFF p. 67
Tagged Image File Format: a family of image encoding formats which can vary the resolution and the number of bits used to represent each cell.
TIN p. 49
Triangulated Irregular Network: a system of terrain representation that builds triangular facets to connect point heights. The points and triangles are chosen to represent a surface within some limits.
TM p. 230
Thematic Mapper, a satellite sensing system with resolution of 30 meters, 16 day repeat cycle, 185 km scene width and seven bands of spectral data; launched by NASA.
topology p. 34
a branch of mathematics concerned with those properties of geometry which are independent of a distance metric and are unchanged by any continuous deformation. In cartography, topology refers to combinatorial topology. The other branch, point-set or algebraic topology, "emerged in the twentieth century as a subject that unifies almost the whole of mathematics" (Boyer and Merzbach 1991, p. 622) and is used as the basis for relational databases.
topology of topography p. 161
qualitative (ordinal) relationships in the structure of a surface. Topography refers to the surface of the earth.
transverse p. 75
a projection oriented at right angles to the equator. A transverse cylindric projection uses a meridian of longitude as its central meridian.
travelling salesman problem p. 203
given a graph connecting a set of nodes, devise a route that visits each node in the graph exactly once and minimizes the total cost accumulated.
trend surface p. 210
result of a statistical fit of the observed values using a selected spatial equation (from a plane up to higher order shapes).

U

unit of measure p. 94
the reference value used to establish a ratio-scaled measure; provides the value of 1 for a ratio scale.
USLE p. 137
Universal Soil Loss Equation; predicts average soil loss (in tons) for a period of time (a storm event or a year) as the product of six factors: Rainfall intensity, Erodability of the soil, Length of slope, Slope gradient, Crop, and Practices.
UTM p. 75
Universal Transverse Mercator; a spatial reference system using a set of transverse Mercator projections six degrees wide that cover the Earth (except for polar regions covered by two Polar Stereographic projections).

V

variable length list p. 59
a data structure that can store a flexible number of elements in an ordered sequence. Frequently implemented with a count followed by the elements of the list.
vector p. 62
a spatial data model based on geometric primitives (point, line, and area), located by coordinate measurements in a spatial reference system; from mathematical term for a direction, or a directed line segment.
vector field p. 158
a multi-component surface whose values have a quantity and a direction in space. Newtonian physics can be expressed as vectors. This mathematical term is the origin of the term 'vector' applied to a geometric data structure, but the connection is indirect.
Voronoi network p. 153
a set of lines that divides a plane into the area closest to each of a set of points. The lines are perpendicular bisectors of the lines connecting nearest points (Delaunay triangulation).

W

watershed p. 161
area bounded by ridges that would converge (downhill) to a single exit point. (see topology of topography)
well-defined point p. 76
a point-like (isolated) feature that can be distinguished on the source and on the ground to sufficient accuracy; in US National Map Accuracy Standards of 1947, implemented as 'plottable to .01 inch'.

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Version of 1 January 1997